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What Lies Beneath: Interior Renovation of Aging Piper Aircraft

What Lies Beneath: Interior Renovation of Aging Piper Aircraft

Renovating an aircraft interior is a prime opportunity to see what’s really underneath all that ugly old upholstery. Whether you do it yourself, hire it out, or choose a combination of the two, you want to make informed decisions. Industrial designer and airplane interior expert Dennis Wolter is here with a series of articles to help you.

What a difference 50 years can make. During my college days back in the late 1960s, I had a job where I would occasionally be sent to ferry a new airplane from the distributor back to Lunken Field (KLUK, now Cincinnati Municipal) for delivery to its new owner. To this day, I can remember the look, feel and smell of a new, ferry-time-only airplane. 

Very few of us have the opportunity to own a brand-new airplane, but there are obviously many older airplanes out there in need of upgrades that can in many ways elevate them to exceed the design, comfort, performance and safety of a new airplane. 

After graduating from the University of Cincinnati as a newly-minted industrial designer, I tried two “real” jobs before deciding to pursue my true passion of renovating airplanes. The key word here is passion; I believe it is the glue that holds the General Aviation community together. 

Folks who own, fly and maintain these wonderful machines do it primarily out of love of flying. I am committed to helping save these irreplaceable machines. In coming months, I will submit a series of articles that will cover in detail the work required to truly renovate an entire cabin, from the firewall to the aft bulkhead. 

I plan to walk readers through each step in the process, with photos showing all of the tricks we’ve learned over the past 44 years—including the tools and supplies required—to thoroughly execute the job.

Common issues in aging aircraft

I often mention in seminars that the time has come in this world of General Aviation to either save these airplanes for future generations of pilots, or let them slowly deteriorate and end up in a salvage yard, where their corroded airframes will be picked clean of usable parts to support the dwindling number of still-airworthy airframes. 

Some readers may feel that this is an overly pessimistic comment, but in my business, I see far too many airplanes that are deteriorating before our eyes. Various aging-airplane problems seem to apply to all makes and models of older airplanes, no matter the value of the airframe. High-dollar Senecas are as susceptible as Cherokee 140s.

Installing a new interior in a Piper presents a great opportunity to really see into every nook and cranny of the cabin area with all interior components and insulation removed. 

About 15 years ago, I realized we were dealing with an increasing number of non-upholstery issues, such as corrosion (Photo 01, Page 55); questionable wiring (Photo 02, Page 55), leaking windows (Photo 03, Page 55), degraded static and fuel lines, and so on. 

These problems were becoming almost commonplace in the majority of airplanes going through our shop, so I decided to make it standard practice to invest the time necessary to repair all of the technical issues we discovered. 

To address corrosion, we started with antiseptically cleaning all floors, bulkheads and cabin skins, followed by a thorough application of corrosion-controlling zinc chromate. (The corrosion mitigation process will be covered in-depth in a future article. —Ed.)

Corrosion isn’t the only aging airplane issue that must be addressed, however. We often encounter other problems such as neglected or poorly-executed maintenance and carelessly-installed upgrades. 


Addressing problems takes time

This total approach to renovation has made a very pronounced change in both the time required as well as the cost involved in fully renovating the interior of a 40-plus-year-old Piper. When Air Mod first opened its doors in 1973, the scope of work required to renovate an aircraft interior was much less than the task we face today. 

I remember one of the first interiors we did in a six-place 1964 Aztec. Working 12-hour days, a part-time assistant and I completed the job in two weeks. This turnaround time was possible for a number of reasons. 

First and foremost was the fact that the airplane was only 10 years old, and it certainly did not present many of the issues we would see today in the same, now 54-year-old airplane. 

A second factor was that, at the time, our customers typically did not add items beyond what we included in a full interior renovation. Today, an extensive list of upgrade items can bring airplanes up to 21st century standards. 

These optional upgrades include safety-enhancing four-point inertia reel shoulder harnesses; more efficient, quieter ventilation systems; LED lighting; super soundproofing; thicker glass; fully-articulating seats; composite side panels with recessed armrests (see Photo 04, Page 56); and custom instrument panels with exotic wood trim, to mention a few.

A third issue adding to today’s longer downtimes involves avionics installations and maintenance. Our company partners with two companies located at Clermont County Airport (I69), and coordinating these projects while the interior is being renovated makes it convenient for a customer. 

But the bottom line is this: the Aztec interior that took two weeks to complete in 1974 has become an eight- to 10-week project in 2018.

Proper intervention is key 

Here’s the good news. For most of the airplanes in the fleet, the level of degradation is at a point where it can be stopped with proper intervention, saving these airframes for the future. 

The other good news is that 95 percent of what is required to mitigate corrosion and upgrade an interior can legally be done by an owner under the FAA guidelines of preventive maintenance, FAR Part 43, Appendix A(c). 

AC 43.13-1B is an excellent “how-to” manual from the FAA, and a must for every handy aircraft owner to have. Armed with this great resource, you will have concise and clear guidance as to how to properly perform many of the tasks that are required to keep your airplane in tip-top shape.

Steps in a cabin and interior renovation project

The following is a list of the steps we take during a typical renovation of cabin and interior in a 40-year-old airplane. In upcoming months, I will work my way through the major items on this list, with descriptions of processes accompanied by photographs. 

1. Remove and secure all documents and personal items.

2. Perform an ergonomic study of the pilot seating station. 

3. Test radios, intercom, autopilot, electrical components and lights for function. Report findings and recommendations.

4. Remove and evaluate existing side panels, seats, headliner, carpet, insulation, floor inspection panels and window trim.

5. Inspect all structures and skins for corrosion. Remove corrosion and glue from inner cabin skins, spar carry-through and related components. Treat all inner surfaces and appropriate components with corrosion-control materials.

6. Clean exposed antenna connections and inspect all systems and controls. 

7. Strip seats to bare frames to perform a complete mechanical and structural inspection. Repair as required.

8. Install heavy sling reinforcement straps on seat frames and install a new seat sling. The reinforcement keeps the sling from stretching or coming loose from the frame and prevents future sagging.

9. If requested, build the height of the seat back structure to accommodate the stature of the pilot. Many of our customers choose this option as an alternative to a headrest. (Note: this requires appropriate FAA approval and a Form 337.)

10. Build new seat foam, shaping with several densities of flame retardant urethane foam. The new foam is contoured to fit the customer as determined by measurements taken earlier.

11. Clean, mask, prime and paint all seat frames to match the new interior color scheme.

12. Sew and fit the new seat upholstery. Shaping is done with hidden sewn-in rods and pulls to insure long-lasting structural integrity, eliminating sagging and shifting. All seams are double-lock stitched to prevent seam failure. All seat panels have backing foam and backing fabric to insure proper fill.

13. Strip side panels to bare metal. Factory cardboard panels are replaced with new aluminum panels; existing metal panels are repaired to be in like-new condition or made new. 

14. Temporarily install the non-upholstered panels. Check for fit and layout of the new design; modify as necessary to ensure ease of installation and removal when upholstered. Fill in ashtray holes if requested.

15. Sew and mount side panels using new high-density, flame retardant backing foam and upholstery material of choice.

16. Prep, paint and placard plastic trim, door and window trim, and cabin components. 

17. Clean, mask, sand, fill and paint all door frames and related interior airframe trim with custom-matched interior paint.

18. Install new insulation behind side panels, in doors and behind headliner. 

19. Clean below floors and behind rudder panels as required.

20. Clean all seat tracks. Buff or paint heater outlets and similar components.

21. Install new windlace cord on doors; install reupholstered side panels using new hardware.

22. Strip headliner panels and repair or replace as required. Plastic headliners are re-formed and reinforced with aluminum as necessary to prevent future sagging or warping.

23. Fit, sew and install new headliner and reupholstered sunvisors, if applicable.

24. Cut and fit new carpets for cabin floor, baggage compartment and related surfaces. Special attention is made to allow for future removal and reinstallation without removing seats.

25. Serge all carpet edges; sew on Velcro and heel pads. Insulate the underside of the floor carpets with heavy density, flame-retardant foam. Bond Velcro to floors and install the new carpet.

26. Reinstall existing lap belts and shoulder harnesses, or install new and/or re-webbed components. 

27. Install cabin seats. Lubricate all door and seat latches.

28. Perform a safety and function check on the interior, radios, flight controls and electrical components.

29. An A&P mechanic will make all necessary logbook entries and weight and balance changes, and check that all placards are in place.

30. Wash and ground-run aircraft to prepare for customer delivery.


Other items that may be addressed

Some additional items that may be addressed during a renovation include: reinforcement and repair of aluminum and plastic cabin components; replacement parts; painting and placarding of instrument panel, pedestal and circuit breaker panels. 

Other items might include installation of special composite insulation and soundproofing; glareshield modification, repair and upholstery; repairs to or replacement of side panel components and repair or replacement of damaged floor boards. 

In addition, the owner may choose to add extra map cases, storage boxes, cup holders and gooseneck maplights and install a new windshield and/or windows.

Things to consider

Some owners will read through this list and realize that some of the work is beyond their ability. (Sewing seats and headliners come to mind.) Fortunately, there are companies who can provide quality interior kits and components with good product support for those wanting to install a mail-order interior. 

I highly recommend that any owner undertaking interior renovation work seek the advice of his or her A&P mechanic and arrange for that mechanic to inspect the stripped-out cabin structure, systems and seat frames for signs of any airworthiness issues. These areas can be hidden from view during routine maintenance when interior components and insulation remain in place.

Many owners may choose to renovate their airplanes incrementally, removing one side panel at a time and cleaning, chromating and insulating the exposed structure of that one area. I have mentored a number of people through this process over the years. 

Whether you are planning to have a professional shop renovate your airplane, or you plan to do part or all of it yourself, stay tuned. Upcoming articles should help guide you in your decision-making. 

Until next time, fly safe!

Industrial designer and aviation enthusiast Dennis Wolter is well-known for giving countless seminars and contributing his expertise about all phases of aircraft renovation in various publications. Wolter founded Air Mod in 1973 in order to offer private aircraft owners the same professional, high-quality work then only offered to corporate jet operators. Send questions or comments to .

It’s Re-Bladder Time

It’s Re-Bladder Time

Contributing editor and A&P Steve Ells recently installed an FFC fuel bladder in the left tank of his 1960 Comanche.

I open the cabin door of my airplane to start my preflight inspection I’ve been catching a whiff of Avgas. 

I checked around the shaft of the fuel selector thinking that fuel was wicking up past the selector shaft seal, but didn’t see the telltale blue stain. I pretzel-ed myself into position to double-check, and risked (more?) brain damage by sucking in a big whiff. 

Nope, that wasn’t the source of the leak.

I looked in the engine compartment—maybe there was a leak there? But a thorough visual inspection didn’t reveal any blue staining. 

Uff da, I thought to myself. This lack of evidence narrows the source of the smell to one of the fuel bladders—one of the “rubber bags” snugly hidden away inside the fuel tank bay of the wing. 

I removed the wing root fairing and saw it: a small blue stain. 

It wasn’t very big, yet it was there.

Bladder backstory

According to one source, fuel bladders were installed in Piper airplanes for two reasons. First, the military used bladders because it thought that the bladder—being flexible—was much less likely than an aluminum or integral tank to burst during a crash. 

And second, installing a bladder—even the semi-stiff bladders manufactured 70 years ago—took less time than manufacturing and welding up a rigid tank or sealing the wing structure to create an integral tank. 


Causes of fuel bladder failure 

The most common cause of bladder failure is porosity of the upper surface. 

It’s pretty well known that fuel bladder life is maximized by keeping the tank as full as possible. This lessens plasticizer loss by keeping temperatures moderated. 

But the breakdown of the upper surface is inevitable and when it gets to a certain point, fuel starts to evaporate out of the tank. 

I had noticed that the fuel level in the left tank had dropped by an inch or so between flights; another sure sign the fuel was evaporating out the porous top surface of the tank. 

I needed a bladder and after checking with a couple of trusted techs for suggestions, ordered one from Floats & Fuel Cells (FFC) in Memphis, Tenn. 

FFC technology

A few days after ordering, a big box arrived. I was surprised at how light it was, but after opening the box and inspecting the new bladder, I understood why. 

New fuel bladders from Floats & Fuel Cells are very flexible and are visibly smoother than the old, semi-rigid bladder. Sort of like the difference between a 1960 Cadillac El Dorado and a 2015 Tesla. 

Each corner of my new bladder tank is rounded and each potential wear point and corner is smoothly reinforced. According to Brewer, this is because bladders from FFC are cured in an autoclave that melds all the parts together by utilizing temperature and pressure to create a one-piece unit.

And it’s light. The weight of the 30-gallon bladder is 4.9 pounds.

Modern bladder-style tanks from Floats & Fuel Cells are constructed of a proprietary P2393 nitrile rubber. Nitrile combines “excellent resistance to petroleum-based oils and fuels, silicone greases, hydraulic fluids, water and alcohols with a good balance of such desirable working properties as low compression set, high tensile strength and high abrasion resistance,” according to one reference book. The only drawback is a low resistance to attacks by ozone. 

FFC’s bladders consist of a four-layer construction. The layers (from inner to outer) are a nitrile layer that retards permeability; a transparent fuel vapor barrier that’s bonded to a nylon fabric layer; and an outer layer of nitrile rubber formulated to resist ozone attacks. 

Remove and replace

I flew the fuel level in the left bladder down to six gallons before siphoning the remaining fuel into grounded fuel cans. (Important: Always establish a ground between the aircraft and the fuel cans to prevent static electricity from spontaneously igniting the fuel vapors.)

I next removed the large access (top) plate and the fuel quantity sender from the top of the wing and removed the screws and bolts connected to the reinforcing/mounting ring molded into the top of the bladder. 

I tried to wrestle the old semi-rigid bladder out and up through the access hole, but soon realized there was a vast difference between the new, flexible, lightweight FFC bladder that I had just unpacked and inspected and the semi-rigid heavy bladder that was leaking. 

I needed enough room to shove most of my right arm down inside the old bladder to free the feed nipple and pull loose the clips holding it in position. 

I took the easy way—I cut the old bladder into pieces. It was a good decision; a box knife and scissors was all it took. That transformed the process of working in the tank bay from an arm-bruising struggle into an easy job. 

The rate-determining step in the removal of older bladder tanks is getting the fuel feed nipples that are molded into each bladder to release from the rigid airframe fuel feed lines. The bladder in my Piper Comanche has a single feed nipple located in the aft inboard corner of the tank.

Due to my prior experience in struggling to pull an old bladder free of the feed line, I spent at least an hour using my box knife to cut away sections of the nipple from the feed line. It wasn’t difficult because the nipple was old and brittle. 

Not content with that, I then spent time spraying Kroil between the nipple and the feed line after I had shoved a small pointed punch between the two surfaces. (This penetrating oil spray is designed to free up frozen hardware, but I figured that since it penetrates so well it would work to break the seal between the nipple and the rigid feed line.)

I worked the punch—and sprayed the Kroil—between the feed line and the nipple from the wing root and from inside the tank. Then I went home for the day.

The next day I applied straight pulling pressure and to my surprise the bladder nipple slid easily off the fuel feed line and over the coarse screen at the end of the line. Removal complete!


They don’t last long? 

There’s a tale that has been passed along at the preflight planning table and in the pilot’s lounge that bladders don’t last long and that they’re very hard to change. Hogwash. 

Six years ago, I changed the right bladder in my airplane. This year, I changed the left bladder. 

The left bladder was manufactured in 1957 and installed in 1959 in my 1960 vintage airplane. There’s a maintenance logbook entry citing, “Replaced fuel tanks” in late 1988. 

This led me to believe that both bladders had been removed, sent out, repaired and reinstalled, since both bladders I removed had the original U.S. Rubber part numbers, serial numbers and acceptance dates. Thirty years before the first repair is not short-lived. (For more information, take a look at “Refurbish, Repair or Replace: What do to when your fuel bladder fails” in the December 2013 issue of Piper Flyer. —Ed.)

Some are repairable (and some aren’t)

There were at least four different bladder tank manufacturers in the past. These companies included industry powerhouses such as U.S. Rubber, BF Goodrich, Firestone and Goodyear Aerospace. 

There were good construction methods and some that weren’t so good—some bladders are very repairable and some are not. Companies in the refurbishment business know which bladders are good candidates for overhaul. (For a link to table on the FFC website with specifics on which brands/types can be repaired, see Resources at the end of this article. —Ed.)

My Comanche’s original tanks had a cotton-based construction. According to Kevin Brewer at FFC, those U.S. Rubber 584 bladders would not be repaired today. The right one lasted 22 years after being refurbished, the left one 28 years. 

There are at least two other companies that produce and sell bladder-type fuel cells. 


Clips and hangers

Bladder-type tanks are fabricated to fit as perfectly as possible within the fuel tank bay inside the wing. The Floats & Fuel Cells bladder arrived with a new gasket, a roll of industrial-strength, fabric-backed tape and nine button-type spring clips. 

The tape is used to prevent tank abrasion. It’s laid over all rivet and screw heads and all seams within the fuel tank bay prior to installing the bladder. The tank bay of my Piper was very clean, so all I had to do was remove the old tape and install new tape.

The clips slip under a reinforced ring attached to the outside surface of the bladder. There’s a ring/clip in each bottom corner of the tank that’s there to keep the bottom of the bladder wrinkle-free. 

The bladder in my airplane holds 30 gallons. It’s shaped like a rectangle except for the forward corner of the inboard section which extends forward; it’s like a triangular piece was grafted onto the rest of the rectangle. 

There are five clips that need to be pushed into receptacles to hold the upper surface of the bladders in position. All of the removal, installation, flow nipple and clip installation work has to be done through the access hole in the top of the wing. 

It’s important to use the new clips and to get them snapped into the receptacles without bending them.

AD 68-13-03 applies to the bladder tanks in my PA-24 Comanche. It requires a visual inspection every 100 hours to check the condition of the clips. So installing new clips—and installing them correctly—is important.

Since the bladder in my Piper is small, I didn’t have any problem with arm length/finger strength issues during clip installation. 

Finishing up

One part of the installation is difficult. Two half-inch (inside diameter) flexible tubes need to be slid over tube ends attached to the metal top plate and tube ends mounted in the wing. These tubes connect the top plate fuel spill drain-off tube and the fuel tank vent line tube. 

Once these tubes are slid into position, the reinforcing/mounting ring of the bladder has to be pulled up into position so the screws connecting the two can be started and torqued. 

My solution to pulling the bladder reinforcing/mounting ring up into position—since the top plate completely fills the access hole, thereby cutting off access to the bladder—is to fabricate two long aligning pins out of bolts. 

These are screwed into nut plates in the bladder mounting ring prior to sliding the flexible tubes onto the wing mounted tubes. 

Once the top plate is in position I pull up on the aligning pins—which are nothing more than long bolts with the heads cut off—and start screwing in the screws that hold the top plate and the reinforcing/mounting ring together. 

Calibrating a dipstick

After the bladder is in and is deemed ready, I like to fill the tank in five-gallon steps—with the airplane on level ground and the landing gear struts and tires at normal inflation—for two reasons: this is the perfect time to make up a fuel tank quantity dipstick (I know I sound like a dipstick salesman, but they work and they are a simple safety tool) and it allows you to make sure you know exactly how much fuel your new bladder holds. 

A new bladder should provide good service for at least 20 years, and likely much longer if steps are taken keep the top surface of the bladder cool. 

Do this by shading the top surface of the wings whenever possible and by striving to top off the fuel tanks after each flight since the fuel will moderate bladder surface temperatures by acting as a heat sink. Hangaring an aircraft is the most effective method for preserving its fuel bladders. 

Know your FAR/AIM and check with your mechanic before starting any work.


Steve Ells has been an A&P/IA for 44 years and is a commercial pilot with instrument and multi-engine ratings. Ells also loves utility and bush-style airplanes and operations. He’s a former tech rep and editor for Cessna Pilots Association and served as associate editor for AOPA Pilot until 2008. Ells is the owner of Ells Aviation (EllsAviation.com) and the proud owner of a 1960 Piper Comanche. He lives in Templeton, Calif. with his wife Audrey. Send questions and comments to


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Floats & Fuel Cells, Inc.
Repair and overhaul chart
Floats & Fuel Cells, Inc.
Other fuel cell suppliers
Eagle Fuel Cells, Inc.
Hartwig Fuel Cell Repair
Airplane Maintenance for the DIYer: Changing a Tire

Airplane Maintenance for the DIYer: Changing a Tire

In this article—the first in a series describing various preventive maintenance actions—A&P Jacqueline Shipe goes through the entire process of removing and reinstalling an aircraft tire. 

All aircraft owners will periodically have to replace a tire. Even planes that don’t get used much eventually require tire replacement due to dry rot and sidewall cracking of the rubber. 

Tire replacement is one of the items the FAA considers preventive maintenance that owners may legally perform on their aircraft. Changing a tire isn’t mechanically complex, but it does require the owner to use some caution. 

Proper aircraft jacking procedures have to be used, and if the plane is jacked outside, first check the weather conditions to ensure that the wind is not going to be too high. Major damage can be done to the airframe structure whenever a plane falls off a jack. 

Once the plane is properly jacked, the removal of the wheel assembly can begin. (For information about aircraft jacks and proper procedures, take a look at Shipe’s “Airplane Maintenance for the DIYer: First Steps” in the May 2016 issue. —Ed.)

If the plane is equipped with wheel pants, the wheel pant has to be removed first. This is usually a pretty straightforward process, just be sure to keep the removed screws and bolts identified as to which receptacle they came out of, because sometimes several different diameters and lengths are installed. 

Removing a main wheel

The next step is to remove the outboard brake backing plate (if the wheel is a main wheel) so that the brake disc will be free to slide off the axle with the wheel assembly. 

There are two or more bolts that connect the backing plates to the caliper that have to be removed. They may or may not have safety wire on them, depending on the design. 

Once the outboard brake is off, the next step is to deflate the tire by removing the valve core. It is important to do this before removing the big axle nut, because if any bolts holding the wheel halves together are loose or damaged, the wheel assembly could blow apart as the axle nut is loosened. 

Most axle nuts have either a clevis pin with a small cotter key or a single large cotter key to keep them from backing off. 

When the axle nut is removed, the wheel assembly will slide off. Some manufacturers employ a spacer that may or may not slide off with the wheel; care needs to be taken to ensure it doesn’t get misplaced.

Removing a nosewheel

Nosewheels usually have a removable axle that slides through the wheel and nose fork. This is generally held in place with a single long bolt and lock nut that secures two cup-shaped retainers. 

Nosewheels are fairly easy to remove unless the steel axle is corroded and frozen in the aluminum fork, in which case the axle has to be driven out. A wooden dowel should be used to drive out the axle because it won’t gouge or mar the aluminum fork.


Breaking the tire bead

Once the wheel is removed, the tire bead needs to be broken from the rim of the wheel halves. Get a piece of plywood to lay the wheel on (so it won’t get marred), then forcefully push the tire down all the way around. It will eventually pop down off the rim. This process has to be repeated on the reverse side as well. 

Some folks use flat blade screwdrivers or pry bars to pry the tire away from the wheel, but this can result in major scarring of the relatively soft cast aluminum that the wheels are made of. 

If the bead is really stubborn and just won’t break loose, you may have to enlist the help of a mechanic and a bead-breaking tool made specifically for the task.

Splitting the wheel and removing the tube 

Most wheel assemblies are two-piece, and the wheel halves are split by removing the through bolts and nuts holding them together. After separating the wheel halves, the tube can then be removed from the tire. 

Some mechanics replace the tube with every tire change, and some re-use the old tube as long as it looks good. Tube manufacturers recommend always replacing tubes when replacing a tire because they stretch while in use. 

Once an old tube is removed, it can be barely inflated—just enough to expand it a little—so the entire exterior can be inspected. Tubes with deep wrinkles or that have signs of damage or age, such as dry rot cracks around the valve stem, should always be replaced. 

When ordering a replacement tube, pilots may want to get the type that doesn’t lose air, such as a Leakguard or Airstop inner tube. They do reduce the frequency of having to air up the tires by quite a bit, especially if they are serviced with nitrogen instead of compressed air. 

Reinstalling the tube

The outside of the tube and inner part of the tire need to be coated in talcum powder before installing the tube. The powder keeps the tube from sticking to the sides of the tire and helps prevent chafing. 

The tube should have a balance mark on it. This needs to be aligned with the balance mark on the tire, which is generally a red dot. 

In the absence of a balance mark on the tube, align the valve stem with the red dot. This matches the heaviest part of the tube with the lightest part of the tire and makes it much easier to balance.


Reassembling the wheel

Once the tube is installed in the tire, the wheel halves (and the brake disc, if it is a main wheel) can be assembled together. Slightly inflating the tube a tiny amount helps to ensure it won’t be pinched between the wheel halves. 

Place the wheel halves together so the bolt holes align. The bolts can then be slid through and the washers and lock nuts installed. Lock nuts should have enough tension on them so that they cannot be tightened by hand, otherwise they should be replaced. 

The correct torque should be observed when assembling the wheel halves. These are made of cast aluminum and are strong, but over-tightening the nuts and bolts can lead to cracking. 


Balancing the tire

A wheel balancer is a fairly expensive tool to buy, and tires aren’t usually replaced on an individual airplane often enough to merit owning one for most folks. Large imbalances in a wheel assembly can be detected by mounting the wheel on the axle and installing the axle nut, but leaving it slightly loose so the wheel rotates freely. 

Once the tire is spun a few times, if the same spot always ends up coming to rest on the bottom, this indicates an imbalance and weights will need to be added to the light side. Using stick-on lead type weights purchased from an aviation parts warehouse or automotive store, add enough weight so that the wheel comes to a stop in random places as it is spun freely on the axle. The balance is more critical on the nosewheel assembly because it will cause a shimmy if there is even a slight imbalance. 


Reinstalling the wheel on the aircraft

Once the wheel is balanced, it is ready for installation. The axle should be wiped off and greased, and any corrosion should be removed with an abrasive cleaning pad. 

A general-purpose Scotch-Brite 7447 pad (maroon color) works well and can be purchased anywhere automotive paint products are sold. These pads are abrasive enough to clean off rust, but not so abrasive so as to scratch the metal. Use elbow grease to scrub the axle until it is shiny.  

The nosewheel axle is prone to rusting internally. Any rust should be removed here too, and the internal part of the axle should be either painted with a rusty-metal primer, or coated in LPS 3 or other corrosion-inhibiting compound. 

During the final installation, the axle nut needs to be tightened enough so there is no free play detected as the tire is grasped and pushed inboard on the top while pulling outboard on the bottom, or vice versa. (This checks for side-to-side free play, and there should be none.) 

The wheel should spin somewhat freely, but there needs to be a slight amount of tension on the axle nut. If the nut is too tight, the wheel won’t spin much at all by hand, and the wheel bearings will be more likely to fail from having too much of a pre-load placed on them. 

Once the correct tension is achieved, align the cotter key hole in the axle with the opening in the nut and install the cotter key. Be sure to bend the edges of the cotter key in such a way that they won’t get entangled in the valve stem or rub on the wheel bearing retainer. 

After the wheel is secured in place, the valve stem should be removed and the tube inflated and deflated two or three times to remove any wrinkles. Then the valve stem can be reinstalled and the tire inflated to its proper pressure. (Correct tire pressures are found in the POH.) 

After the outboard brake and/or wheel pant are reinstalled, the tire change will be complete. The aircraft owner will also have the satisfaction of having completed the work himself (or herself)—and will have hopefully have saved a few bucks in the process.


Jacqueline Shipe grew up in an aviation home; her dad was a flight instructor. She soloed at age 16 and went on to get her CFII and ATP certificate. Shipe attended Kentucky Tech to obtain her A&P license. She has worked as an airline mechanic and on a variety of General Aviation planes, and has logged over 5,000 hours of flight instruction time. Send question or comments to .


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Wheel Bearing Service: Why & How

Wheel Bearing Service: Why & How

A&P Jacqueline Shipe describes how to service wheel bearings in this article, the second in a DIY series for pilots who wish to take on preventive maintenance of their aircraft.

FAR 43 Appendix A lists the preventive maintenance items owners may legally perform on their planes. This list is fairly long—and some of the items are a little involved for a person to perform the first time by themselves, while other tasks on the list are pretty straightforward. 

There are several preventive maintenance tasks pertaining to the landing gear, including tire changes, strut servicing and servicing the wheel bearings. (Last month, Shipe discussed the steps involved in changing an aircraft tire. See the June 2016 issue for more information. —Ed.) 

Bearings: small but mighty

While cleaning and greasing wheel bearings doesn’t seem like too difficult a task, there are some guidelines that need to be followed. The failure of a wheel bearing can cause major damage to the wheel and can even allow the wheel assembly to slide off the axle.

Wheel bearings are relatively small, but are incredibly strong. They have to support the weight of the plane while allowing the wheel to spin freely in all types of temperatures and conditions. In addition, wheel bearings and races on airplane wheel assemblies also have to be capable of withstanding hard landings and both vertical and horizontal loads without failing. 

Types of bearings

The bearings on most airplane wheel assemblies are the tapered roller-type. The outer part of the bearing is larger than the inner part, and the rollers are installed at an angle. 

The bearing itself rides in a metal cup called a race. The race has a “pressed in” fit in the wheel half, and is tapered on the inside to match the bearing. The biggest advantage of tapered bearings is the high load capacity that they can withstand. 

Automotive wheel bearings, on the other hand, usually use spherical rollers (i.e., balls). Ball bearings can withstand prolonged high speeds without building up too much heat, but cannot take high impact loads. 

Tapered bearings will bear up under the not-so-good landings that occur from time to time with an aircraft. In addition, proper servicing of these bearings will keep the wheels spinning freely and will last for a long time. 

Removing the clips

Once a wheel assembly is removed from the axle, the wheel bearings are easily removed by taking out the metal retaining clips that secure the bearings and grease felts. 

There is an indention in the outer part of one end of the clip to allow a screwdriver to be used to pry it out. The clips don’t have a lot of tension on them and can be easily removed. 

Once the clip is off, the bearing, metal rings and grease felts can all be lifted out together. 

Be sure to keep all the rings and clips organized so they can be reinstalled into the same wheel half and in the same place. The metal rings that retain the bearing are sometimes slightly smaller on the outer half than the double rings used on the inner half, and can be easily mixed up. 

Cleaning the parts

A small bucket with 100LL Avgas works well to clean the bearings. Swishing the bearing around and spinning it by hand while it is submerged will clean all of the old grease and gunk out. 

The metal rings and clips should also be cleaned, but the felt material needs to be set aside; it should not be submersed in anything. There is really no way to clean the felt, anyway—as long as it is still in one piece, it’s good to go. Any grease felt that is torn or missing a section needs to be replaced. 

Once all the parts are cleaned, they should be blown out with compressed air (if available) or laid out on paper towels to dry. The parts need to be thoroughly clean and dry before fresh grease is applied. Inspecting the parts

After the bearings, metal rings and clips are clean and dry, the bearing and race should be inspected for pitting or damage. If the race is smooth and has no corrosion, the bearing is generally corrosion-free as well. 

Races that have light surface corrosion can sometimes be smoothed out with a piece of light grit sandpaper (800 to start and 1200 to finish). Deep pits in a race mean replacement is needed. 

Discoloration on the bearing or race, such as a rainbow or gold color, can be a sign that these parts have generated excessive amounts of heat, in which case they should be replaced.

Preventing corrosion

Wheel bearings typically fail for two reasons: corrosion or overheating. 

The greatest threat to airplane wheel bearings is usually corrosion. Almost all bearings and races will eventually require replacement due to water getting past the grease seals and accumulating in the bearing cavity, causing rust and pitting. 

When cleaning a plane, strong degreasers should not be used on wheel assemblies and wheels should never be sprayed with a water hose. The pressurized water will get past the grease seals and ruin the bearings. 

Folks that want their wheels clean can wipe them out with a rag that is lightly moistened with a little Gojo original white cream hand cleaner (the non-pumice kind). Then the wheels can be wiped clean with a dry rag. 


Replacing the races

Wheel bearing replacement is easy, but replacement of the races is a little tough to do without the proper tools. 

Because the race has a pressed-in fit in the wheel half, it has to be driven out. This can be accomplished by using either a hammer and punch or a bearing driver tool. 

Occasionally a person encounters a wheel assembly with a race that has broken loose and is spinning in the wheel half itself. In this case, the wheel assembly has to be replaced; there is no permanent way to hold the race in place if the wheel assembly has lost enough metal that the race is no longer fitting tightly. 

The wheel is made of cast aluminum. When reinstalling the steel race, it is very important that it be driven in straight. If it gets cocked—even a little—the much softer aluminum will be gouged and damaged. 

The best tool for the job is a bearing driver, as it allows each blow of the hammer to be applied equally around the circumference of the race. 

Once the race is almost near the bottom of its recess, very light blows should be used to seat it in the wheel half. Many mechanics have driven the race in too far and cracked the fairly thin aluminum ring that retains the race. 

The wheel should always be thoroughly inspected for any sign of cracking on the front and back sides, whether or not a race is replaced.


Packing the bearings and reinstalling

Once all of the races are installed and the wheel halves are inspected, the bearings are ready to be packed and installed. A high-quality wheel bearing grease that has good water resistance should be used. 

The grease has to be pushed up through the bearing until it comes out the top between each roller. If it doesn’t squeeze through each opening, the inside of the bearing will have gaps and inadequate lubrication. 

It takes a little while to pack a bearing by hand. There are bearing packers sold in almost any automotive store that make the job a little faster and a little less messy. 

Once the bearing is packed, apply a layer of grease to the entire surface of the race to ensure it is covered as well. 

The bearing can then be reinstalled along with the correct order of retaining rings and grease felts. 

Lastly, reinstall the clip. It is a good idea to make sure the clip is pressed into place all the way around by pushing it outward with a screwdriver. 

After all the clips are in, the wheel bearing service is complete.


Jacqueline Shipe grew up in an aviation home; her dad was a flight instructor. She soloed at age 16 and went on to get her CFII and ATP certificate. Shipe also attended Kentucky Tech and obtained an airframe and powerplant license. She has worked as a mechanic for the airlines and on a variety of General Aviation planes. She’s also logged over 5,000 hours of flight instruction time. Send question or comments to .


Strut Servicing: The Ins & Outs

Strut Servicing: The Ins & Outs

In the third article in a DIY series for pilots, A&P Jacqueline Shipe goes through the steps an owner can take in order to properly service the struts on their aircraft.

Among the preventive maintenance items listed in FAR 43 Appendix A that pilots may legally perform on an airplane that they own is strut servicing. 

The struts on any airplane serve a critical purpose. They provide the shock absorption necessary to prevent the airframe structure from enduring too much stress from the impact loads incurred on landings. 

Even taxi operations impose stress on an airframe every time the gear hits a bump or uneven surface. 

The strut absorbs the bulk of these loads and prevents them from being transmitted to the airframe. 

Types of struts

There are several different kinds of struts used for shock absorption. Over the years aircraft manufacturers have used different materials to limit the stress from the impact of landing. Some have used rubber biscuits, bungee cords and spring steel. 

The most common type found on most planes (and the only type used on fairly heavy planes from light twins all the way up to airliners) is the hydraulic air/oil cylinder, also referred to in some manuals as oleo struts. The oleo strut is very reliable, can withstand tremendous loads and is fairly simple in its design. 

The oleo strut uses air pressure and hydraulic fluid to create a spring effect. The strut consists of an outer housing called a cylinder and an inner piston that is connected to the nose fork or to the main wheel axle. The piston portion of the strut is the part that actuates up and down. 

There are different styles and configurations, but all struts house hydraulic fluid in the lower section of the strut and compressed air (or nitrogen) in the upper section. As the piston is driven into the cylinder upon landing, the fluid is forced through an opening called an orifice that slows the rate of the flow. 

Some manufacturers make use of a metering pin connected to the piston. The pin is mounted so that it is forced upward through the orifice along with the fluid. It protrudes up through the orifice, is slim in the middle and wider on both ends. 

Its shape is tapered so that as the piston reaches the top portion of its travel, less and less fluid can fit through the opening. This gradually slows the fluid flow and decelerates the piston. Meanwhile the pressure of the compressed air is being steadily increased as the piston travels upward and reduces the volume of space in the upper chamber. 

Eventually the increased pressure of the compressed air overcomes the decreasing fluid pressure and forces the piston to extend. As the fluid flows in the other direction, its flow is impeded at a steady rate by the opposite end of the metering pin, gradually slowing the fluid flow in the opposite direction. This results in dampening out any oscillations and returning the airplane to its normal static height above the ground.

Some models don’t use metering pins but have metering tubes with various sized holes in them that slow the rate of flow as the piston reaches either end of its travel. Some manufacturers don’t use either metering pins or tubes, but instead use restrictor plates with orifices in them to produce the same effect. 

Fluid and air: both are vital 

On any model, the strut has to have the correct amount of fluid and air to work properly. The fluid used for strut servicing is MIL-H-5606 (red) mineral-based hydraulic fluid. 

5606 is sold by the gallon and in quarts. It is nice to keep a supply on hand not only for struts, but also to refill brake and gear reservoirs. Typically it takes around a gallon of hydraulic fluid to service three struts.

Nitrogen is better than compressed air for strut servicing because it is drier and doesn’t vary in pressure as much as air; it is also less corrosive to the inside of the strut housing. 

However, nitrogen is not always readily available. A person needs a regulator and high-pressure hose in addition to a nitrogen bottle, and the cost for all the items can exceed $500. 

If nitrogen is not available, air pressure from a standard air compressor is usually sufficient to air up a nose strut. Nose struts don’t require as much pressure as main struts. 

Typically the main struts on Piper aircraft require at least 200 psi to inflate the strut to its proper level. Most standard air compressors don’t get that high. There are boosters that a person can purchase for around $200 that will increase compressor air to a high enough level to inflate the struts.

It would be best on a twin Piper (or any twin, period) to always use nitrogen, due to the increased weight of a twin engine plane and the more extreme temperature changes.

Servicing a strut

The tools a person needs to service a strut include about three feet of clear flexible tubing with a ¼ inch (inside diameter) opening to fit over the Schrader valve; a valve stem tool; and an empty gallon size container to catch the old fluid. 

To properly service a strut with fluid and air, the airplane needs to be jacked, or at least have the nose raised if only the nose is being serviced. 

With the airplane jacked, remove the valve stem slowly from the filler valve in the top of the strut. It is best to loosen it enough to release the air pressure, and then remove it the rest of the way after the pressure has bled off. A small spray of hydraulic fluid comes out with the air pressure, so it’s a good idea to have a rag handy. 

Once the valve stem has been removed, push the tubing over the open Schrader valve and insert the other end into the empty container. Next, push the strut up to its fully collapsed position. Any old fluid will be shoved out. 

Then remove the container with the old fluid and insert the hose into a can with at least a half-gallon of clean new hydraulic fluid. Next, pull the strut down to its maximum extended position. The suction will pull in the fluid; it will continue to siphon for a few seconds after the strut is fully extended. 

Next, slowly push the strut up to its fully collapsed position. As some of the fluid is pushed back out, air bubbles will come out too. 

Extend the strut again, and repeat the process until all of the fluid comes out as a solid stream on the compression stroke. 

Once all of the air bubbles are removed, the strut will be considerably more difficult to push up to its collapsed position. Once the strut is fully collapsed, the hose should then be removed from the valve and the valve core reinstalled. 

This process is called bleeding the strut, and it’s the only way to get the proper amount of hydraulic fluid into the inner chambers of the cylinder. 

There is no way to simply pump a little fluid in to the strut; the strut must be filled using this bleeding process. If the process isn’t followed, large air pockets in the lower chamber can cause the strut to collapse under a load. 

Once the strut is filled with fluid, it can then be aired up with either nitrogen or compressed air through a strut booster. 

After the airplane is lowered off the jacks, the final adjustments can be made by releasing a little of the pressure by depressing the valve core for a split second at a time. 

Generally main struts should be inflated so that around five inches of the piston is exposed, and nose struts to around four inches. 

The exact range for each model can be found in the service manual. The struts should be inflated so that they are within the proper range even when the airplane is fully loaded. 


Troubleshooting struts

Properly serviced struts should have a certain amount of buoyancy about them. 

Struts that are filled with air pressure but are low on hydraulic fluid tend to stick in place. Struts that stay extended for a period of time after a plane has landed and then suddenly collapse are also typically low on fluid. 

Any sort of a knocking noise from the nose strut during taxi operations or upon landing is an indication that it is bottoming out due to it being low on fluid, air, or both.

If a strut is low on fluid, it is usually because the rubber seals have gotten old and hardened. There is generally a rubber wiper ring and a large rubber O-ring with one or more backup rings in the strut housing. These rings harden and become brittle over time, especially in cold weather. 

Granville’s Aircraft Hydraulic and Strut Sealant is an FAA approved product that can be mixed with hydraulic fluid and added to the struts during the servicing process. It doesn’t cause the seals to swell, but it does cause them to soften and become more flexible, much as they were in their original state. 

This additive works well, once enough of it gets into contact with the seals. After it is first added, the strut may still go flat a time or two and need to be re-aired before it finally holds. 

Properly serviced struts help to soften landings and prevent damage to the airframe, and keeping the struts in good shape will pay off big in the long run.


Jacqueline Shipe grew up in an aviation home; her dad was a flight instructor. She soloed at age 16 and went on to get her CFII and ATP certificate. Shipe also attended Kentucky Tech and obtained an airframe and powerplant license. She has worked as a mechanic for the airlines and on a variety of General Aviation planes. She’s also logged over 5,000 hours of flight instruction time. Send question or comments to .


Appendix A to Part 43, “Major Alterations, Major Repairs and Preventive Maintenance”
Riding the Tornado

Riding the Tornado

The author delves into the secrets of aerodynamics to find out why adding bumps on a wing can improve controllability.

I always assumed that a smooth airflow over a wing would naturally provide the most lift and the least drag; it seems to make sense, doesn’t it? A good smooth flow of low pressure air on top of the wing sucking it into the air, with a push from higher pressure air down below, would obviously be the ideal. 

Aircraft designers worked for years to eliminate rivets and any other obstructions from wings to give them the smoothest surface and reduce drag. Competition glider wings are highly polished and constantly cleaned of the slightest dirt, even dust. 

So why would anyone want to install a row of metal vanes standing like fence posts along a wing’s leading edge? Well, aerodynamics is a mysterious thing that no normal person can understand. Even aeronautical engineers have a secret pact to keep anyone from realizing they have no idea, either.

A whirlwind on my wing

But, like Dorothy’s tornado, which one wouldn’t think could turn her Kansas farmhouse into a bizjet bound for Munchkinland, it seems a little tornado on your wings can give you some extra lift.

Although the term “vortex generator” sounds like a device that would feature in a time-travel movie, in reality they are just those simple, metal vanes you may see standing like marching soldiers, usually on the leading edge of a wing. 

The vanes are placed at an angle to the airflow so that, in flight, the air moving past them creates a “vortex,” a little horizontal whirlwind that improves the effectiveness of the wing. They’re often installed on control surfaces as well.


Six degrees of airflow separation?

I decided to avoid the technospeak I’d get from an aeronautical engineer—“the viscosity of the fluid and the friction of the object surfaces generates high transverse velocity gradients”—and just talk with someone who spends Monday through Friday, 9 to 5, with vortex generators (VGs). 

Anni Brogan is the president of Micro AeroDynamics, a company that does nothing but design and manufacture VGs for General Aviation aircraft. In simple terms that I could understand, she explained, “the vanes are mounted to the wing, usually between two to 12 percent aft of the leading edge, and at a slight angle to the oncoming air. 

“As the air flows by,” she continued, “it’s given a spin by the vanes—and this swirling air will stick to the wing longer, especially at lower speeds.

“On a typical wing without vortex generators, the air reaching the trailing edge separates and roils around—even flowing forward at times—and this turbulent air doesn’t provide any lift from that last bit of the wing,” Brogan said. “On ailerons, elevators and rudders, the vortices provide better controllability, and that improves safety.”

Since the air is flowing smoothly over more of the wing’s surface area, although spinning all the while, it provides more lift and better performance. For wings, the additional lift means the wing can fly slower before it stalls and it will fly earlier during the takeoff roll. For control surfaces, the extended air flow across the surface increases the forces it generates, which increases its controllability.

I don’t need to know the physics of exactly how it all works, but just that it really does. There is very little argument from anyone that VGs aren’t effective.

Aerodynamic benefits, and other pros and cons

The most often mentioned benefits of VG installation are lower liftoff speeds and lower stall speeds. Certainly, a lower stall speed is a great safety improvement and might prevent some of those deadly, low altitude, turn-to-final stalls. Although, don’t rely on that: never get too “low and slow.” 

A lower liftoff speed would certainly help those taking off from rough, short fields, which is great for aircraft owners flying into unimproved, backcountry airfields. The increased effectiveness of ailerons, elevators and rudder, which improves controllability, especially at slow speeds, is another safety benefit.

The installation of these vanes has very many pros and very few cons. On the plus side is the fact that they are relatively inexpensive (in aviation terms), they’re easy to install and don’t interfere with any other aircraft systems or parts.

The most often cited con is that some pilots think they’re ugly, and they do make the wing harder to wash. Tests by some pilots of their planes, before and after the installation of the VGs, have found a slight reduction in cruise speed of one to two knots; but, in the same breath, many of those independent reviewers have called the reduction “negligible.”

Aftermarket VGs for Pipers

There are various companies making VGs for experimental aircraft, but Micro AeroDynamics Inc. is one of the few manufacturers whose products have an STC for use on certified aircraft. The company has been making VG kits for most General Aviation aircraft since 1989 at their Anacortes, Wash. factory. 

Micro AeroDynamics VGs are manufactured from aircraft grade aluminum and coated for improved corrosion resistance and paint adhesion. A kit includes the VGs, installation tools, adhesive, templates, detailed instructions and the ever-important STC. 


Although the installation of VGs is often described as “quick and easy,” it is a job that requires care and accuracy. 

Experimental aircraft owners can install the VGs and sign off their own work. On certified airplanes, the VGs must have an STC, and the work must be approved by someone with FAA inspection authority. Anni Brogan recommended aircraft owners consult their mechanic before starting the installation. Some are willing to sign off on an owner’s work; some are not. 

The vanes, which are glued on, are not difficult to install, but they must be placed in a precise line with accurate spacing. Micro AeroDynamics provides a peel-and-stick template that helps—but it, too, needs to be accurately placed. A good compromise she suggested is having the mechanic apply the template to the wing and then the owner can do the tedious part of gluing all the vanes into place. 

Sometimes the VGs are placed singly and sometimes as a pair, with the two angled toward each other in a “V” shape. For example, a Piper Super Cub wing takes 36 pairs of vanes across the wing, and 14 pairs on the underside of the horizontal stabilizer.

A typical installation involves these steps:

Paint the vanes. They come treated with Alodine, which improves corrosion resistance and paint adhesion, but gives the vanes an odd greenish-yellow color. You can paint them to match the current color of your aircraft, or use a contrasting color if you like. (Micro AeroDynamics will paint them your desired color before shipping, for a fee.)

Determine the correct placement and apply the template to the wing or control surface, and then remove the cutouts where the vanes will be placed. Double-check the position now, before you start gluing.

Scuff the exposed aircraft surface and the vanes with the included Scotch-Brite pad to ensure optimal adhesion. The glue used to attach the vanes is a two-part adhesive: one part is applied to the wing and the other to the vane. There’s no need to clamp or weight the vanes. The vanes are custom made for each airplane and may either have a curved or flat surface for gluing, depending on the spot where they’re to be attached. The vanes can be glued to any material—even fabric—as the glue really sticks to the paint, not to the surface material.

Carefully glue the vanes in place. How long it takes will depend on the installer’s skills, confidence and the total number of vanes to be installed, but usually the entire installation can be completed in one day.

Have your work inspected. Leave the template in place so the inspector can see it and know the vanes were positioned according to the template, especially if the mechanic didn’t place the template.

Remove the template. (You might wish to store it along with the extra vanes.)

Once the aircraft has been returned to service, go fly.


The VG kits cost from $395 for experimental aircraft up to $3,950 for a large, certified, twin-engine aircraft. A Piper Cub kit from Micro AeroDynamics costs $695; a kit for a PA-28-150/160/180 Cherokee costs $1,450; a kit for a Seneca II, for example, list at $1,950. Add taxes and shipping to those prices, and if you want your VGs painted to your specifications, that’s an additional $250. 

The kits can be bought directly from the company, through your mechanic, or from aircraft supply shops. Assuming that an owner would perform the installation, the VG kit, STC, tax and shipping are the only costs, but on certified aircraft the installation must be inspected and approved, so that’s a cost that must be considered.

For $695, plus a small amount of paint and the inspection fee, I’d consider this modification for my Super Cub, especially if I decide to fly into more backcountry airfields. Currently I can land in just about any field around here, so it’s not a necessity for me. But for pilots who need maximum performance from their aircraft, the installation of VGs is a simple, yet very effective improvement.

Dennis K. Johnson is a writer and a New York City-based travel photographer, shooting primarily for Getty Images and select clients. He spends months each year traveling, flies sailplanes whenever possible and is the owner of N105T, a newly restored Piper Super Cub Special. Send questions or comments to .


VG kits – PFA supporter
Micro AeroDynamics Inc.
Other VG kits
(PA-31 and PA-34 series only)
BLR Aerospace
Aircraft Instruments: What You Need to Know

Aircraft Instruments: What You Need to Know

Do you know what instruments you can rely on to provide accurate information when the unexpected happens? A&P Mike Berry discloses what you absolutely need to know about your aircraft instruments.

Aircraft instruments have been a part of aviation since the first flight of the Wright Flyer, which was equipped with a stopwatch, an anemometer (to measure wind speed) and a tachometer. 

With the increase of flight activity in the early years of aviation, aircraft instruments were invented to provide necessary information to pilots for precise control and navigation of their aircraft. 

As a pilot and aircraft owner, it is important to understand not only how aircraft instruments work, but also to be knowledgeable of the systems that they interface with. 

The maintenance and care of an aircraft, including its systems and required inspections, are tasks that the aircraft owner is responsible for—and they are not easy. 

In this article I will give some insight into instrument repair and replacement options as well as the maintenance and repair of systems these instruments are operated by. 

The basics, and some important questions

All modern aircraft, whether the aircraft has digital or analog instruments, share the same basic pitot and static systems. These systems deliver a very slight pressure to the instruments that they serve, and instrument accuracy is impacted by even slight variations. Leaks, disturbed air or even partial blockage in the lines serving instruments such as the altimeter, airspeed, and vertical speed indicators will certainly affect accuracy. 

There are other systems that are electrical or mechanical in nature and for the most part are self-energized such as the tachometer, oil pressure and oil temperature gauges. While the latest models of aircraft have electrically powered instrumentation, the majority of General Aviation aircraft still retain the self-powered instruments as a matter of reliability and economics. 

It is important as an aircraft owner and pilot to know the basics. In case of a total electrical failure, what instruments can you rely on to continue to provide you with accurate information? For example, fuel quantity gauges on most aircraft require electrical power and will not be reliable with the electrical system shut down. 

Consider the vacuum system that powers most General Aviation gyroscopic instruments such as an artificial horizon (AH) and gyroscopic heading indicator (DG). When a vacuum pump fails, what instruments can you rely on? 

Will your autopilot work? Will a failure of one vacuum instrument cause the other vacuum instruments to fail shortly thereafter? How about the old turn-and-bank or more modern turn coordinator instrument; how are they powered? 

Turn coordinators are electrically powered—and the most important aspect of any gyroscopic instrument is that a failure may not be immediately noticeable unless the aircraft is equipped with a warning system. 

In the case of a failed pump supplying vacuum pressure to gyroscopic instruments, the instruments will decelerate and become inaccurate over a minute or two, not in mere seconds. This inaccuracy over time can cause a pilot to lose control of the aircraft by following a slowly dying gyro into the ground. Several fatal accidents have occurred over the years for just this reason, and a low vacuum warning can be a lifesaver. 


The rules concerning aircraft instruments

FAR 91.205 specifies required instruments for VFR flight for the most basic aircraft. These consist of an airspeed indicator, altimeter, compass, fuel quantity, oil temperature and pressure, and tachometer. These instruments must be operational for an aircraft to be considered airworthy. 

There may be additional required instruments associated with the specific operations of the aircraft (such as instrument flight rules) and even some instrument requirements specified by ADs, Type Certificate Data Sheets, flight manuals or supplements and STCs. 

It is up to the pilot in command to determine that the required instruments are operational before flight, and that the instruments are certified for the operation intended. While some instruments may legally be inoperative, consideration must be given as to how an inoperative instrument will affect the operation of the aircraft. 

Additional rules concerning aircraft instruments according to 14CFR 65.81, General Privileges and Limitations, are that “… a certificated mechanic… is not permitted to… accomplish any repair to or alteration of instruments. These activities are reserved for certificated repairmen at an authorized repair station.” 

This means that anything other than an external adjustment of an instrument—including installing a compass repair kit—is not authorized. 

Static systems test and inspection for IFR flight is required by FAR 91.411 and must be accomplished every 24 months or “Except for the use of system drain and alternate static pressure valves, following any opening and closing of the static pressure system, that system has been tested and inspected and found to comply with paragraph (a), appendix E, of part 43 of this chapter; and (3) Following installation or maintenance on the automatic pressure altitude reporting system of the ATC transponder where data correspondence error could be introduced, the integrated system has been tested, inspected, and found to comply with paragraph (c), appendix E, of part 43 of this chapter.” 

This means a certificated mechanic with the proper test equipment can certify only the static system (checking for leaks) and not the altimeter or transponder portion which is referenced in FAR 43 appendix E.

How instruments operate, and why they fail

Traditional (steam gauge) aircraft instruments can be grouped according to their operating systems. 

Pressure instruments

Pressure flight instruments operate off of the static and pitot system, are self-powered and extremely sensitive diaphragm-type instruments relying only on variations in pressure to operate. These pressure variations are transmitted mechanically by gears and a jeweled movement as a result of the extension and retractions of the diaphragm. 

As with anything mechanical, age takes its toll on the accuracy of pressure instruments such as the airspeed, altimeter and vertical speed indicator (VSI). These instruments are affected by moisture as well as dust and dirt, and should be kept clean. 

Cloudy or dusty-looking instruments may mean that the system is contaminated and the static system must be purged of moisture or dust and the instruments promptly repaired or replaced. Leakage sometimes occurs between the instrument glass and outer case as well as inside system fittings. Sealants become inflexible over time and lose their ability to keep the system closed. Leakage must not be tolerated, either, as the accuracy of all the instruments in that system is compromised. 

Aircraft instruments are delicate and require special equipment and training to be successfully repaired. 

Vacuum instruments

Vacuum operated (gyroscopic) instruments have been very reliable over the years, with very few actual failures of the instruments themselves; however, these instruments are subject to malfunction when an aircraft vacuum system fails. 

Vacuum system failures can be prevented with proper care and maintenance (or replacement of components) as specified by the aircraft manufacturer. 

One often-overlooked procedure is to check the vacuum gauge reading in your aircraft against a calibrated gauge. This ensures that the actual vacuum/pressure is set correctly, as over-pressure or under-pressure compromises accuracy, increases wear and creates an opportunity for failure of instruments or the entire system. Another often-overlooked but recommended procedure is to replace both pressure and vacuum filters on an annual basis. 

When replacing a vacuum pump due to a failure, ensure that all hoses, filters and fittings are checked for contamination from foreign material as not only is the newly-installed pump at risk of failure, the instruments may also fail due to foreign material contamination. 

Vacuum instruments are mechanical devices that operate with a gyro spinning at high speed powered by jets of vacuum or pressure impacting on small cups machined into the gyro rotor. The precision-balanced rotor is suspended by a shaft and supported by tiny bearings which are lubricated when the instrument is assembled. There is no provision for lubrication other than when the unit is disassembled during maintenance or repair. 

Gyros rarely fail without some type of warning which may be indicated by excessive drift or precession, noisy or erratic operation. Inactivity really takes its toll on these instruments as the lubrication that is on the tiny bearings tends to drip or wick away from the actual bearing surfaces when the instrument is at rest for long periods of time. 

Electric instruments

Electrically powered instruments can be of several different configurations, from a simple fuel quantity sender or flap position sender (variable resistor) and indicator, to an electrical tachometer powered by a small generator (though a flexible mechanical cable between the engine and the gauge in the instrument panel is more common). 

EGT and CHT gauges are usually self-powered relying on dissimilar metals in the sender or sensor to generate an electrical signal directly to the gauge on the instrument panel. The color coding of the wires is important as senders with different color coding than the instrument will not be compatible. 

Sending units and wiring for CHT/EGT gauges must not be repaired, spliced, or in any way modified from the original configuration—including length. If it’s broken, replace it. 

Anything electrical is subject to the effects of vibration, corrosion and broken (open) connections; remember this in your troubleshooting routine. 

Also significant in any electrical instrument installation is that individual components of a system are in most cases not interchangeable. For example, a Rochester brand gauge must be connected to a specific type of sender unit intended for use with the Rochester gauge; a Stewart Warner brand sender may not work properly with a Rochester gauge. 

Mistakes can be costly; check the schematic diagrams for the proper wiring, refer to the parts manual for the compatible component, and physically check that the item is what is actually installed in the aircraft you are working on. 

Electrical components do wear out and/or deteriorate over time and malfunction, even if the item is rarely used. Good preventive maintenance practices—such as keeping moisture off of connections, proper routing and attachment of wiring, and reducing airframe vibration—can go a long way in avoiding premature instrument and electrical failures. 

Repair options

Finding a shop that will work on older instruments is becoming difficult if not impossible, and often owner-pilots are left with no option but to replace an instrument. 

The rules of requiring approved technical data covering repairs and overhauls, approved parts sourcing and proper repair and test equipment are alive and well in the aircraft instrument arena. For this reason, many instruments that were original equipment on General Aviation aircraft 30 to 50 years ago are no longer supported and are not repairable. 

Authorized shops

Instrument repair shops operate as FAA approved repair stations and while all instrument shops adhere to the same FAA rules, some shops may be authorized to do repairs while others may not. 

Do some checking around to see if you can find a shop that does repair an older instrument. There are some, such as Air Parts of Lock Haven, that specialize in older aircraft instruments and in fact have repair station authority to do extensive repairs. 

Air Parts of Lock Haven also has access to repair parts sources that other shops may not have. Not only does this shop repair older instruments, it also can duplicate original instrument dials and faces (such as those original to the Piper Cub). 

Radioactive components

Many instruments that were supplied as original equipment in the 1940s and 1950s and even into the 1960s came with luminous dials and markings which happen to be radioactive and are now considered hazardous material. 

If you have one of these instruments, it must be shipped as hazardous material with all the markings, shipping labels and details that pertain to hazardous material. Few shops are equipped to handle this material and will refuse the shipment. 

At last check, Air Parts of Lock Haven can receive these instruments and has authority to handle the material, but the instrument will not be returned with the radioactive dials. 

General shipping information

Any instrument that requires shipment to a repair shop must be packaged properly—as if you were shipping eggs—and the package should be marked as fragile and insured. 

It would be prudent to call the instrument shop you are shipping to and ask for a carton to ship an instrument in and wait a few days for the container to arrive rather than risk damage to the instrument in shipping. 

Unfortunately, shipping companies can and do damage aviation material—and an insurance adjuster’s value of the instrument may be much less than what a functional instrument may actually cost. 

Buyer beware

A word to the wise: if you are buying an instrument at a flea market or on eBay, not only ensure that it can be repaired and certified, but make certain that it is appropriate to your aircraft.

Markings on a replacement airspeed indicator, for example, must be specific to the make and model of aircraft, and the details may be found in an official flight manual, TCDS, STC-related flight manual supplement, or even AD notes or Service Bulletins. 

If you send in an airspeed indicator with a specific aircraft manufacturers’ part number, what you will get back is a repaired or a replacement airspeed indicator that will have the markings appropriate to that particular part number—which may or may not have the correct markings for your aircraft. There is no choice here as to changing the markings, and adding or deleting marks is not permitted by the FAA. 

The importance of accuracy for performance

The performance listed for your aircraft was obtained when the aircraft, engine and propeller were new, and the aircraft was rigged properly, loaded to the most favorable center of gravity location and flown by a test pilot under optimum atmospheric conditions with accurate instrumentation. While it is possible to duplicate the published performance numbers with an older aircraft, everything must be nearly perfect to do so, and accurate instrumentation plays a big role. 

Although digital instrumentation is replacing analog instruments and equipment, much of the instrumentation still relies on precise pitot or static system pressure which is then delivered to the computer or other device to indicate airspeed, altitude or vertical speed. 

So, unless you have precise pressure, the 78 knots indicated you are using to achieve best rate of climb may not be exactly 78 knots. In addition, mechanical tachometers, whether due to age or inactivity, have a history of being inaccurate. 

Inaccurate readings from just these two instruments—airspeed and tachometer—can have a very definite impact on performance and overall safety, as the aircraft will not achieve published performance numbers. 

Most, if not all, aircraft maintenance shops have tachometer checking equipment and the calibrated tachometer checker should be used to compare required static rpm listed on the TCDS to the aircraft’s actual full-throttle revolutions per minute. An aircraft tachometer can easily differ from the published requirements by 100 rpm or more and some aircraft are rejected during annual inspection because of this. 

Practical application

Any aircraft owner knows that aircraft are expensive to maintain and there is no indication that costs will come down. Aircraft instruments are no exception; however, there are some economical ways to determine if you do have instruments that are in need of repair or replacement. 

System leaks

Static system leaks, for example, can often be discovered by some simple tests. Does the VSI, airspeed or altimeter needle move when a door or window is closed or opened while on the ground with the engine not running? When you open the cabin heat valve or a window in flight, do any of the three instruments just mentioned move abruptly? 

Unusual temporary indications may indicate a leaking system component such as an alternate static port, leaking instrument glass or a broken or cracked moisture trap. 

Altitude discrepancies

Also consider the effect of modifications to your aircraft, as these may impact the static system and overall instrument accuracy. An example of this was an aircraft that was modified with a cargo pod and several electronic sensors for aerial survey operations. 

When the modifications were completed, the aircraft was test flown and at higher altitudes (in the teens). An instrument accuracy check revealed a 900-foot error in the actual altitude versus indicated altitude. 

Errors such as this are rare, but can happen, so be especially vigilant when multiple modifications are made to an aircraft. The possible combined effect these may have on actual versus indicated altitude is worth examining. 

An unofficial altitude comparison can be made between a GPS unit’s derived altitude and the indicated altitude while in flight. Large errors—such as a difference of a few hundred feet or more—should be cause for further investigation into pressure instrument (altimeter) and static system accuracy. 

Electrical fuel quantity

Fuel gauges are another set of instruments that are known to be inaccurate, yet pilots rely on them. A typical electrical fuel quantity system on General Aviation aircraft consists of three parts: the sending unit (using a variable resistor attached to a mechanical arm/float), electrical wiring, and an indicator in the cockpit. 

The sending unit attached to the fuel tank can fail mechanically or electrically, or provide inaccurate readings as both parts can wear or age. The float can absorb fuel and partially sink, providing an erroneous indication. Electrical wiring can become corroded or disconnected, and if a complete circuit is not maintained, may indicate full all the time 
(or empty all the time). 

Some basic troubleshooting by a technician with a voltmeter and schematic can determine the offending component fairly quickly—especially when the plane is opened up for annual inspection. 

A fuel gauge indicating the quantity of fuel in each tank is one of the required instruments according to FAR 91.205, and most (if not all) components—even on the most ancient aircraft—can be repaired or replaced to make the system work properly. 

Be proactive

As a pilot or aircraft owner/operator it is very important that you properly maintain aircraft instruments and associated systems as well as seek repairs or replacement at the first sign of any deficiency. Operating an aircraft with a faulty or inoperative instrument can have serious consequences. 

Maintenance personnel conducting an annual or 100-hour inspection should not return an aircraft to service, and pilots should not conduct flights with inoperative instrumentation or equipment required by FAR 91.205. 

Michael Berry, a former aircraft repair shop owner, is a multi-engine rated ATP (757/727). In addition, he’s a turbo jet flight engineer, an A&P/IA mechanic, airplane owner and 121 air carrier captain. Berry has 15,000-plus pilot hours. Send questions or comments to .



Air Parts of Lock Haven
–PFA supporter

Further reading

FAR 91.205
“Powered civil aircraft with standard category U.S. airworthiness certificates: Instrument and equipment requirements”
FAR 91.411
“Altimeter system and altitude reporting equipment tests and inspections”
Appendix E to FAR Part 43
“Altimeter System Test and Inspection”
14CFR 65.81
“General Privileges and Limitations”
All of the above documents are available at the FAA website: rgl.faa.gov
Getting Current: Troubleshooting a Landing Light Circuit

Getting Current: Troubleshooting a Landing Light Circuit

Finding and repairing a broken circuit is the subject of this fourth installment of A&P Jacqueline Shipe’s DIY series. 

Among the many preventive maintenance items listed in FAR 43 Appendix A that a pilot may legally perform on his or her plane is “troubleshooting and repairing a broken landing light circuit.” This specific entry is the only reference to electrical circuit troubleshooting on the list. 

Most electrical circuits for lights or pitot heat, etc. are fairly straightforward, while a wiring harness for a unit like a panel-mounted GPS can be very complex. This article will focus on the tools and expertise required to successfully troubleshoot a landing light.

Study the diagram

On any electrical circuit, the best troubleshooting tool is always the current wiring diagram pertinent to the model and serial number of the airplane. Learning how to read a wiring or system schematic can help a pilot not only in performing repairs, but also in understanding how a unit or system actually works. 

Everything electrical has to have a power source and a ground to operate. Some circuits contain numerous switches and circuitry that work in conjunction with each other to provide the needed power or ground. 

When a fault occurs, knowing how to dissect that circuit into sections—and understanding when and where voltage or a ground is supposed to be present—is essential. The wiring diagram provides all the needed information. 

There are standard symbols used on these diagrams to indicate different components in a circuit. There is always a symbology chart somewhere in the maintenance manual wiring section that lists the symbols and the components they represent. 

Some of these symbols are drawn to look somewhat like the component they represent, such as a circuit breaker. Switches, contactors and relays are generally shown on diagrams in the open (or “relaxed”) condition unless otherwise noted. 

To get familiar with a specific circuit, follow the flow of a circuit on a diagram and consult the chart when you see an unknown symbol. It doesn’t take long before the symbols all become familiar.


The parts of a circuit

Exterior light circuits are some of the most straightforward circuits on any plane, and the diagrams provide good practice for folks first learning to read a wiring schematic. Generally, it is best to start at the power source in the diagram and read down from there. 

In figure 9-35 on page 24, the Piper Comanche landing light circuit, power comes from the bus bar to a 20-amp circuit breaker, which is a shared breaker for two separate landing light circuits. The number 14 indicates that the wire size for that section is 14 gauge. 

L1A and L2A indicate the wire numbers; “L” representing a lighting circuit. (Each original wire on a plane is stamped multiple times along its entire length with the appropriate wire number, which helps tremendously.) 

Both switches are shown in the open or “off” position. Coming out of each switch is the “B” section of each wire, still 14 gauge, up to a knife connector. Past the knife connector is the “C” section of each wire up to the terminal on the bulb itself. 

The “D” section starts at the opposite terminal on the bulb and goes to ground. This section of wire is a little heavier duty as indicated by the fact that it is 12-gauge wire. 

The wire gauge refers to how big its cross-section is; the smaller the number, the fatter and heavier duty the wire is. Starter and battery wires, for example, are large and heavy duty, generally six gauge or lower. Larger diameter wires can carry much more current than smaller diameter wires.

The airframe itself is generally used to provide a ground on most planes. The exact point where a wire for a circuit is connected to the airframe for a ground is usually not too far from the electrical component itself. 

The airframe should be clean, and the wire terminal should be free of corrosion to ensure there is a resistance-free path for an electrical flow to ground. 

Check the bulb first

First, be sure that any wires being checked are not touching any other wires or the airframe. Don’t allow any metal tools to touch both a live wire and the airframe at the same time. 

In the landing light circuit, the quickest and easiest place to go to troubleshoot an inoperative bulb is to check the bulb itself. 

After removing individual bulbs from the plane, measure the resistance across the terminals with a multimeter set to ohms. This is a very simple task for landing or taxi light bulbs. 

Navigation lights and several of the interior light bulbs utilize a base that inserts into a socket. The small raised area at the bottom of the base is the positive contact point, and the base of the bulb is the ground contact point on these bulbs. A good bulb will show continuity; the resistance varies a little depending on the type of filament it has. 

Strobe bulbs are different; they can only be tested by an operational check. To check a strobe bulb, put the suspect bulb in a known good circuit, turn it on and see if the bulb works. If one wing strobe bulb works but the other doesn’t, switch the bulbs and you’ll immediately know if it’s the bulb or something else in the circuit that’s causing the trouble.

Check these things next

The two main checks that are required when troubleshooting a circuit are for voltage and a ground. Voltage is a measurement of the potential amount of electric power coming in to a certain point. It is not an indication of how much power is actually flowing. 

Amperage measurements give an indication of the actual amount of power (current) that is flowing. Most meters only measure voltage, but some do have amperage settings. 

Voltage is easier to measure because it can be checked anywhere in the circuit by placing the meter in parallel with the circuit. Amperage is harder to read because the meter has to be placed in series within the circuit, and the circuit has to be complete so that current is actually flowing. 


Voltage and resistance can be easily measured with an electrical multimeter. There are many different manufacturers of multimeters; even the most inexpensive ones are generally good enough to troubleshoot most electrical issues. 

Before taking any measurements, it is a good idea to set the meter to the lowest ohms setting and touch the test leads together. This tests the connection of the leads to the meter and also the continuity of the test lead wires as well as the internal resistance of the meter. The reading should be zero ideally, but in any case it shouldn’t be much over one ohm. 

When the aircraft battery is on and the landing light switches are closed (i.e., turned on), there should be 12 volts at the terminal of the L1C and L2C wires. There should also be a very low resistance path to ground, which is measured on the L1D and L2D terminals. 

In the landing light circuit, voltage can easily be measured by placing the positive probe of the multimeter on the light terminal for the L1C or L2C wire (depending on which bulb is being checked) and the negative probe on a clean, bare metal area of the airframe for a ground. (Some owner-pilots use a small alligator clip to connect the black lead off the multimeter to a spot on the airframe. —Ed.)

With the appropriate switch flipped on, the voltage reading should be approximately the same as battery voltage if the circuit is working properly.



Most of the time, a voltage measurement is all that is needed to be assured that power is being received, but it is good to know how to check amperage. 

An electric motor that is operating a little slower than normal or is on the verge of shorting out typically begins to draw an excessive current load. A high amperage reading will also be present if there is too much resistance to motion in the mechanical apparatus the motor is trying to move. 

Flaps that are binding, or a landing gear retraction or extension mechanism that is not properly adjusted will cause a motor to draw a high current load and get hot. Checking for a higher than normal amperage reading can allow you to detect a malfunction and fix it before it causes a total failure.

Amperage measurements are also useful to confirm that a component is receiving the full amount of electrical energy it needs to operate. Most of the time, voltage readings are sufficient for this, but there are some circumstances where a voltage indication can be misleading. 

A wire that is barely connected or a switch that has badly burned contacts can still make enough of a connection to show full battery voltage at points in the circuit beyond, but will not be able to actually carry enough amperage to operate different components downstream. This can cause a lot of confusion, but is a fairly rare circumstance.

If all other checks pass with proper voltage and a good ground being indicated, and a known unit that is operable still won’t function, it would be prudent to see how much amperage the unit is getting; there could very well be a poor connection in the circuit somewhere upstream, even though the voltage readings are correct. 

Ground connection

Ground connections are measured in ohms of resistance. Generally a reading of two ohms or less is indicative of a good connection to ground; readings that are five ohms or higher are cause for some concern. 

The airframe itself is used to ground most electrical circuits. The airframe often develops corrosion, which can cause excessive resistance in ground connections. Usually disconnecting a ground wire and cleaning the terminal and contacting airframe with 220 grit sandpaper or an abrasive pad (i.e., Scotch-Brite) clears it right up. 

With a little practice and persistence, pilots will be able to interpret wiring diagrams, a multimeter will become easier to use, and electrical problems will seem less complex. 

Most electrical issues can generally be traced to a problem that is fairly easy to fix. Knowing how to troubleshoot a circuit and read a schematic will save a pilot/owner both time and money in the long run. 

Know your FAR/AIM and check with your mechanic before starting any work.


Jacqueline Shipe grew up in an aviation home; her dad was a flight instructor. She soloed at age 16 and went on to get her CFII and ATP certificate. Shipe also attended Kentucky Tech and obtained an airframe and powerplant license. She has worked as a mechanic for the airlines and on a variety of General Aviation planes. She’s also logged over 5,000 hours of flight instruction time. Send question or comments to .


Voltmeter and multimeter tools
– PFA supporters

Aircraft Spruce & Specialty Co.
Chief Aircraft
Aircraft Battery Care: "Learning your ABCs"

Aircraft Battery Care: "Learning your ABCs"

Jacqueline Shipe, A&P, explains the technology and preventive maintenance for aviation batteries in her sixth DIY article targeted to owner-pilots. 

The bulk of the items listed in FAR 43 Appendix A, paragraph (c) that an owner may legally perform on his or her owned aircraft are primarily maintenance tasks that have to be performed on a fairly regular basis. 

This is definitely true concerning aircraft battery maintenance, and “servicing or replacement of aircraft batteries” is included on the list of 31 preventive maintenance items. 

All batteries begin to degrade in performance from the moment they are placed in service. The constant chemical reactions that take place cause an ever-increasing lack of efficiency within the battery. This is especially true of batteries that are allowed to run down and remain in a low or depleted state.

Lead-acid batteries are the type used in almost all General Aviation planes and are becoming more common for turbines employed in low-cyclic applications like medevac. (Turbine powered planes in high-cyclic applications (i.e., airliners) often have nickel cadmium or “NiCad” batteries installed. These batteries are costly, and the servicing requirements are much more complex than for the lead-acid batteries. NiCad batteries should only be serviced by a professional.) 

Anatomy of a battery

A lead-acid “flooded” battery consists of multiple cells enclosed in a plastic case. Each cell consists of alternating sets of lead plates. 

Half the plates contain lead oxide, and the other half of the plates contain soft spongy lead. The plates are set in an alternating arrangement; each lead oxide plate is next to, but not touching, each spongy lead plate. 

The plates are immersed in an electrolyte solution of sulfuric acid and water. Removable caps allow an owner to inspect and adjust the electrolyte level of the battery. 

Each battery cell produces roughly two volts of electric power. A 12-volt battery has six cells (and six caps) and a 24-volt battery has 12 cells (and 12 caps).  


The chemical reaction

Sulfuric acid produces a chemical reaction between the opposing plates, causing the lead oxide plates to become positively charged and the spongy lead plates to become negatively charged. 

As a battery discharges, the sulfuric acid in the electrolyte solution is converted into lead sulfate on both the positive and negative plates. Lead sulfate is not conductive. As it grows on the plates, covering more and more of the surface area, it reduces the efficiency and output of the battery. 

The discharge process also makes the electrolyte far more watery as the sulfuric acid is depleted. Batteries not only discharge under an electrical load, but they also self-discharge when not being maintained in a fully charged state.

If a battery is left for a prolonged length of time in an uncharged state, it will eventually completely discharge once the plates become so coated in lead sulfate that no more exchanges of electrons or ions can take place.  

The charging process

During the charging process, the chemical process is reversed: the lead sulfate on the plates is converted back into sulfuric acid; lead oxide is redeposited back on the positive plates; and pure lead is deposited back on the negative plates. 

A battery which remains in a depleted state of charge for a prolonged period of time forms lead sulfate that eventually hardens and crystallizes on the plates to the point that it can’t be converted back into its original components of lead oxide, pure lead and sulfuric acid—no matter how long the battery is left on a charger. 


Maintenance-free batteries

Maintenance-free or “sealed” batteries have non-removable covers and the electrolyte level cannot be adjusted. These sealed batteries go by a variety of names: RG, or recombinant gas; AGM, or absorbed glass mat; and VRLA, or valve regulated lead acid. 

These batteries use a fireproof glass mat separator between the positive and negative plates. The glass mat is saturated with electrolyte and the mat’s microporosity allows the hydrogen and oxygen to recombine. 

VRLA batteries are designed to recombine the gases generated during the charge-discharge process and to maintain electrolyte throughout the lifespan of the battery, which makes them maintenance-free for the aircraft owner. 

Extending battery life

The best thing any owner can do to extend the life of his or her battery is to keep it fully charged. The alternator or generator on a plane that is regularly flown helps to keep the battery in a good state of charge. 

A plane that sits for extended periods, however, needs an external charging source to keep the battery maintained in good shape and prevent permanent sulfating of the plates. The Achilles’ heel on any battery is to allow it to completely discharge, especially if the discharge occurs slowly over a long period of time.

Handling a vented aviation battery

Battery acid is harmful to the skin and eyes, so rubber gloves and safety glasses should be worn any time you are charging or servicing the battery in your aircraft. 

To prevent electric shock, ensure that any metal tool that is in contact with the positive battery terminal is not allowed to touch any metal structure on the battery box or airframe.

Anytime the battery is charged or serviced, the best thing to do is to completely remove it from its compartment. 

This can be difficult to do depending on the location of the battery, and all batteries are heavy and can be tough to lift out of the box. The 24-volt batteries are particularly cumbersome. 

The straps that are occasionally installed on the tops of the batteries are only there to aid in the removal from and installation into the battery box. 

Once it is out of the aircraft, the
battery should be supported from underneath; very often the plastic or rope-like straps weaken over time and can easily break. 

Taking care of the battery box

The complete removal of a vented battery from the airplane not only makes it easier to service, but also allows the battery box to be cleaned and inspected. 

A solution of baking soda and water will neutralize any acid residue in the box. 

The drain line should be inspected to be sure it is still attached properly and is clear of any clogs. 

Any corrosion should be thoroughly cleaned off, and the box should be painted with either a zinc chromate primer topped by a good quality epoxy paint or with a bituminous or acid proof paint that is specially made for battery boxes. (Battery box modifications for Piper aircraft are available by STC from Bogert Aviation. —Ed.)


Adjusting electrolyte levels

In addition to charging the battery, the electrolyte level should be inspected on flooded batteries. The electrolyte will be low if the battery is in a discharged state and will increase as the battery is being charged; therefore, the final adjustments of the electrolyte level should take place once the charging process is complete. 

Most service manuals recommend adding only distilled water to cells that are low on electrolyte after the battery is fully charged. 

During initial servicing of a new battery, however, only aviation electrolyte should be used and the cells should not be diluted with water. The specific gravity of the electrolyte on a charged battery is 1.285 while electrolyte for an automotive battery has a specific gravity of 1.265. 

When adding fluid, a syringe or a bulb-type battery filler works well so that fluid can be removed if too much is added. 

Any spills can be cleaned and neutralized with a little baking soda and water, but only do so after the battery caps are reinstalled and tightened. Care should be taken to make sure none of the baking soda enters the battery.

Upon reinstallation, be sure not to overtighten the battery terminals. The terminals on a sealed battery require a relatively low torque, and overtightening can cause them to leak.


External charging of a battery

When using an external charger to charge a battery, it is best to use an aviation-specific charger. Always charge the battery to the manufacturer’s specifications. 

Aircraft batteries have thinner plates than automotive batteries and are more susceptible to damage from overcharge. They also require lower charging voltages than automotive batteries. This is also true of float chargers that are typically left plugged in any time a battery is not in use.

Teledyne Battery Products, the company that makes Gill batteries, lists four chargers for its various battery products on its website; these are available through Gill distributors.

The charger recommended by Concorde for use on its batteries is the Battery MINDer brand. This company has aviation-specific float chargers for aircraft batteries that are temperature compensated voltage regulated. These chargers provide a higher charge rate in colder temperatures and a greatly reduced rate of charge as temperature increases, preventing an overcharge. 

Once the battery reaches a fully charged state, the charger shuts itself off. Battery MINDer also has some solar powered versions for planes that are parked out on the ramp.

Float chargers are nice and lots of folks permanently install them on the battery. If you do, be sure to use FAA approved components like those available from Audio Authority Corp. that are designed for aviation use. 


Not designed to last forever

Even with the best care, batteries by design have a fairly short lifespan of usefulness. Periodic replacement is a given—around five years if unmaintained and up to 10 years if properly maintained. When choosing a new battery, pick a high quality product. 

Some folks like flooded-style batteries best, some prefer VRLA. Flooded batteries are typically messier than sealed batteries and cause corrosion, but they are slightly more forgiving of being overcharged since electrolyte levels can be adjusted. Flooded batteries are also less expensive.

Sealed batteries are less corrosive, and they self-discharge at a slower rate than flooded batteries. Sealed batteries typically cost more than flooded batteries.

With either style, the best thing an owner can do to extend the life of his or her battery is to keep it fully charged. With the improved chargers on the market today, that is becoming easier to do. 


Aviation batteries
– PFA supporters
Concorde Battery Corp.
Teledyne Battery Products
(Gill batteries)
Replacement battery boxes
Bogert Aviation
– PFA supporter
Temperature compensated voltage regulated chargers

Battery MINDer
Airframe interface kits and accessories
Audio Authority Corp.
Tech Tips from Flightline Technical Services: Troubleshooting a Flaky Ammeter

Tech Tips from Flightline Technical Services: Troubleshooting a Flaky Ammeter

Last month, we published a letter from Piper Flyer member Rich Schwartz. In addition to a stubborn prop vibration, he is having difficulty with the ammeter reading on his Arrow III.

Here is the second half of his letter:

Q: The ammeter [in my 1978 Piper Arrow III] consistently reads about one-half of a needle width off the zero line. The picture of the Seneca panel on page 45 of the October 2016 issue [of Piper Flyer] depicts this very well.

The indication is the same no matter what the load is, the needle will jump one needle width when a high-load item is first turned on (such as the fuel pump), then returns to zero.

The ammeter should work like a load meter and show the total load on the electrical system.

The shunt has been installed according to the Service Bulletin that came out. The original ammeter was replaced with a new unit from Air Parts of Lock Haven, and the results are the same.

This leads me to believe that the wrong shunt may have been installed and the ammeter is, in a sense, being bypassed.

I have used a multimeter to check the battery and alternator parameters, and all were normal.

I have my A&P [certificate], but do not work professionally in that capacity. I work exclusively on this airplane and a Stearman.

Any assistance you can offer would be most appreciated.

Thanks again,

Richard Schwartz


A: The original configuration of the ammeter in the PA-28R-201 was an internally-shunted meter, thereby all the power from the alternator went directly through the meter.

Piper decided that was not the best way to do things so they issued Service Bulletin 811A which changed the meter to an externally-shunted meter. This way all the power went through a shunt and the meter is hooked up in parallel to the shunt and actually reads minivolts.

The ammeter is calibrated to display amps.


I suspect that the connections at the shunt may be the issue, since you have already replaced the meter.

The shunt in this case is just a long piece of #6AWG copper cable.

I have several documents showing the before and after wiring, specs on testing the shunt and a copy of SB811A.


Hope this helps,

Tom Pentecost

Distribution Service
Administrator (DSA),

Flightline Technical Services

(a Piper Aircraft Dealer and Service Center)

The information noted in this exchange can be found on the forum at PiperFlyer.org under “Magazine Extras.” Members can download wiring diagrams and Service Bulletin SB811A there. —Ed.

No Appointment Necessary: DIY Oil Change

No Appointment Necessary: DIY Oil Change

A&P Jacqueline Shipe details the process of changing the oil and filter on an aircraft in this fifth installment in Piper Flyer’s DIY series. 

One of the items that is labeled as preventive maintenance by the FAA that a pilot may perform on his or her own airplane is the cleaning or replacing fuel and oil strainers or filter elements.

An oil change performed at a regular interval is one of the best things that can be done to prolong engine life. Clean oil lubricates and carries away harmful deposits better than dirty oil; plus, inspecting the contents of the removed filter or screen often helps to detect a malfunction before it becomes catastrophic. 

Draining the oil

Warm oil drains faster than cold oil, so it is nice to run the engine a little before beginning the oil change to cut down on the amount of time it takes to drain. (If you choose to do this, remember that the exhaust components will be especially hot. Use care.) 

The first step in the oil change process is draining the old oil. The container the oil is being drained into needs to be large enough to accommodate it. An old five-gallon bucket works well. 

Most planes have a drain valve on the bottom of the sump to facilitate oil changes. There are two primary types of valves: the style that pushes straight up to lock in place, and the type that has to be pushed up and turned. 

If it is possible to reach the valve through an opening in the lower cowling, an old rubber hose that fits snugly over the drain end of the valve works great to port the old oil into a bucket and saves having to remove the entire cowling. (It makes a huge mess if the hose pops off the drain valve midstream, so be sure it is secure.)

If the valve is inaccessible, the lower cowling will have to be removed, which isn’t such a bad thing because it allows access to give the engine a good looking-over. 


Removing the filter

Filter removal begins with cutting the old safety wire. The wire should be cut in the loop that goes through the safety hole on the engine, never pulled through. 

If the wire is pulled, it will cut through the soft, very thin tab on the engine and the tab will then be useless. Once this happens, the safety wire has to be attached at another point on the engine. 

After the safety wire is removed, the filter is loosened by using a one-inch size wrench on the end adapter to remove the filter. It is best if the wrench is the six-point style because the adapters are fairly thin; if the filter is stuck on the engine from being overtightened the time before, a lot of torque will be required to break it loose. The ears on the adapter can be easily rounded off if this happens. 

The space between the filter and the other parts on the accessory case is very limited on some engines; a short wrench may be required in these situations. 

The filter usually drains a little oil as it is removed. An empty oil container turned sideways with the top cut off can catch any oil that dribbles out as the filter comes off. Once the old filter is off, it can be placed aside and allowed to drain.


Installing the new filter

The new filter needs to be double-checked to be sure it has the correct part number. 

Some folks jot the tail number and tach time on the side of the filter using a permanent marker when it’s installed. This helps to determine when the oil was last changed without having to drag out the logbooks. It also ensures that the old filter won’t get mixed up with one that came off another engine or airplane. 

The new filter also needs to be inspected to be sure there are no leftover pieces of packaging or debris laying in it. If the filter has any dents or other signs of mishandling, it should be discarded or returned. 

After the filter is inspected, the gasket should be lubricated to protect it during installation. Some mechanics also partially fill the filter with clean oil before it is installed to help prevent a dry startup. 

Dow Corning DC-4 is the lubricant most filter companies recommend for greasing the gasket on the filter prior to installation. The lube helps with removal the next time, but mainly it keeps the rubber seal from being broken loose from the filter during installation It usually costs around twenty bucks for a tube of this lubricant, and one tube can last for several years.

The oil filter adapters are designed to provide a place to grab the filter with a wrench and are spot-welded on during the manufacturing process. If the new filter is overtightened during installation, one or more of these spot welds may break loose, which can cause a severe leak initially or later on down the road. 

Some mechanics never use the wrench adapter to install a filter because of this. At any rate, care should be taken to not over-torque the new filter. 

Once the new filter has been installed and properly tightened, it is ready for safety wire. Wherever the new wire is routed, it should be situated so that it can’t chafe into the filter or into anything else. Some mechanics slip a piece of heat shrink or something similar over the new wire to help prevent it from rubbing into the filter. 

The wire should have a positive pull on the filter and be fairly taut. 


Service the oil screen

Oil screens are removed in a similar fashion as an oil filter. The screen is small (compared to a filter) and generally has a larger area for access. The screen is safety-wired in the same manner as a filter. 

A copper crush washer is used as the seal on the oil screen housing, and the crush washer should be replaced at each oil change. 

Engines that didn’t originally come with an oil filter as standard equipment and have an aftermarket oil filter kit installed often will have the original screen as part of the oil system. 

The oil screen should be regularly removed and cleaned in addition to replacing the filter on these engines because the dirty oil goes through the screen before it reaches the filter. If it’s neglected, the screen can actually become clogged with debris and restrict oil flow to the engine.


Perform a leak check

Changing the oil and filter is fairly straightforward, but there are some precautions to take. The oil pressure at the point where it flows through the filter or screen is very high; a leak in a gasket can quickly lower the oil level if the airplane is flown with a leaking filter or crush washer. 

Any time the oil filter is replaced or the oil screen is cleaned, the engine should be run on the ground afterward and then checked for leaks. After the proper oil amount is added, the engine is ready for a ground run and leak check. 

Inspecting the oil filter and/or screen

The contents of a removed screen or old filter should always be thoroughly inspected for any metal. A clean white paper towel works well to catch the residue off a screen as it is cleaned. Most mechanics use a solvent or parts cleaner to flush a screen. 

An old filter should be cut open using a filter cutter so the inner part of the filter can be removed. The paper element needs to be completely cut out of the inner metal housing and unfolded so it can be inspected. 

When the pleats are still wet with oil it can be hard to see very small particles embedded in the paper. Some folks let the filter drain overnight because of this; the element can be folded sideways and compressed in a vice if a person is in more of a hurry. A vice will squeeze out the old oil but leave any contaminants in the pleats. 

A magnet gently drug across the filter pleats or the paper towel will pick up any steel particles. Finding any metal is cause for concern, but finding steel contaminants is a major concern. The source of the steel should be tracked down and remedied if there is more than a trace amount. 

Aluminum flakes or slivers are generally caused by wear on one or more of the piston pin plugs in the cylinders. These plugs are made of aluminum in most engines; some are now made of brass. 

They contain the free-floating piston pins on either side and keep them from contacting the cylinder walls. Aluminum and brass are relatively soft compared to steel and are designed to wear without damaging the cylinder wall. 

If just a few particles of aluminum or brass are found, it is best to change the oil at an earlier interval next time in order to inspect the filter contents. Generally the problem goes away on its own because the plug wears itself in a little and stops, but not always. 

Increasing amounts, or large initial amounts, of any metal is grounds for disassembling the cylinders—and possibly the entire engine—to discover the cause. It is best to let an experienced mechanic inspect oil filter contents if any metal is present to get a second opinion about what action, if any, should be taken.

Regular oil changes will save money in the long run. Owners that change the oil, filter and/or screens themselves will not only save money on labor costs, but also have a good understanding of the health of their airplane engine.


Know your FAR/AIM and check with your mechanic before starting any work. Always get instruction from an A&P prior to attempting preventive maintenance tasks. Jacqueline Shipe grew up in an aviation home; her dad was a flight instructor. She soloed at age 16 and went on to get her CFII and ATP certificate. Shipe also attended Kentucky Tech and obtained an airframe and powerplant license. She has worked as a mechanic for the airlines and on a variety of General Aviation planes. She’s also logged over 5,000 hours of flight instruction time. Send question or comments to .

I Found This in my Oil

I Found This in my Oil

What is, and what isn’t, typical.

Engine oil has several functions. Its primary purpose is to reduce friction and wear of internal parts by preventing metal-to-metal contact. Oil also helps to coat the bare steel internal surfaces and prevent corrosion inside the engine.

It performs several other functions, too. First, the oil system provides some cooling for the engine. Circulating oil distributes heat by cooling the hotter sections and warming the colder sections; it eliminates part of this heat through the oil cooler. 

The oil system also cleans the engine, as it suspends various particles of metal, silica, combustion by-products and other contaminants, then deposits them in the filter or screen.

Regular interval oil changes are one of the single most important things an airplane owner can do to help ensure lengthy and trouble free service from his or her engine. In addition to being excellent preventive maintenance, the oil change also provides a golden opportunity to get a diagnosis of the internal health of the engine. 

The examination of the removed oil, the sump screen and especially the oil filter or engine screen, can reveals a great deal about any internal engine wear or malfunctions that have occurred in the hours of operation since the last oil change.

Examining used oil

The removed filter should be treated as evidence, and drained into a small clean container, such as an empty oil container that has been cut open—so that the inner contents of the filter and the removed oil can be examined for contaminants. (See photo 01 below and photo 02, page 32.) 

The inner section of the oil filter contains a metal spool around which an accordion-style paper element is installed. The element has to be cut away from the spool with a utility knife and removed with pliers in order for it to be examined. Any tools used on the element should be clean and free of any contaminants. 

Once apart, the element should be placed on a clean layer of paper towels and allowed to drain further, around eight hours or overnight. It is important to remove as much of the excess oil as possible because the leftover, wet engine oil saturating the pleats makes inspection of the contaminants more difficult and sometimes hides small areas of trapped metal particles. 

If time is an issue, the element can be folded closed, wrapped in a clean shop towel or thick paper towel and compressed in a vise. This quickly removes the excess engine oil, allowing an accurate examination of the trapped particles. 

Care needs to be taken to ensure that all the surfaces the removed element comes into contact with are clean and free of any metal shavings or debris. Also, it is a good idea to wear disposable gloves when handling anything with used motor oil on it to prevent direct contact with your skin.

As an alternative to squeezing or draining the excess oil from the filter pleats, some mechanics recommend rinsing the element in a clean container of Varsol or mineral spirits and then straining the rinsed contents through a clean white paper towel or coffee filter. This process helps to free virtually all of the trapped residue in the element, allowing it to be clearly inspected. 

Oil filter screens are cleaned in a similar fashion by rinsing them in a container of Varsol and straining the rinsed contents through a filter. (Photo 03, below, and photo 04 on page 33 show an oil screen, and the residue removed from the screen, respectively.)

The drained element, towel, coffee filter, etc. should be taken out and carefully examined in a well-lit area, preferably in sunlight. (See photo 05, page 33.)

What you might find


Carbon is usually always present in the filter element. (See photo 06, page 34.) Some carbon is normal, but large amounts of carbon in the filter/screen are usually the result of excessive blow-by past the rings. Oil that rapidly turns black soon after an oil change is also an indication of blow-by. 

Carbon flakes are black and can appear shiny, but are easily distinguishable from metal. Carbon flakes found in an oil filter/screen can effortlessly be broken apart with a small pick or even between fingernails. Metal flakes will remain intact. 


Aluminum or bronze

Aluminum shavings or flakes are occasionally found in filters and are almost always the result of wear from a piston pin plug. (See photo 07, page 34.) The piston pins themselves are free floating, and the plugs installed on each end are made of a fairly soft metal like aluminum or bronze, so that as the piston pin plugs occasionally come into contact with the cylinder wall, they wear without damaging the cylinder barrel. The piston pin plugs are designed to wear slightly, and will sometimes wear a little then stop once they are no longer contacting the cylinder wall. (For more information about piston pin plug wear on Lycoming engines, see Resources at the end of this article. —Ed.)

Suddenly-occurring or large amounts of aluminum (or bronze, if the bronze-style plugs are installed) can be a sign that the plugs have become excessively worn and need to be replaced. (See photo 08, page 35.) It can be tough to figure out which cylinder needs to be pulled because oftentimes the compression is still good since the piston pin is below the rings and the relatively soft plug metal usually doesn’t leave appreciable wear marks on the cylinder walls. 


Brass-colored nonferrous slivers are usually generated from a wearing rocker arm bushing, or possibly on some engines, a starter adapter bushing. A tiny sliver or two is generally not a big deal, but regular occurrences or large amounts would merit finding out where it is coming from and fixing it. 

Magnetic particles

The presence of magnetic particles can be detected with a mechanic’s magnet. Sweeping the magnet across all the pleats of the element or towel/filter, just above the surface, will generally pull and reveal most of the magnetic debris. (See photo 09, page 35.) 

Common contaminants consist of bits of carbon, silica or dust particles that have been ingested by getting through a leaky air filter intake, or a very small amount of nonferrous metal. Engines that are past the break-in period (having 50 hours or more) should not have any significant amount of visible metal. 

Magnetic particles, flakes, slivers, etc. are always a cause for concern. Magnetic deposits can occur due to excessive wear or a malfunction in some component, but are most commonly caused by corrosion. 

Engines that have sat without being run for extended periods of time are susceptible to corrosion formation on several of the internal steel surfaces. Once an area of rust forms, the part affected no longer has a uniform smooth surface, and subsequent use will generate metal in the oil system. Steel cylinder barrels or steel piston rings occasionally rust after a time of inactivity. 

Camshafts and their corresponding lifter bodies are especially vulnerable to damage caused by rust. (See photo 10, page 36.)

Rust formation on lifter surfaces causes small areas of pitting that then grind on the camshaft lobe and produce rapid wear. (See photo 11, page 36.) 

Once this begins to occur, there is no hope that the lifter and lobe will stop wearing; things will only get worse and worse with every hour of subsequent engine operation. (See photo 12, page 37.)

Eventually the worn cam lobe will cause a loss of power due to the affected valve not opening as far as it should. A problem with the camshaft and/or lifter body is a very expensive repair because the engine must be pulled and the case split to gain access to the camshaft. 

The importance of engine temperatures

One of the best things aircraft owners can do to ensure long engine life is to run the engine(s) often, and to be sure that the oil temperature during operation is at least 180 degrees. Engine oil systems must reach at least 180 degrees to effectively evaporate all condensation from the oil system. 

During cold winter months, some owners restrict part or all of their oil cooler openings to help temperatures reach 180. Care should be taken not to allow oil temperatures to get too high, and it is also advisable to double-check the oil temperature gauge against a known source to be sure it is accurate. Lycoming recommends placing the oil temp probe in a container of water along with a thermometer and heating the water to 180 degrees, then confirming the oil temperature reading on the gauge, or even placing a reference mark on the gauge.

Also, for owners that use an electric engine oil preheater, it is best to not leave it plugged in continuously, but only for a period of time before the engine is to be operated. Most folks plug them in the night before the plane is to be flown. Leaving a preheater plugged in continuously can cause condensation to form inside the engine, especially in humid climates.

Oil analysis

If metal (especially magnetic metal) is present in the filter, the first step in deciding what corrective action to take is to determine which part of the engine is actually producing the metal. This is where an oil filter analysis comes in handy. The filter element can be bagged and sent to an oil analysis lab to have the metal contents diagnosed to determine their source. 

Generally, the results are in the form of aerospace material specification (AMS) numbers. The lab result containing all the AMS numbered metals and quantities can then be sent (generally via email) to the engine manufacturer to determine which engine sections are represented by the applicable AMS numbers. This is a great help in determining what, if any, corrective action should be taken. 

Tiny amounts of engine wear occur over time as an inevitable result of the engine running. Over time, all moving components in the engine are subjected to wear. The metal generated from normal everyday wear and tear tends to be microscopic and occurs in small amounts, with little or no visible portions of it showing up in the oil filter. These microscopic particles can be detected and catalogued in an oil analysis report. 

Regular interval oil analysis checks are a helpful trend-monitoring tool if they are sent in regularly. The sample needs to be taken from the old oil midstream during the draining process. Oil sample reports are useful for detecting a change. Oil sample reports should be kept so that a baseline of normal wear patterns exists. Any abnormalities can be detected as a change from the normal quantities of each type of detected material present in the oil. 

As a general rule, the owner should use the same lab and operate the engine with the same type of oil. If one or the other has changed, the reports may differ a little at first, but they should become consistent again as time goes on. 

Regular oil changes, along with consistent operation of the engine—at least one hour per week—will help ensure long engine life. Avoid allowing an aircraft to sit dormant for extended periods. 

In addition to these consistent operating practices, regular oil analysis and oil filter inspections serve as two good opportunities to catch any problems that do manage to occur before they become catastrophic. 

Know your FAR/AIM and check with your mechanic before starting any work. Always get instruction from an A&P prior to attempting any maintenance tasks.

Jacqueline Shipe soloed at age 16 and went on to get her CFII and ATP certificate. She obtained an airframe and powerplant license and has worked as a mechanic for the airlines and on a variety of General Aviation planes. Send question or comments to .


Oil analysis
AOA by ALS Labs 
Aviation Laboratories (AvLab)
Blackstone Laboratories
Lab One, Inc.
Engine operator’s manuals and service information
Continental Motors Group

Further reading

“No Appointment Necessary: DIY Oil Change” by Jacqueline Shipe
Piper Flyer, October 2016

Lycoming Service
Instruction No. 1492D
“Piston Pin Plug Wear Inspection” 

PiperFlyer.org/forum under “Magazine Extras”

Hone in the Range: Lycoming Oil Pressure

Hone in the Range: Lycoming Oil Pressure


Engine oil provides lubrication and cooling for an aircraft’s engine. Ensuring your oil pressure remains “in the green” is one of the most important things you can do for your engine’s health and longevity. Oil pressure in an engine is like blood pressure in a human. Both are important indicators of internal health, and both should be kept within proper parameters to ensure longevity.

Operating pressure

The normal oil pressure range for most Lycoming engines is between 60 to 90 pounds per square inch (psi). This range is indicated by the green arc on the oil pressure gauge. The maximum oil pressure allowed for short durations is 115 psi on most models. The maximum allowable pressure has increased over the years from 100 to 115 psi. The top red line on most oil pressure gauges is 100 psi. The lowest allowable limit for oil pressure with the engine operating at idle with hot oil is 25 psi, which is indicated by the lower red line on most oil pressure gauges.

Lycoming generally sets the operating pressures for cruise rpm on their factory-rebuilt engines to between 75 to 85 psi. Most new, rebuilt or overhauled engines require a slight adjustment of the oil pressure to finalize the setting once the engine break-in process is complete. 

Oil flow through a typical Lycoming engine

Lycoming engines use a “wet sump” oil system. This simply means that the oil sump is mounted under the engine and oil flows by means of gravity back to the sump after it has been pumped through the engine. The sump is completely open on the top so that all areas of the engine can drain back into it, and it functions like a large drain pan. “Dry sump” systems have a separate dedicated oil tank. Oil is routed to the tank once it has completed its course through the engine. 

The Lycoming oil pump is located in the accessory housing. It consists of an aluminum outer body and two steel impellers, one of which is gear-driven off the crankshaft. (See photos 01, 02 and 03 on this page.) It produces oil pressure in direct proportion to how fast the gears spin. At higher engine rpm, the pump produces more oil pressure than at low engine rpm. 

Oil is drawn up through the suction screen in the sump and through the oil pump impellers. The oil is then routed to the thermostatic bypass valve (also called a vernatherm valve). 

Oil continues to flow to the oil filter adapter on the accessory case and through the oil filter (or screen if the engine is not equipped with an oil filter). From the filter, oil is routed to the oil pressure relief valve. The oil pressure relief valve is located on the top right side of the crankcase. It relieves excessive oil pressure by opening a drain port to the sump to bypass some of the oil flow if oil pressure gets too high. 

Oil then travels to the crankshaft bearings and through predrilled passageways in the case to lubricate the internal engine parts through either pressure or splash lubrication. After completing its course, the oil drains back to the sump.

Thermostatic bypass valve

The thermostatic bypass valve is similar to a thermostat in an automotive engine cooling system. (See photo 04, far left.) The valve remains open when the oil is below 180 F, allowing the oil to bypass the passage to the oil cooler. As the oil heats up past 180 F, the vernatherm expands and eventually contacts its seat, forcing oil to pass through the oil cooler.

An engine that has abnormally high oil temperature may have a thermostatic bypass valve that is not expanding as it should with increased temperature, or that is not seating properly due to a worn seat. The valve seat wears over time and typically gets a worn groove that gets slightly worse every time it closes. If the valve gets excessively worn it allows some oil to bypass the oil cooler even when the oil is hot. (See photo 05, left.)

Some of the older bypass valves had retaining nuts that were improperly crimped during manufacture. Lycoming issued Mandatory Service Bulletin 518C that contained instructions for performing a heat treatment using a special Loctite to permanently secure the nuts in place. Valves that have had the Loctite treatment are typically inscribed with an “L” near the part number to indicate they have been repaired. 

As of August 2016, Lycoming no longer recommends this repair. Mandatory Service Bulletin 518D supersedes 518C and states that valve repair/rework is no longer allowed. Older-style valves with loose crimp nuts should be replaced.

Engines that suddenly develop an oil temperature problem may have one of the older-style valves with an improperly crimped nut that has come completely loose. Lycoming Service Instruction 1565 provides the procedure for replacement.

Oil pressure relief valve

The oil pump is a direct drive pump. This means that the pump impellers spin in direct relation to engine speed and produce oil pressure that also varies directly with engine speed. At high engine rpm, the pump produces far more pressure than the engine is designed to handle. Therefore, a pressure regulator must be incorporated into the system to keep pressures high enough at low engine speeds to protect the bearings and low enough at high engine speeds to prevent rupturing or damaging any of the engine components. 

The oil pressure relief valve (or oil pressure regulator) is located on the top right side of the crankcase; behind the number three or the number five cylinder, depending on whether it’s a four- or six-cylinder engine. (See photo 06, page 34.)

The oil pressure relief valve is very basic in its method of relieving excessive oil pressure. It consists of an aluminum housing with a strong spring, which presses against a steel ball. The spring keeps the ball seated. As oil pressure builds beyond the amount the spring is adjusted to maintain, the ball is forced off its seat by the excessive pressure. This exposes a passageway (bypass) that directs excess oil back to the sump, relieving some of the oil pressure. 

There are three types of housings. The latest style has an adjustable spring seat that can be cranked in or out as needed by means of an attached castellated nut on the end of the shaft. The older styles were adjusted by removing the housing and spring and adding or subtracting washers behind the spring to increase or decrease pressure. (See photos 07, 08 and 09 on page 34.)

The oldest style housing was short and had an adjustment of zero to three washers maximum. (See photo 10, page 36.) The longer housing allowed up to nine washers maximum to increase spring tension. (See photo 11, page 36.) Each added washer increases oil pressure approximately 5 psi. On the externally adjustable models, one turn in (clockwise) increases oil pressure approximately 5 psi. 

There are also springs of varying tensions and lengths which can be interchanged if the above adjustments do not yield the desired results. Some of the springs are color-coded to help differentiate them from one another. The most commonly used ones are the white LW-11713 springs (thick, heavy springs that are used to increase oil pressure at all settings), the 68668 (purple springs that are short and have much less tension than the others), and the 61084 non-color-coded spring that is standard equipment on most regulators. (See photo 12, page 36.)

One of the more common problems with the oil pressure regulators is with the seat that the steel ball contacts every time it closes. The seat is simply a machined aluminum section of the crankcase itself on most engine models, and over time it can become worn, especially if the ball is not contacting the seat dead in the center. If oil pressure varies excessively with engine rpm, especially at lower engine speeds, the regulator ball and seat may not be closing properly. Poor contact allows some of the oil to bypass back to the sump when it shouldn’t. (See photos 13, 14 and 15 on pages 36 and 38.)

If the cast aluminum seat has an irregular wear pattern in it, Lycoming recommends rigging up a makeshift tool out of an old ball welded to a steel rod that is thick enough to be struck with a hammer, then inserting the newly made tool squarely against the seat and giving it a couple of sharp hammer strikes to reform the seat, allowing a tighter fit between a new ball and the seat. 

The field method of repairing a worn or non-concentric seat that most mechanics employ is to use the same tool mentioned above, but instead of striking it with a hammer, they use a tiny bit of valve grinding compound on the ball to re-lap the seat. Care must be taken to prevent the compound from getting into any of the oil passageways during the process, but overall this method tends to work well to reform the seat and regain a good seal between the ball and seat. (See photo 16, page 38.)

Some of the earlier engines did have a replaceable seat insert that could be changed out and replaced if it was worn, but the most common seat is the cast aluminum type mentioned above.


Oil pressure gauge

The oil pressure gauge on many airplane models consists of a mechanically-actuated “Bourdon tube.” The Bourdon tube is a somewhat rigid, coiled, hollow tube. The tube is connected to a small oil pressure line and as oil pressure increases, the tube is stretched to a straighter, uncoiled position. The amount that it stretches varies directly with the pressure. An attached needle and gear mechanism allows the varying pressure to be read on the oil pressure gauge. These mechanisms can get dirty and stick, or the gearing mechanism can get worn and not indicate correctly. A shaky needle is often caused by a worn gear mechanism in the gauge.

Some aircraft use an oil pressure transducer or sending unit that looks similar to the oil pressure switch used for Hobbs meter installations. It is a unit that has an oil pressure line piped into one side, and electrical wires connected to the other side. Pressure is converted to an electrical signal and wires are run to a gauge that displays the oil pressure reading.

The oil pressure in most Lycoming engines is taken off the top rear accessory case. The oil pressure fitting has a reduced orifice in the outlet to the gauge. This helps prevent catastrophic oil loss if the oil pressure line or gauge begins to leak. Carbon or dirt can sometimes clog the orifice and cause an abnormally low oil pressure reading. 

Troubleshooting oil pressure problems

Most oil pressure problems can be adjusted back to normal with the regulator or traced to a malfunctioning regulator or gauge. Sometimes, the trouble is a little more difficult to repair. 

The first step in correcting abnormally high or low oil pressure should be to double-check the pressure reading with a separate pressure gauge to confirm that the oil pressure really is too high or low. Check the oil temperature, too. Low oil pressures will produce increased oil temperatures, and vice versa; overly high temperatures thin the oil and can cause a lower-than-normal oil pressure reading.

Excessive internal engine clearances due to excessive wear or a bearing failure can become so great that the output of the pump is insufficient to fully pressurize the oil system. This is typically a worst-case scenario and lower oil pressure readings occur gradually over time. 

Excessive oil pump clearance between the impellers and the housing can also cause degraded oil pressure output.

Oil viscosity plays a role in oil pressure as well. A slightly lower than normal oil pressure may be caused by using too thin an oil depending on where the plane is operated. 

A clogged suction screen or partially blocked passage between the screen and pump can also cause low oil pressure.

A higher-than-normal oil pressure reading, especially one that occurs suddenly, can be indicative of a blockage somewhere in the system, usually downstream of the pump.



Oil pressure readings should be consistently monitored so that any deviation from normal operation can be detected and remedied quickly. Consistent, normal oil pressure from startup to shutdown helps assure that an engine will run reliably for a long time.

Know your FAR/AIM and check with your mechanic before starting any work. 

Jacqueline Shipe grew up in an aviation home; her dad was a flight instructor. She soloed at age 16 and went on to get her CFII and ATP certificate. Shipe also attended Kentucky Tech and obtained an airframe and powerplant license. She has worked as a mechanic for the airlines and on a variety of General Aviation planes. She’s also logged over 5,000 hours of flight instruction time. Send question or comments to .


Lycoming Mandatory Service Bulletin 518D

Lycoming Service Instruction No. 1565A


No Harm, No Foul: Spark Plug Maintenance

No Harm, No Foul: Spark Plug Maintenance

In the last of Piper Flyer’s series on owner-performed preventive maintenance, A&P Jacqueline Shipe looks at the servicing and replacement of aviation spark plugs.         

Aviation spark plugs need to operate while subjected to the wide temperature ranges that are possible in an aircraft engine. A spark plug with a 0.020-inch gap must be able to handle around 14,000 volts and fire reliably during its lifespan. 

Regular cleaning, gapping, and rotation of spark plugs helps ensure that the longest and most reliable service life for each plug is obtained. Regularly pulling and inspecting the plugs also helps diagnose cylinder health. 

Under Appendix A, paragraph (c) of FAR 43, the items “spark plug cleaning, gapping and replacement” are on the list of maintenance items an owner can perform on their own aircraft. 

Anatomy of a spark plug

Aviation spark plugs have a positive center electrode that is connected to the ignition lead terminal through a resistor. This center electrode assembly is housed in a ceramic insulator, which prevents the high voltage electrical current generated by the magneto from grounding out against the metal outer shell, which contains the negative electrode(s). 

These plugs are designed to withstand severe operating conditions and typically provide a long service life if they are properly maintained.

Removing the plugs

The first step in spark plug maintenance is removal of the plugs. Once the engine cowling is removed to the extent necessary so that access to all the plugs is achieved, the ignition leads can be disconnected from the spark plugs. 

The inner part of the lead needs to be held stationary as the outer nut is removed to prevent the lead from being twisted as the outer nut is turned. The leads should be gently pulled straight out and not cocked as they are removed from the plugs. 

A good deep-well six-point 7/8-inch socket is required to remove the plugs. Aviation spark plug manufacturers, including PFA supporter Tempest, makes and sells a specialized aviation spark plug socket that works well. Be sure the socket is properly seated on the plug before attempting to break it loose.   

It is important to keep track of which position each plug is removed from. This helps for diagnosing cylinder health and for plug rotation during the reinstallation. 

Homemade spark plug trays with marked receptacles for each plug are easy to make, or plugs can be laid out on a piece of marked cardboard. Tempest highly recommends using a spark plug tray to keep plugs from rolling of the workbench and to assist with proper plug rotation.

Avoid laying a plug on top of the cylinder, or any place where it could roll off and hit the floor. Dropped plugs often have cracked insulators or damaged resistors—and even if they pass a resistance check afterward, they could still have defects that can result in malfunctions and misfiring later on. Any plug that is dropped should be discarded. 

Inspecting the spark plugs

Plugs should be inspected after removal for excessive wear and general condition. 

Oil-soaked plugs

Any bottom plugs that are wet with oil aren’t a cause for concern, but if the top and the bottom plug in a cylinder are wet with oil, it can be a sign that there is either excessive piston ring wear, the ring gaps are lined up and/or the plug is malfunctioning. It wouldn’t hurt to take a compression check on the cylinder in question. 

Plugs that are misfiring will be oil-soaked simply because they aren’t firing enough to clean off any deposits; a top oil-soaked plug could simply be the result of the plug itself malfunctioning.    

Oil-fouled plugs should also be inspected for cracks and/or chips in the core nose insulator, according to John Herman at Tempest. Cracks or chips here may indicate a broken ring, which may result in cylinder damage from the broken piece of ring scoring the cylinder wall during piston cycles.  

Cylinders with insulator plug damage should be borescope inspected to be sure the cylinder has not been damaged or there is no evidence of foreign object damage or debris (FOD).

Taking note of buildup

Normally, any removed plug has a deposit residue of some sort on it and will be a little sooty just from the normal combustion process in the cylinder. 

Plugs that have virtually no deposits on them (i.e., too clean) or that have a slight reddish-brown tint on the insulator are indicative of a cylinder that is running too hot, or too lean, or both. 

If this is noticed only in one cylinder, the intake gasket and tube should be inspected for leaks. A partially clogged fuel injector on fuel-injected engines can also cause a cylinder to run lean. 

The most common deposits on spark plugs are lead and carbon. Lead buildup forms hardened balls that can eventually bridge the electrode gap and cause a plug to not fire. Carbon is jet black and sooty in appearance.

Excessive lead and carbon buildup on several plugs is a sign that an engine is being run too rich and not leaned properly. A good practice, endorsed by the folks at Tempest and others, is to lean on the ground any time the rpm is below 1,000. Always be sure to richen the mixture prior to takeoff. 

Cleaning the plugs

Once the plugs are removed and organized as to which position they came from, the next step is to clean the plugs. 

Lead deposits can be very built up and hardened, making them difficult to remove. Safety glasses, a dust mask, and chemical resistant gloves should be worn to protect eyes, lungs and hands during spark plug cleaning.

Vibration cleaning

Champion makes a machine that uses two cleaning prongs that vibrate at a high frequency to break loose the lead and pulverize it into fine particles that can be shaken out. Avoid breathing any of the dust generated from this process, as it contains lead particles.

These two-prong machines can be a little pricey, but there are handheld single-prong models that retail for a little over 20 dollars. (See Resources for a list of PFA supporters that sell the handheld spark plug vibrator cleaners. —Ed.) 

Abrasive blasting

In addition to getting the lead out of a plug, some shops clean the firing end of a spark plug in a sand or glass bead blast cabinet. 

Tempest does not recommend glass bead blasting on its plugs because some of the glass bead residue can become lodged between the center electrode and the ceramic insulator. As engine temperatures heat up, the glass beads melt into a conductive coating which can cause the plug to misfire. 

If a plug is to be blasted, Champion and Tempest both recommend using an abrasive grit that is made specifically for cleaning plugs. These companies advise lightly blasting only the tip of the plug; excessive blasting erodes the electrodes, causing premature wear. 

Some mechanics don’t recommend any kind of abrasive blasting to clean plugs due to the electrode erosion it can cause, especially on fine wire plugs. Tempest doesn’t recommend abrasive cleaning for its fine wire spark plugs for this very reason. 

Manual cleaning 

If plugs are oily, a little solvent (e.g., Varsol or other traditional mineral spirits) works well to clean the residue out of the firing end. Note: the plug should not be fully immersed in the fluid; it should only be used on the firing end. 

For stubborn lead deposits on the firing end, a good gun cleaning solvent, such as Hoppe’s #9 Bore Cleaning Solvent, is recommended by Tempest.   

A swab soaked in Methyl ethyl ketone (MEK, or butanone) works well to clean the insulator and ignition lead contact in the opposite end of the plug. Note: never use Tetra ethyl chloride on the terminal well area of the spark plug; rubbing alcohol will work just fine, according to Tempest.  

The threads on the firing end can be cleaned using a wire brush; just be sure not to clean the electrodes with the wire brush, as this can damage them. 

Gapping the plugs

Once the plugs are cleaned and dried, they are ready to gap. There are a few different styles of gapping tools, but they all essentially work the same. 


The plug is threaded into a receptacle on the tool, and a prong is pressed or screwed against the ground electrodes to move them closer to the center electrode. The recommended gap varies according to the plug and can be located on the spark plug manufacturer’s website. 

A wire-style feeler gauge is used to measure the gap between the center and outer electrodes. Care needs to be taken to not close the gap too much, as the electrodes can’t be spread back apart. 

Do not leave the feeler gauge between the electrodes when setting the gap. This can put a load on the insulator and cause it to crack.

Fine wire plugs typically don’t require re-gapping too often. Champion makes a specialized gapping tool for use on fine wire plugs if they do need to be reset. Tempest doesn’t currently have a similar tool, but is in the process of expanding its spark plug tool product line.  

Bench testing

Bench testing the plugs helps to detect and prevent reuse of a faulty plug. 

Both Tempest and Champion recommend the use of a bomb test to check a plug’s ability to fire under pressurized air. These types of testers are expensive and are usually found only in an equipped maintenance hangar, but it should cost only a few dollars to have the shop do the checks for you. 

A resistance test can be performed in addition to the bomb test, but it’s not a replacement for the bomb test. 

Tempest recommends using an electrical multimeter to check the resistance value between the ignition lead terminal in the upper part of the plug and the center electrode. The electrical resistance should not exceed 5,000 ohms on Tempest plugs. Any plugs with readings higher than 5,000 ohms should be discarded. 

Reinstalling the plugs

After the plugs are gapped, they are ready for reinstallation. 

Replacing gaskets

The copper gasket that seals the plug against the cylinder head hardens as engine temperatures heat and cools the gasket over a period of time. 

A hardened gasket does not seal as well as a soft gasket does, and can also keep the plug from properly seating against the cylinder head. Therefore, copper gaskets should be replaced before reinstalling the plugs. Spark plug manufacturers recommend that the gaskets are replaced each time the plug is removed and cleaned. 

Plugs that have thermocouple gaskets attached to CHT monitors do not require a copper washer in addition to the thermocouple washer.

Anti-seize lubricant 

Before the plug is threaded into the cylinder, a thin coat of a high-quality anti-seize material should be brushed on the threads. 

The first two threads closest to the electrodes should not be coated to prevent the conductive anti-seize compound from getting on the electrode and causing a misfire. 

Champion and Tempest make specialized anti-seize lubricants that they recommend for use on their plugs. 

A high-quality graphite- or copper-based anti-seize works well. Nickel-based anti-seize has always worked well for me. Aluminum-based anti-seize lubricants typically don’t work well because they don’t hold up under the severe heat. (Per Lycoming Service Instruction No. 1042AH "Use a copper-based anti-seize compound or engine oil on spark plug threads starting two full threads from the electrode, but DO NOT use a graphite-based compound.")

Never use a general or all-purpose graphite-based lubricant; use only lubricants that are designed for spark plugs.

Rearranging the plugs

Aviation spark plugs should not be reinstalled in the same location they were removed from. 

Ignition leads are polarity-sensitive on all magnetos (other than some of the dual magneto models); this means that the north and south poles of the spinning magnet in the magneto generate a negatively-charged spark that is sent down one lead, alternately followed by a positively-charged spark sent down the next lead. 

Plug electrodes wear in predictable ways. The plugs connected to the positively-charged leads always fire from the positive center electrode to the negative electrodes, eroding the center electrode. The plugs on the negatively charged leads always fire from the negative electrodes back to the center electrode, eroding the outer electrodes. 

Keeping the plugs rotated so the positive and ground electrodes wear evenly will double spark plug life. They should also be rotated from top to bottom, as the bottom plugs usually incur more deposit material. A rotation that a lot of mechanics use is top-to-bottom, and next in firing order. 

Torque values

Proper torque values should be used when reinstalling the plugs. Lycoming recommends 30 to 35 foot-pounds (420 inch-pounds); Continental recommends 25 to 30 foot-pounds (300 to 360-inch pounds). 

The ignition leads should be installed with care, and the leads should not be allowed to twist as the outer nut is tightened. 

Mag check and troubleshooting

An engine runup and magneto check should always be performed to ensure that all of the plugs are firing properly. A smooth runup and magneto check indicate a job well done.

A rough-running engine during the magneto check is most likely indicative of a little debris or excess anti-seize on the electrodes of one of the plugs causing it to misfire or not fire at all. Take note of which magneto the engine is running rough on. 

Once the engine is shut down and cooled off, check to see which plugs are fired by the magneto in question by visually following each ignition lead from the rough magneto all the way out to each plug. 

These plugs can then be removed, and any debris can be gotten out with a small pick. Any anti-seize lubricant that has gotten on the electrode can be cleaned off with a little degreaser.

Over the last seven months, I’ve given you some general tips and step-by-step ways you can work on your own aircraft according to what’s allowed in FAR 43, Appendix A, paragraph (c). 

This DIY series, along with guidance from a trusted mechanic, should give you a better understanding of preventive maintenance on your airplane—and might even save you a little money in the long run.

Know your FAR/AIM and check with your mechanic before starting any work. Always get instruction from an A&P prior to attempting preventive maintenance tasks.

Jacqueline Shipe grew up in an aviation home; her dad was a flight instructor. She soloed at age 16 and went on to get her CFII and ATP certificate. Shipe also attended Kentucky Tech and obtained an airframe and powerplant license. She has worked as a mechanic for the airlines and on a variety of General Aviation planes. She’s also logged over 5,000 hours of flight instruction time. Send questions or comments to .


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Camshaft & Lifter Wear

Camshaft & Lifter Wear

Corrosion attacks camshafts and lifters before most other bottom-end engine parts, especially when an aircraft doesn’t fly frequently. Learn how to prevent, identify and treat common camshaft and lifter problems before they cost you! 

Many folks think of engine wear and tear and overall condition as being represented by the amount of operating time since the last overhaul. The “time since major overhaul,” or TSMOH for short, is typically entered by mechanics into an aircraft’s logbooks at every major inspection and annual. One of the first questions that buyers ask when considering an aircraft for purchase is “How much time is on the engine?”

Cumulative engine operating time does matter. Moving parts, particularly in the high heat and high stress cylinder environment, do wear with use. However, one of the main reasons that engines have to get torn down and rebuilt isn’t due to wear from operating regularly, but from corrosion that occurs during prolonged periods of non-use. 

An engine’s camshaft and lifters are particularly susceptible to damage from corrosion. Corrosion and the resulting wear on the camshaft and lifter bodies are one of the primary reasons engines get torn down before reaching the manufacturer’s recommended time between overhauls (TBO).

Camshaft construction and function

Camshafts are a simple-looking part, but they play a major role in engine operation because they control and drive the entire valve train. The camshaft is gear-driven and typically turns at one-half the speed of the crankshaft. 

Camshafts are made of carbon steel and have varying numbers of “lobes” along their lengths. Each lobe has an elliptical shape and is put through a hardening process called “carburization” during its manufacture. 

The carburization process involves heating the camshaft in a specially-designed furnace or oven and exposing it to carbon monoxide gas as it is heated. This process causes the exposed surfaces on the camshaft (primarily the lobes) to absorb extra carbon, which makes the surface of the lobes very hard. The depth of the hardened layer on the lobe is about .015 inch (fifteen-thousandths of an inch). 


Lifter construction and function

The lifter consists of a cast-iron outer body that houses a hydraulic unit. There are a couple of Lycoming engines that still utilize solid lifters with no hydraulic unit, but the vast majority of engines have hydraulic lifters. Most mechanics use the term “lifter” when referring to both the lifter body and the hydraulic unit inside the body. 

Lifters are also sometimes referred to as “tappets.” The face of the lifter body is the part that is in continuous contact with the camshaft lobe that it rides on. The lifter face is made of chilled cast iron which gives it increased wear resistance. It is tough, but not nearly as hard as the carburized surface on the cam lobe. 

The lifter is designed to be a little softer than the lobe, so that the two are compatible with each other. If deterioration does start to occur, it generally occurs first on the lifter rather than the camshaft lobe. 

The hydraulic unit in the lifter body is designed to act as a solid unit as it opens the valve. It must also be able to expand or contract as needed to take up all extra clearance in the valve train. There are some variations in the exact design, but the basic function is the same. 



Valve operation

The cylinder valves are linked to the camshaft through the lifter, push rod and rocker arm. The camshaft rotates and the top of the lobe raises the lifter to its highest point. The push rod in turn raises the rocker arm end to which it is joined.

Since the rocker arm is mounted near its center on a steel shaft, it acts as a fulcrum and raising one end causes the other end to lower. The opposite end of the rocker arm that is in contact with the top of the valve stem then lowers the valve to its full open position. 

The time that the valve opens in relation to all other moving engine components, and the length of time that it stays open are both determined by the height and position of the camshaft lobe. 

The valve clearance, or valve “lash” as it is often called, is measured with both the exhaust and intake valves closed with the piston at top dead center (TDC) on its compression stroke. The clearance is measured with the hydraulic unit of the lifter “bled down,” or in its flattest position with no oil in the reservoir of the lifter. 

The clearance is checked between the valve stem and rocker arm with the push rod socket on the rocker arm pressed in toward the camshaft. The engine manufacturers set a minimum and maximum clearance for the valves. Too much or too little clearance can cause increased wear on the valve operating mechanism. 

The clearance is adjusted on engines with solid lifters by means of an adjustment screw on the rocker arm itself. On engines with hydraulic lifters, the adjustment is made by replacing the push rod with another of a different length. 

The valves are held in the closed position by multiple strong valve springs. The camshaft and lifter must overcome the valve spring tension every time the valve is opened. This generates a large amount of pressure between the lifter face and the camshaft lobe. 



Lifter spalling

The camshaft and lifter bodies typically are lubricated in most engines by what is called “splash lubrication.” There are no pressurized oil ports on standard engines that deliver oil to the cam and lifter surface; they run only in the oil that is thrown off the crankshaft and rods as they rotate. 

At engine start-up, it typically takes a few revolutions of the crankshaft to begin getting any significant amount of oil to the camshaft. The lack of pressure lubrication along with the high pressure between the interface of the camshaft lobe and lifter face create an environment that is unforgiving of any defects in the surfaces of either the lobe or lifter. 

Engines that sit dormant for extended periods of time (between two to six months, depending on the environment) are subject to developing corrosion on both the lifters and on the cam lobes themselves. 

The camshaft on Lycoming engines is located in the top of the crankcase above the crankshaft. Its position in the top of the case means that oil drains off it first once the engine is shut down. On Continentals, the camshaft is located beneath the crankshaft, but it is still above the oil sump and residual oil eventually drains off it as well. 

The crankshaft is not as vulnerable to corrosion because its metal-to-metal contact surfaces are encased in bearings. The bearings typically hold a film of oil on the crankshaft surface. The camshaft has no bearings. It spins inside machined grooves in the crankcase. The lobes and lifters have a lot of their surfaces fully exposed to the air that drifts through the engine. 

Aircraft engine breathers are simply vents designed to prevent pressurization of the crankcase with the engine running. However, they also vent and expose the internal engine components to any humidity that may be present in the outside atmosphere as the airplane sits parked.

Condensation occasionally occurs inside the engine after shutdown as parts of the engine cool at varying rates. Condensation also occurs with different weather conditions during times of non-use. Moisture can cause small freckles of rust to form on the lifter face. As the corrosion progresses, the rust eats into the lifter surface. 

Once the engine is started again, the small patches of rusted material fall out as the lifter face is pressed against the camshaft lobe. After this process starts, the constant pressure between the cam lobe and lifter wears off more and more material even though the engine is being lubricated during operation. This process is referred to as “spalling” of the lifter. 

The removed material and the rough lifter face will eventually wear the camshaft lobe. If lifter spalling is caught early, the problem may be fixed by replacing the affected lifters. However, if any of the camshaft lobes have begun to wear, the entire camshaft—in addition to the damaged lifters—must be replaced. 


Lifter inspection on Continental engines

Most Continental engines use cylindrically-shaped lifter bodies that can be removed without splitting the crankcase. Lycoming engines use mushroom-shaped lifter bodies that have a larger diameter at the face of the lifter. These types of lifter bodies cannot be removed without splitting the case. 

On the larger Continentals (O-470 series, IO-520, etc.), lifter removal only requires removal of the affected cylinder valve cover, rocker arm, push rod and the push rod housing. The housing is removed by compressing the tension spring on the inboard part of the housing to release it. 

Once the housing is out of the way, the lifter can be removed by gently using a pick to grab it by the oil galley in the push rod socket and sliding it out. Engine manufacturers do not recommend using a mechanic’s magnet to remove a lifter because it could magnetize the ball in the check valve of the hydraulic unit and cause the valve to malfunction.

After the lifter has been removed, the camshaft lobe can be inspected using a bright light. Continental Service Information Directive SID 05-1B contains inspection criteria which should be used to determine if a camshaft is still airworthy. Continental recommends using a mechanic’s pick to probe any surface deformities. 

If the pick catches, the camshaft most likely will need to be replaced. If the camshaft lobe looks OK with no surface damage, the lifter itself can be replaced with a new one if it shows any sign of wear. 

Some mechanics are leery of replacing lifters without replacing the camshaft because the lifters and cam lobes do wear into each other slightly during the initial engine break in, but it is primarily the lifter, not the camshaft lobe that wears. 

Aircraft owners who have large Continental engines that have been dormant for an extended period of time would be wise to inspect their lifters before they run the engine again. The process of doing so may very well catch a corroded lifter before it has a chance to wear a cam lobe. Six months would be considered an extended period of time under usual conditions; two months in humid or salty areas.


Clues that a camshaft may be wearing

Most camshaft and lifter wear goes undetected until fragments or small magnetic filings show up in the oil filter during an oil change. Even though amounts may be very small, the camshaft and lifter bodies are usually the culprits if an engine suddenly starts making magnetic material. Steel cylinder barrels or steel cylinder rings also are prone to rusting a little, but they generally stop shedding any rusted debris as soon as the airplane is flown a time or two. 

Regularly-occurring magnetic filings are most likely coming from the lifter bodies, and once lifters start shedding surface material, they continue to degrade and wear with subsequent use. The good thing is that other than possibly causing a tiny bit of engine roughness, cam lobe and lifter wear is not something that causes sudden stoppage of an engine. 

If small amounts of filings are found, the oil filter element and its contents should be sent to an aviation oil analysis lab to determine the source. An oil-only analysis will probably show nothing unusual in these cases because when the lifters start spalling, the flakes and filings they shed are so large that they get caught in the filter rather than being suspended in the oil itself. Most labs can examine the filter element itself to determine where the metal is coming from. 

Some mechanics that find very small amounts of magnetic material (less than enough to cover the end of a mechanic’s magnet) will recommend flying the plane for another 10 hours or so, then checking the filter again. The hope is that it will stop on its own, but an engine that is past the break-in period should not be making magnetic material. 

If the filings are coming from rusted cylinder barrels, an owner might get lucky and the oil filter might be clean on the next check, but it’s been my experience that almost every time I’ve found magnetic debris in the filter, it only gets worse and worse with use. 



Reground versus new camshafts

A camshaft or lifters that are worn badly enough to necessitate an engine teardown will require replacement. Many engine rebuilders in the field have no problems using a reground camshaft (one that was still within serviceable limits that has the lobes re-machined to a smooth finish.) Lifter body faces can also be reground to a smooth finish. 

If an owner chooses to use a reground camshaft or lifters, they should only get them from a high-quality aircraft machine shop experienced in the regrinding process. The carburized hardened layer on the camshaft lobes is only around .015 inch deep, and the layer’s depth can vary a little. 

If a machine shop accidentally gets below the carburized layer at any time during the regrinding process, the camshaft lobe will wear down rapidly once it is put back into service. The initial hardening process of carburization is completed only at manufacture. It can’t be duplicated in the field because it is almost impossible to keep the camshaft from warping during the carburization process. 

Additionally, the apex (or top) of the camshaft lobe is tapered slightly. One side of the lobe is slightly (around .003 inch) higher than the other side. This design causes the lifter body to spin as it contacts the lobe. This prevents the camshaft lobe from contacting the lifter face in the same place every time, and helps prevent wear on the lifter face. The exact taper may be hard for some shops to duplicate.

I personally would rather have a new camshaft and lifters if the engine is being torn down to the extent that they were accessible. However, most reputable engine builders that I spoke with had no problem using reground cams and lifters, depending on where they were done, and I know of more than a few engines that have easily made and even gone past TBO with reground parts.





The best thing an owner can do to prevent camshaft and lifter problems is to regularly fly his or her plane—at least once a week in humid climates. Each flight should be long enough to get the oil temperature up to at least 180 degrees to evaporate any water in the oil system. Changing the oil regularly and often is a good preventative maintenance practice. The addition of an anti-corrosion additive also helps. 

The initial cost of prevention generally is greatly rewarded with a long-lasting healthy engine. 

Know your FAR/AIM and check with your mechanic before starting any work. Always get instruction from an A&P prior to attempting preventive maintenance tasks.

Jacqueline Shipe grew up in an aviation home; her dad was a flight instructor. She soloed at age 16 and went on to get her CFII and ATP certificate. Shipe also attended Kentucky Tech and obtained an airframe and powerplant license. She has worked as a mechanic for the airlines and on a variety of General Aviation planes. She’s also logged over 5,000 hours of flight instruction time. Send question or comments to .



TCM Service Information Directive 05-01B, “Inspection Guidelines for CM Camshafts and Hydraulic Lifters”


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Is your Engine Worn Out? How to Tell & What to Do About It

Is your Engine Worn Out? How to Tell & What to Do About It

Smart owners who monitor key performance indicators can tell if an engine is still good or whether “it’s time.” If your engine is due for an overhaul or replacement, STEVE ELLS has a list of options which can save you time, money and maybe even both.

The day before the start of what I’m now calling the best EAA AirVenture Oshkosh ever (See page 52 for Steve’s AirVenture report. —Ed.), I stood before an enthusiastic group of Piper Flyer Associ-ation members at the annual Gathering at Waupaca, Wisconsin. It was 7:30 a.m. Sunday morning. I made sure everyone was awake by asking a scary question.

I asked how many owners thought they had an engine overhaul looming on the horizon. Seven hands went up. Those owners reflected the concerns of many owners. Engine overhauls are expensive; not to mention they can be time-consuming and stressful.

It is difficult for owners who don’t deal with overhauls on a daily or weekly basis to be able to tell when “it’s time.” An engine can be worn out, but it will still start, develop power and appear to be operating normally. On the other side of the coin, it’s also certainly possible for an engine to be running well and in good condition far beyond the manufacturer’s recommended time between overhaul (TBO).

I’m going to provide a few guidelines for determining your engine’s health.

The engine’s bottom end (and why it matters)

The air-cooled direct-drive engines we fly behind are stout; especially the “bottom end” portions. The bottom end includes the case, crankshaft, connecting rods, camshaft, lifters and accessory gears and accessory housing.

Just because the compression is low in one, two or all cylinders does not mean the engine is ready for an overhaul. Cylinders can be removed and rebuilt, or replaced with new cylinders without disturbing or compromising the bottom end. But when an engine’s bottom end is worn out, nothing short of an overhaul will restore it to airworthy condition. 

Oil pressure

Idling oil pressure when the engine is hot is an excellent indicator of the health of the bottom end of an engine. The hot idling oil pressure of our engines should always stay above the lower red line on the oil pressure gauge. 

The oil pressure limits and acceptable range are in every owner’s manual and pilot operating handbook (POH). As a rule, Lycoming engines have a 25 psi low oil pressure limit and Continental engines have a 10 psi low oil pressure limit. 

One of the most important factors in maintaining oil pressure is the clearance between the crankshaft journals and main crankshaft bearings. The spinning crankshaft in an engine is supported by a cushion of lubricating oil under pressure. 

Since there is a gap between the outside diameter of the journals of the crankshaft and the inside diameter of the main bearings surrounding each journal, the oil that’s pumped in also flows out through the gap between the two. The size of the gap is a major determinant of idling oil pressure. When the gap grows due to wear, the leakage through the gap increases and idling oil pressure goes down. Low idling oil pressure almost always signals that the bottom end of your engine is worn out, or that there’s another problem with the bottom end.


Oil consumption limits

It’s rare for an air-cooled Avgas-burning engine to not use any oil. Manufacturers are tasked with producing engines that must perform in conditions ranging from below zero F outside air temperature (OAT) to 100 F-plus OAT. The engines must produce rated power in missions where the aircraft may take off from extremely hot temperatures on the ground, only to climb rapidly to altitude where OATs are below freezing. Given all the metallurgical expansions and contractions that take place due to these extremes, air-cooled aircraft engines are intentionally built to larger tolerances than any automobile engine.

Oil usage is one of the trade-offs that result from building air-cooled engines that perform as well as ours do. 

If your aircraft’s engine uses oil, that’s normal. But how much is too much? Luckily, there’s a formula for that. 

Lycoming’s Service Instruction 1427C, “Lycoming Reciprocating Engine Break-In and Oil Consumption,” provides the following formula: 

0.006 x BHP x 4 ÷ 7.4 = quarts per hour.

Let’s find the allowable oil consumption for a 180 hp engine. BHP is an acronym for brake horsepower, so the formula works out like this: First, multiply 0.006 x 180 x 4 = 3.6. Dividing that by 7.4 yields a maximum oil consumption for a 180 hp engine of 0.58 quarts per hour, or a quart every 1.7 hours. 

The same formula applied to a 300 hp engine yields a maximum oil usage of 0.97 quarts per hour. 

The only drawback with very high oil consumption is that it limits flight leg length. If your engine has a 4-quart sump, you’re not going very far if your engine is going through 2 quarts an hour.

Many owners are unaware that each engine and airframe combination has an oil level “sweet spot,” where consumption slows. 

Above this level, much of the oil is discharged out the crankcase breather tube. The oil is not being consumed; it’s simply being pumped out the breather tube. If you see a lot of oil on the belly of your airplane aft of the breather tube, you are probably over-oiling your engine.

The sweet spot in my 1960 Piper Comanche 180 with a Lycoming O-360-A1A is 6 quarts in the 8-quart sump. The consumption rate for my current Lycoming O-360 is 1 quart every five hours. My average cross-country leg is around four hours so I just carry some oil and add about a quart at every stop. 

The key is to first fill to the sweet spot for your airframe/engine and then use consumption from that level to determine your engine’s oil consumption. 

Oil leaks

Damaged engine cases can cause persistent, hard-to-find oil leaks. Cases can and do crack, leading to loss of oil. 

Lycoming narrow-deck engines—the standard configuration before the mid-1960s—can develop a difficult-to-find leak when the engine case through bolts are loosened and then retightened during a cylinder change or top overhaul. The sealing O-rings between the case halves often fail to reseal the through studs after the cylinder(s) are reinstalled and torqued down. The result is a persistent oil leak past one or more of the through studs. 

There’s no way to stop that leak, nor is there a way to fix a leaky crankcase crack short of engine disassembly. 

Section 6-4.12 of Continental Motors Publication M-0, “Standard Practice Maintenance Manual for Spark Ignited Engines,” covers crankcase inspections and allowable cracks. There is a provision for continued operation of certain engines with limited cracks in noncritical areas of the crankcase. However, the engine will continue to leak oil through the crack. 

I once found a leak in my engine by thoroughly cleaning the outside of the engine, then adding a small amount of fluorescent dye to the oil. I bought the dye and a black light at the local auto parts store. I waited for a dark night, then after a ground run, found the leak by shining a black light on the engine. I rebuilt the engine soon afterward. (Be aware of all regulations and the potential hazards before introducing a foreign substance into an aircraft’s engine or oil. —Ed.)

Oil screen and oil filter inspections

Always cut open the spin-on oil filter and inspect the filter media for contamination. I cut the paper media at the edges so I can unfold it for visual inspection. 

Engines that don’t have a spin-on filter will have a pressure screen. Remove it at every oil change and flush it.

If the filter media or screen reveals a quantity of metal that exceeds a quarter teaspoon, Lycoming mandates grounding the airplane until the cause can be found. Lycoming Service Bulletin 480F describes proper procedures for oil filter or screen inspections as well as corrective actions if the inspection shows contamination. 

Jacqueline Shipe’s article “I Found This in my Oil” (May 2017 issue of Piper Flyer) provides a pictorial guide to oil filter inspection. —Ed.

Black oil

If the engine oil turns black in the first 10 hours after an oil change, yet the compression readings are good, combustion gas byproducts are blowing past the pistons and piston rings into the bottom end of the engine. The oil will continue to lubricate, protect and cool the engine, but due to the contamination from combustion byproducts, it’s a good idea to shorten the oil change interval. 

Compression tests and borescope valve inspections

Never pull a cylinder based on one compression reading. Compression test results can vary from flight to flight. Always fly the airplane to bring temperatures up into normal operating range. If you have a low reading, go fly a bit, and then perform a second, and possibly a third compression test. 

Lycoming’s guidelines specify that each cylinder’s compression reading should be above 70/80, and within 5 psi of the engine’s other cylinders. When compression readings fall below 70/80, Lycoming says that’s the result of wear and should be further evaluated. 

There are very detailed instructions in Continental Publication M-0, Chapter 6-4.11.1 through 11.3 describing procedures and guidelines for compression tests. For instance, tests are only valid if a calibrated compression testing tool is used. The calibration procedure provides a low limit compression reading number for that specific testing tool.

Any cylinder with a compression reading above that limit is airworthy, provided a borescope internal inspection of the cylinder does not show cylinder wall scoring or extreme wear and the exhaust valve does not show any signs of burning. 

Many A&P technicians are not aware of the proper compression testing procedure for Continental engines. If your mechanic calls saying your compressions are too low, make sure he reads and understands the Continental procedures which are spelled out in detail in Chapter 6-4.11.2 of Continental Motors Publication M-0.

I strongly recommend that all airplane owners download this manual (it’s free) from the Continental website. There’s a wealth of general information that, in my opinion, is useful to all air-cooled airplane engine operators.

Now what?

Let’s assume that you’ve gotten some bad news from these tests. You’re facing an engine overhaul or replacement. What are your options?

There’s a choice of factory new, factory overhauled, factory rebuilt, repair station overhauled or field overhauled engines.

This is also an excellent time to research the STC data on the FAA website to find out if there are any engine upgrades such as installing a more powerful engine in place of the original engine. Some airframes may be eligible for engine upgrades via STC. An upgraded engine may be able to give you better performance and/or reliability. As an example, RAM Aircraft, LP offers a couple of popular STCs to upgrade Piper PA-28s. 

Finally, you may want to consider replacing your worn-out engine with a lower-time used engine. 

Factory engines

Obviously, buying a new “zero-time” engine from Lycoming or Continental will be the most expensive option. A factory rebuilt zero-time (exchange) engine is usually the next most expensive, followed by a factory “time since major” overhaul where the manufacturer overhauls your current engine. 

There are some very good reasons to deal directly with Lycoming and Continental. First, the price quoted is fixed, meaning there won’t be any unexpected price “modification” phone calls. 

Second, it’s broadly accepted that a factory zero-time engine will add value to any airplane. Remember that there are two flavors of factory zero-time engines. A brand-new factory engine is built from all new parts. A rebuilt engine is built with a combination of new parts and used parts which meet new limits. Both come with fresh, zero-time logbooks.

Third, and maybe the most important, is that you can continue to fly your airplane until the day your new engine is drop-shipped to your hangar or the nearest maintenance shop.

It’s a great advantage to have the removed engine and the new engine side-by-side during an engine change. This ensures that all the fittings are available and that routing questions can be answered without having to rely on memory or digital photos taken prior to engine removal. 

All Continental and Lycoming factory engines are sold with a core charge. The core charge for a Lycoming O-360-A1A engine is currently $16,400. If a buyer wants to keep the engine that’s been removed, or can sell it for a better price than the core charge, he/she is free to do that. However, the core charge must be paid if an engine is not returned to the factory.

The window to return the removed core engine is usually 90 days. 


Repair station or field overhaul

There are excellent non-manufacturer overhauls and not-so-good non-manufacturer overhauls. The excellent ones are built to new limits. The not-so-good are built to what’s called service limits. It’s legal for a shop to build an engine to the worn end of the manufacturer’s service limits guidelines. Of course, the engine won’t last as long as a “new limits” overhaul. When you’re gathering quotes from overhaul shops, make sure that you specify that you want your engine overhauled to new limits. 

Choosing a factory overhaul means your airplane will be down while your engine is removed, boxed for shipping, overhauled and shipped back. During a repair station overhaul or field overhaul of your engine your airplane will be down while the engine is disassembled, the parts inspected and certified, and the engine is reassembled and tested. Smaller repair stations and field overhaul shops typically must box and ship the ferrous parts and the engine case to a specialty shop for inspection and certification. 

There are 77 Type 1 (less than 400 hp) engine repair stations listed in the FAA’s repair station directory. Repair stations have submitted organizational plans and plans for parts accountability and quality assurance to the FAA.

What is included in an overhaul?

Factory engines typically come with a carburetor or fuel injection system, two magnetos and ignition harness, spark plugs, starter, oil cooler and engine-driven fuel pump. This is also the case with most non-factory overhaul options, but you’ll want to double-check to make sure these items are included.

It’s important to take notice of and budget for what’s not included. Time and money must be devoted to inspecting, purchasing, repairing and in some cases overhauling the turbocharger and wastegate (if installed), the exhaust system, the engine mount, the cooling baffles, the generator or alternator, hoses, engine mount and vibration isolators, propeller, prop governor, vacuum pump and fuel boost pump. 

Though you don’t necessarily have to replace or rebuild all of these items at the same time as the engine overhaul, it’s certainly more cost-effective to address them when the engine is already off the airplane. Access is easier, and you can minimize installation and removal hours. 

Most of the larger overhaul shops have worked out favorable pricing with over-the-road shipping companies but shipping costs must also be included during overhaul budget planning. 

It’s also critical to compare the warranties offered by each vendor as there is no industry standard for coverage. 

Can I overhaul my engine myself?

The FAA considers the overhaul of all except a very few engines to be minor repairs, not major repairs. This assumes that the person doing the work adheres to the procedures in the manufacturer’s engine overhaul and inspection manuals. 

You as the aircraft owner (or anyone else) may overhaul your engines, as long as a certificated A&P mechanic oversees the work and he/she is willing to sign off the overhaul. 

If you or your mechanic aren’t ready to do it yourself, there’s no reason a local machinist with years of engine building experience can’t build your engine. Again, this assumes the factory overhaul procedures are adhered to and an A&P is willing to supervise and sign off.

There are some caveats to this approach. 

• Your A&P must agree to this option, and must oversee it to the extent that he/she will sign it off.

• You (or the builder) must use aircraft quality parts.

• You (or the builder) must comply
with applicable engine manufacturer Service Bulletins.

• You (or the builder) must comply
with all applicable Airworthiness Directives (ADs).

• You (or the builder) must follow the machining processes outlined by the engine manufacturer. 

• You (or the builder) must follow the engine manufacturer’s break-in procedures.

If you’re not sure about the details involved in a light aircraft engine overhaul, there’s a 36-minute video on rebuilding a Lycoming engine on YouTube. (See Resources for the link. —Ed.)

Once you watch the video, it’s easy to see that these engines aren’t complex, nor are they difficult to overhaul. 

Aircraft owners have another option to enhance their knowledge prior to attempting an overhaul. Lycoming offers engine teardown and assembly classes throughout the year in Pennsylvania.


Used guaranteed engines

Another option to get your aircraft back in the air is to buy a used, serviceable engine from an aircraft salvage yard. This is not as radical an option as it may sound. All reputable salvage yards guarantee (warranty) their engines.

Ideally, you’re looking for a first run or first overhaul engine which is mid-time or less. For instance, as of the writing of this article, Wentworth Aircraft had an O-360-A3A with 217 hours since major overhaul for sale for $15,500. Though this engine wouldn’t do me any good (I have an -A1A, not an -A3A, and the two aren’t interchangeable), it does illustrate that there are cost-effective used engines available. The used route is dependent on finding the correct engine. 

An advantage of installing a used engine is the lack of core charge. The $15,500 cost mentioned above could be reduced by a few thousand dollars if you’re able to find a buyer for your core. Your worn-out engine may be just what another owner or kitplane builder is looking for.

Another source for used serviceable engines are engine upgrade specialists. Check the Piper Yellow Pages online or call PFA for more information about
Piper Flyer supporters. 

You can also often find good engines on the buy-and-sell pages of various online forums, via the For Sale/Wanted thread on the PiperFlyer.org forums or through the advertisers in this magazine. 


Not every engine showing trouble signs needs an immediate overhaul. However, if you and your mechanic have determined an overhaul or replacement is needed, there are several options. Take your time, do your research and you’ll be back up in the air soon. 

Steve Ells has been an A&P/IA for 44 years and is a commercial pilot with instrument and multi-engine ratings. Ells also loves utility and bush-style airplanes and operations. He’s a former tech rep and editor for Cessna Pilots Association and served as associate editor for AOPA Pilot until 2008. Ells is the owner of Ells Aviation (EllsAviation.com) and the proud owner of a 1960 Piper Comanche. He lives in Templeton, California with his wife Audrey. Send questions and comments to .


Further reading and research

Lycoming Service Instruction 1427C, Lycoming Service Bulletin 480F, and Contintental Motors’ publication M-0
under “Magazine Extras”


FAA STC data


FAA Repair Station directory


Factory engines/rebuilds/
factory overhauls – PFA supporters
Lycoming Engines

Continental Motors Group


FAA Repair Stations – PFA supporters
Airmark Overhaul, Inc.


Poplar Grove Airmotive


Other rebuild and overhaul resource
– PFA supporter

Progressive Air


Millennium cylinders, PMA parts
– PFA supporter
Superior Air Parts

Salvage yards – PFA supporters
Dodson International Parts, Inc.


Preferred Airparts, LLC


Wentworth Aircraft


Engine rebuild video
SkywardTech Inc.
Engine Management 101: Understanding Cylinder Baffling & Keeping your Engine Cool

Engine Management 101: Understanding Cylinder Baffling & Keeping your Engine Cool

The following is an excerpt from Bill Ross’ new book “Engine Management 101.” Published by Superior Air Parts, Inc., “Engine Management 101” is a compilation of what Ross has learned during his 35-plus years of experience as a pilot, aircraft owner, piston aircraft engine industry leader and A&P/IA. (To get a free copy of Ross’ book, see the sidebar on page 34. —Ed.)

A major topic of interest for any aircraft owner should be the proper maintenance of their engine’s baffling. I’ve witnessed too many instances where an aircraft owner or their mechanic has installed a $30,000 to $90,000 engine only to use the old deteriorated baffling. Why not spend a little bit more and do all you can to protect that big investment? 

In reality, I don’t think it is so much a question of economics but rather a lack of understanding about the functions and performance of an engine’s baffling.

With fuel system calibration being number-one (on fuel-injected Continental engines), neglecting baffling condition is probably the number-two most common maintenance issue I see under the cowling. The fact is, engine baffling that is left in a state of disrepair will likely result in reduced cylinder life and other operational performance issues. 

Demystifying baffling

Engine baffling serves a crucial role in cooling the cylinders evenly and keeping the entire engine within specified temperature parameters. Small imperfections or problems in the baffling can result in ill effects to the cylinder and oil temperature. 

Some of the detrimental effects can include accelerated wear of cylinder and valve train components, glazing of cylinders, and in some cases, reducing detonation margins to dangerous levels.

Over my many years as an A&P and technical representative, I have assisted in solving cylinder head temperature (CHT) issues many times. In the majority of cases, abnormal CHT is usually not the result of some manufacturing or material defect in the cylinder itself. 

In fact, as I cover in many parts of “Engine Management 101,” there are many things that can influence CHT, including fuel system calibration, engine timing, flight profile, climate and most of all, damaged or improperly installed baffling. 

I have witnessed many mechanics chase CHT issues by adding fuel on the injection adjustment well above the manufacturer’s recommendations. This is merely placing a bandage on the root of the problem!

Owners and mechanics can be quick to blame the cylinder’s manufacturer, but in my 30-plus years, I have found only a couple of instances where the cylinder was indeed the cause of the problem. 

When troubleshooting engine temperature problems, I always ask the owner or mechanic about the condition of the baffling. Without question, 100 percent of the time it is reported as “OK,” “good” or even “perfect.” As I probe further into the problem, more times than not, the condition of the baffling is, in fact, nowhere close to being OK. 

Bird: 1; baffling: 0

I remember one instance where the owner reported high CHT problems. We assisted in troubleshooting over the phone for several days and were told the baffling was perfect. Finally I traveled to his home airport and when I examined the airplane, I actually found the remnants of a bird’s nest in the baffling. The bird had long ago moved out. 

I guess birds don’t like CHTs above 400 degrees either! I just had to ask the mechanic if this was his idea of “good” or “perfect” baffling. He and the owner were ready to remove cylinders, change the fuel pump, etc.—anything but address the actual problem.

Anyway, we cleaned and resealed the baffling and that immediately brought CHT down to normal levels.

The owner was embarrassed and offered to pay all my expenses for the trip. I declined and advised him we would use this time to work with his mechanic. Today—not because of me—this mechanic is one of the best in the industry. I encouraged him to attend many of the recurrent training classes on engines and airframes. While experience is good, training is sometimes better.

Don’t be bumfuzzled by baffling

In order to appreciate the importance of engine baffling, I think we must understand its function and how it works. The accompanying illustration (above) shows how the airflow moves through most General Aviation aircraft cowlings. In the majority of installations, the cooling air flows down through the cylinders and out through the bottom of the cowling. 

This airflow is accomplished by creating a confined high-pressure area through the proper placement and installation of the baffling. The higher pressure on top of the engine allows for the air to flow directly to the low-pressure area on the bottom. 

The baffling constrains the air so it flows directly around the cylinders. The intercylinder baffling directs cooling air to the vital portions of the cylinder head, and in some cases helps to balance the cooling airflow to eliminate hot spots.

When properly installed, the baffling is designed to create a seal between the upper and lower portions of the engine. It is very important that the rubber pieces of the baffling conform to the engine cowling, allowing no air to escape over or around. 

This is why it is vital that all air gaps be sealed properly. The general rule of thumb for most installations is to direct as much cooling air as possible down through the cylinders. The residual heat is carried away through the bottom of the cowling. 

For those of you who fly aircraft equipped with cowl flaps, you are able to regulate the amount of air flowing through the cylinders simply by regulating pressure within the cowling.

Check your engine baffling carefully during preflight inspections and make sure your maintenance provider checks it closely during the next 100-hour or annual inspection. 

You do not have to remove the cowl and do a detailed inspection during preflight; just a quick glance in the cowling will let you know if the baffling is problematic or perhaps folded the wrong way. 

You can review with your mechanic how the baffling should look during your preflight inspections. Here are just a few items to look for:

• Cracks

• Incorrect fit

• Incorrect positioning

• Gaps not sealed properly

• Torn or cut rubber seals

During engine installation, make sure your engine baffling is either in pristine condition or is replaced. This small investment in new or repaired baffling will protect the larger investment you made under the cowling! 

For example, I recently overhauled the engine in my father’s Alon Aircoupe and that overhaul included new baffling. Now, thanks to the new powder-coated baffles and rubber seals, the trusty Continental C90 gets lots of cooling-air love. 

The attention given by you and your mechanic to ensure your aircraft’s engine baffling is installed and maintained in accordance with manufacturer’s instructions will go a long way in providing cylinder and overall engine longevity.

Bill Ross is a graduate from the University of South Alabama and was employed by Continental Motors for 15 years holding positions in engineering, analytical, air safety and technical product support. Bill is now Vice President of Product Support for Superior Air Parts and committed to the company goal of making flying affordable. When not working at Superior, Ross can be often found flying his family’s 1941 Boeing Stearman, working on antique aircraft or exposing young people to the joys of flight and potential careers in aviation. Send questions or comments to .

Dissecting a Dry Air Pump

Dissecting a Dry Air Pump

A look inside your aircraft’s vacuum system. 




When the earliest airplane gyroscopic instruments were introduced, the only available source for air pressure to spin them was an outside-mounted air venturi. The venturi accelerated the ram air pressure produced by forward flight through a narrowed opening. The instrument hoses were connected to the venturi at the point of lowest pressure, creating a vacuum that pulled a steady stream of air through the instruments. 

Although some VFR-only planes still use this arrangement, the trouble with this setup is that the amount of vacuum is low until certain airspeeds are reached, and the venturi can become ineffective due to ice buildup during inflight icing conditions. 

In the late 1930s, air pumps were developed that were engine-driven, creating air suction (or pressure) as soon as the engine was started. These early air pumps were lubricated with engine oil and would later be called “wet style” pumps. 

In the 1960s wet pumps were largely replaced with the “dry” pumps. Dry vacuum pumps are self-lubricating and have an oil-free exhaust flow that reduces belly deposits and provides a much cleaner source of air pressure on aircraft models that use the vacuum pump exhaust for inflating de-ice boots. The dry air pumps are also a little less expensive and weigh about half as much as the older wet style pumps.



How the pump works

The standard dry air vacuum pump consists of a rotating carbon vane assembly housed in an elliptically-shaped aluminum housing. The carbon vane assembly is powered by the engine accessory drive. The outer pump housing has an air inlet port on the front of the pump and an exhaust port on the rear. 

The rotor portion of the carbon vane assembly has slots that house the carbon vanes themselves. The vanes are free to slide inward and outward as the rotor spins. Centrifugal force keeps the vanes in contact with the inner wall of the pump housing. 

As the rotor spins, the vanes in the rotor slide in at the narrow section of the housing and slide outward to their maximum extension at the widest points of the elliptical housing they travel in. 

The intake air from the instrument system is routed through the pump fitting to ports in the forward section of the pump housing. The ports are open at the bottom and sides of the pump housing to allow air to flow in as the carbon vanes are beginning to move outward in the rotor slots. 

The air is then compressed as its compartment is compacted while the vanes rotate toward the narrow part of the housing. It is then accelerated out of exhaust ports located in the narrowest part of the ellipse. This all occurs through the first 180 degrees of rotation. 

As soon as the vanes move past the exhaust openings, they scoop in intake air from a second set of intake air openings—and the entire process is completed again in the second 180 degrees of rotation. 


How the vacuum system works

The airflow through a common single-engine aircraft vacuum system begins under the instrument panel. Air enters the system through a central pleated paper filter. The filter is located under the instrument panel. Ambient air is drawn into and through it solely due to the suction of the attached hoses going to the vacuum pump. It then flows through the attitude and heading indicators before reaching the system regulator. 

The system regulator combines additional air as needed to the intake of the pump so that the system suction stays within the parameters the regulator is adjusted to maintain. (The regulator has a slipover “sock” style filter to protect the pump from any particles that might be drawn in with the ambient air.) Airflow continues through the pump and then is exhausted into the engine compartment on most models. Aircraft with de-ice boots utilize the pump’s exhaust air to inflate the boots. 

The artificial horizon and directional gyro flight instruments are usually connected to the vacuum system in parallel with each having its own connections to the intake and vacuum air so that even if one instrument were to fail or become clogged, the other one still functions because it has its own connection to the air source. 

A suction gauge is connected in the system so that it measures the air pressure difference between the supply line from the central paper intake filter and the outlet of one of the instruments prior to reaching the system regulator. The pressure drop from the intake air (which is close to atmospheric pressure on nonpressurized planes) and the air being drawn into the regulator is measured in inches of suction. 

Most vacuum systems are designed to operate with around five inches of suction with the engine rpm at or near a cruise setting. If the system suction is too high, it can cause excessive wear in the gyros and the vacuum pump. If the vacuum system suction is too low, the instruments will not give reliable indications. 

Twin-engine aircraft with two vacuum pumps also utilize various check valves so that a failure of either pump doesn’t cause the system to lose vacuum. 




Vacuum pump failure

Vacuum pumps are built to run for several hundred hours—but one of the biggest downfalls of dry air vacuum pumps is that when they do fail, it is usually suddenly and without warning. 

The carbon vanes, by design, will wear down over time as the pump operates. Eventually, the vanes can become so short that they will either hang up, or come completely out of their slots as they rotate through the wide part of the ellipse and cause the sudden stoppage of the rotor assembly in the pump. 

Also, the inner wall of the aluminum housing is prone to developing indentions and slight deformities as the vanes slide in and out against it. These indentions can cause one or more of the vanes to hang up and break apart. 

Some vacuum pump manufacturers have incorporated a wear indicator port on the side (or on some models, the rear) of the pump. The ports allow access to check the length of the vanes, which can help catch an impending failure. (The pumps are designed with a nylon drive coupling that shears in two if the pump does lock up, so that the gears in the engine accessory case are not damaged when a pump fails.) 

Contamination within the vacuum pump can also cause a sudden failure. The hoses used in the vacuum system can become dried and brittle over time. If internal pieces of hose begin to flake off, or if any contaminants get into the vacuum system downstream of the central paper filter, they go straight through the pump. The small sock filter on the regulator only filters the ambient air that is added to the flow as the pressure is regulated—not the already-filtered air from the instruments. 

Some mechanics use Teflon tape or some type of sealing compound on the pipe-threaded instrument and pump fittings in the vacuum system. Teflon tape and other sealants are not recommended for use at all in the instrument system, because pieces of the tape or sealant can make their way into the system as the fittings are threaded into place and these may eventually get sucked into the pump. 

The filters themselves can become sources of contamination over time if they aren’t regularly replaced. The sock filter in particular can become so dried-out and brittle that pieces of it may be ingested into the airflow. Most vacuum pump manufacturers require replacement of all filters at the time of installation in order for the pump warranty to be valid. 

Solvents used to wash down the engine during maintenance are very damaging to vacuum pumps. If any of the solvent material gets into the pump, it causes the graphite powder—which is always present from normal wear—to turn into a paste that gums up the inside of the pump. Pump manufacturers recommend completely covering the pump with a resealable plastic bag and tie wraps before washing down the engine.

Oil contamination is also a big culprit in premature vacuum pump failures. One of the biggest sources of oil contamination typically comes from a leaking oil seal on the engine accessory case adapter drive. The adapter drive gear in the accessory case is made with a splined hollow shaft that spins the vacuum pump drive coupling. 

There is an oil seal that the vacuum pump drive gear is inserted through. It naturally wears out over time because the shaft is spinning inside of it. Once the oil seal begins to degrade, it allows oil—under pressure—to head straight for the pump drive coupling and into the pump itself. This excess oil causes a gummy paste to form that eventually binds the pump. 

Kinked fittings or hoses can cause excess wear on a pump by forcing it to work harder than it should to maintain vacuum suction. (If suction levels begin to degrade, lots of mechanics simply increase the suction by adjusting the regulator—instead of determining the exact cause of the suction loss. If a system starts to become sluggish, the root cause should be determined before simply cranking up the regulator.) 


Vacuum pump replacement

Vacuum pumps are typically straightforward to replace.

The hoses and fittings on the old pump should be removed before the mounting nuts are removed, so the pump is held tightly in place as the hoses are pulled off the fittings. 

Most vacuum pumps are mounted on four studs and secured with plain nuts and lock washers. Typically one or more of the nuts are difficult to access with a normal type of wrench, and vacuum pump manufacturers make a specially-curved wrench that helps gain a little access. 

The old mechanic’s trick for breaking loose the nuts that are in a tight place involves using a long flat blade screwdriver placed on the loosening side of the nut. The screwdriver is then gently tapped with a small hammer to break the nut loose. 

A new oil seal should be installed anytime a new vacuum pump is installed—whether the old oil seal is leaking or not. The seals wear out over time and require periodic replacement. They are also reasonably priced (around three dollars each), so cost is not a consideration. 

If an owner is having a shop replace the pump, it is best to specifically request that the oil seal be replaced in addition to the pump, because some mechanics don’t replace them unless they are leaking. 

There are two gaskets that require replacement: the one between the vacuum pump and its drive housing, and the one between the drive housing and the engine accessory case. 

Be sure to check the aircraft maintenance manual to be sure the new vacuum pump being installed is the correct model number. Pumps rotate either counter-clockwise or clockwise as viewed from the rear of the pump and case. (The rotation is specified as “CC” or “CW” in the part number.) Putting the wrong pump on will cause it to spin opposite the direction the rotor slots and vanes are designed for, and the pump will fail in short order—if not immediately. 

The vacuum system hoses and system regulator that are just upstream from the vacuum pump must be checked for contamination whenever a failed pump is being replaced. Pieces of the old pump vanes often get sucked backward into the suction hose—against the normal direction of airflow—as the pump fails because the system still retains a lower pressure for a few seconds even though the pump has stopped. 

If the hose isn’t cleaned or replaced after a sudden pump failure, carbon and vane parts will be sucked into the new pump upon startup.

If compressed air is used to blow out the lines, be sure all the instruments are disconnected so they don’t get blasted with excessive pressure or contaminated by unfiltered particles. Also, as the manufacturers specify, all of the vacuum system filters should be replaced at each pump replacement.

Owners that fly a lot of hard IFR might consider periodically replacing a vacuum pump based on time in use alone, even if that pump is operating properly. 

Many new aircraft are shifting toward a glass panel configuration, but the benefit of the vacuum pump system is that it will still power the pneumatic instruments even in the event of a total electrical failure. A little preventive maintenance and upkeep on the vacuum system can help owners be assured that the indications on the gauges can be trusted in the clouds.

Know your FAR/AIM and check with your mechanic before starting any work. Always get instruction from an A&P prior to attempting preventive maintenance tasks.




Jacqueline Shipe grew up in an aviation home; her dad was a flight instructor. She soloed at age 16 and went on to get her CFII and ATP certificate. Shipe also attended Kentucky Tech and obtained an airframe and powerplant license. She has worked as a mechanic for the airlines and on a variety of General Aviation planes. She’s also logged over 5,000 hours of flight instruction time. Send question or
comments to .

The Right Mix: An Aircraft Carburetor Overview

The Right Mix: An Aircraft Carburetor Overview


Many Piper aircraft depend on a carburetor. Piper Flyer contributing editor and A&P Jacqueline Shipe explains the operation of this fairly simple—and very reliable—invention.

One of the most recognized carburetor manufacturers for the GA fleet is Marvel-Schebler. The company has been around a long time, having its beginnings in the early 1900s when George Schebler and his friend Burt Pierce worked together to design the first carburetor using a tin can with a flap to regulate airflow. 

They both went on to patent their designs, with Pierce calling his carburetor the “Marvel.” Both the Marvel and the Schebler designs were successful and used on a variety of engine types. 

In the early days of General Motors, the two merged and became known as Marvel-Schebler Carburetor Co. (Author’s note: Burt Pierce also designed the still-popular Marvel Mystery Oil through Marvel Oil Co., which he founded in 1923.) In the beginning, the Marvel-Schebler Carburetor Co. made carburetors for cars, boats, tractors and airplanes. 

The company has since changed hands several times, being purchased and resold by Facet Aerospace Products, Zenith Fuel Systems, Precision Airmotive and the Tempest Group (who called it Volare Carburetors until it acquired the Marvel-Schebler trademark in 2010). Today, Marvel-Schebler Aircraft Carburetors LLC produces a complete line of aviation carburetors and parts.

Although Marvel-Schebler is the most recognized brand for aviation carburetors, there are other FAA approved manufacturers, including AVStar Fuel Systems in Florida. 

AVStar was formed in 2007 and has gone on to become the supplier for Lycoming Engines as well as numerous individual customers. AVStar manufactures a line of carburetors as well as kits and parts for use in almost all carburetor models in the General Aviation fleet.




How a carburetor functions

Aircraft engines rely on a steady source of fuel to provide the energy needed to support combustion. Liquid fuel must be vaporized and mixed with the proper amount of air in order to burn properly in the cylinders. 

Many General Aviation planes depend on a carburetor to provide a continuous, reliable source of properly mixed fuel and air to each cylinder. The aircraft carburetor has a relatively simple design and is typically very reliable.

Most aircraft carburetors are fairly straightforward in construction. A top part, called a throttle body, houses the throttle valve, mixture control and venturi; a lower bowl section, called a reservoir, holds a consistent volume of fuel. 

Almost all aviation carburetors are float-style carburetors. This means that a float mechanism regulates the fuel level in the reservoir (i.e., bowl). 


The float mechanism

The float is hinged on the rear, allowing it to pivot up and down. A pencil tip-shaped float valve is attached to the top rear of the float. 

Fuel enters the carburetor through the inlet screen, flows down through the float valve and its seat, and into the carburetor bowl. As the fuel level rises, the float and the attached float valve also rise until the float valve is implanted in the seat, shutting off the fuel flow. 

As the fuel level in the bowl drops, the float and float valve also descend, allowing fuel to once again flow into the bowl.

The float travel from full-up to full-down is relatively short; it is stopped on the descent by a tab on the rear hinge. The level to which it rises up is stopped by the attached float valve and seat. 


Adjusting fuel level

It is important to maintain a correct fuel level in the bowl. If the fuel level is too low, the engine will run too lean; if it is too high, the engine will run rich and fuel may leak continuously from the discharge nozzle. 

The fuel level is adjustable by adding or removing washers under the float valve seat to extend or lower it, or by bending a tab on the float itself at the point of contact with the float valve to extend or lower the valve.





Airflow through the carburetor throat begins at the aircraft air filter and proceeds through the airbox into the throat of the carburetor. 

A venturi in the carburetor throat narrows the airflow opening, increasing the speed of the air, thereby lowering its pressure. (This is based on Bernoulli’s principle of airspeed and pressure being inversely proportionate; the same principle explains how an airfoil generates lift.) The outlet for the fuel discharge nozzle from the bowl is placed in the center of this low-pressure area. 

The air chamber on top of the fuel in the carburetor bowl is vented to atmospheric pressure. The pressure difference from the atmospheric pressure on top of the fuel in the bowl versus the low pressure on the fuel discharge nozzle causes fuel to flow out the fuel discharge nozzle. 

A throttle valve (i.e., a butterfly valve) located just downstream of the venturi controls mass airflow through the carburetor throat. As airflow increases, the suction effect on the fuel discharge nozzle also increases proportionately, allowing more fuel to flow. 


Fuel flow

Before fuel flows from the bowl out the fuel discharge nozzle, it is routed through the mixture control valve. The mixture control valve is attached to the mixture control arm. 

The mixture control valve on most models contains a shaft (also called a stem). The bottom of this shaft is shaped like a half-cylinder. It rotates in a cylindrically-shaped sleeve with an opening on the side. 

When the mixture is set at full rich, the open part of the shaft/stem is aligned with the opening in the sleeve, allowing full fuel flow through the valve and out of the nozzle. As the mixture control is pulled back to leaner settings, the opening becomes more and more narrow until it is completely closed at cutoff.

When the mixture control valve is open, fuel flows from the mixture sleeve through the main metering jet (this is a fixed orifice that controls the maximum amount of fuel allowed to exit the main discharge nozzle once the mixture control is set to full rich) and into the discharge nozzle well, where it begins to be mixed with air from bleed holes in the nozzle. From there, it flows up and out the main discharge nozzle and into the intake pipes for the cylinders. 

At low throttle settings with the throttle valve nearly closed, there is not enough suction on the main discharge nozzle to cause fuel to flow out of it, but there is a slight amount of airflow between the edge of the throttle valve and the wall of the throttle body. 

This small area of airflow around the edges of the throttle valve acts as a venturi, forcing airflow to speed up as it passes between the edges of the throttle valve and the carburetor throat and lowering the air pressure. 

In order to provide adequate fuel for idling, small openings are made in the throttle body in this area of low pressure. Ports connect the openings with the inner section of the main fuel nozzle and draw fuel from the nozzle at low throttle settings. This arrangement provides an adequate fuel supply for idle speeds. 



Idle adjustment

The idle speed and mixture are adjustable, and are the only two adjustments that can be made on most carburetors. Most planes should idle at speeds of 600 to 650 rpm. The idle speed adjustment is simply a stop screw that limits the rear travel of the throttle arm. (It screws in to increase idle speed; moving the screw counterclockwise decreases idle speed.)

The idle mixture adjustment is a large screw on the top rear of the carburetor that screws a needle closer to or further from its seat, which allows more or less fuel to flow through the idle passageways. 

The idle mixture is made leaner as the screw is turned in and richer as it is backed out. It should be adjusted so that there is a 25 to 50 rpm rise in engine speed when the mixture control is pulled all the way back to shut down the engine. 

If there is no rise when the mixture is pulled back to cutoff, the idle mixture is too lean. If there is a rise of more than 50 rpm, it is too rich. 

There have been instances where the idle mixture screw has vibrated loose and fallen out. If this happens, the engine won’t idle at all, but will try to shut down when the throttle is reduced to idle settings.

Basic maintenance and troubleshooting

Aircraft carburetors are generally reliable and seldom require much attention. The internal parts of a carburetor rarely need maintenance if the airplane is flown regularly and clean gas is used. 

An inlet screen that the fuel supply line attaches to can be removed for cleaning. Generally it stays pretty clean, because most debris gets caught in the aircraft fuel strainer before it has a chance to enter the carburetor. 

Over time, the throttle shaft bushings wear, especially on training aircraft that endure several power changes and throttle movements every hour. Worn bushings can allow a slight intake leak and cause an overly lean mixture. 

Most carburetors have an accelerator pump that squirts a stream of extra fuel into the intake air as the throttle is advanced so the sudden burst of extra intake air doesn’t create a lean condition and cause the engine to stumble, especially if the throttle is opened suddenly. The accelerator pump has a plunger that gets worn with use and periodically requires replacement.

Any leaks coming from a carburetor are cause for concern. A carburetor that leaks when sitting with the engine off most likely just has a tiny bit of debris trapped between the float valve and seat. Draining the fuel from the carburetor bowl and then flushing it by allowing it to refill and draining it again will most likely clear it up. 


Long-term storage of an aircraft

A carburetor on a plane that has sat with the aircraft fuel shut off may not allow fuel to enter the bowl when the fuel is turned on due to a stuck float valve. Gently tapping the side of the bowl with a small rubber mallet sometimes jars it loose and allows fuel to re-enter the bowl. 

If a stuck valve is suspected, momentarily crack open the supply line with the fuel turned on to be sure gas is getting to the carburetor, then re-tighten. Next, slowly remove the drain plug to see if there is fuel in the bowl. An empty bowl indicates a stuck valve or an obstruction in the inlet.

For folks that have an auto gas STC, it is best to never leave a plane with auto fuel sitting in the tanks, lines or carburetor for extended periods. Auto fuel causes deposits of varnish to form on the inner surfaces of the fuel system and often seizes the mixture control valve in place. 

If a plane is left sitting for a season, it will be far better for it to sit with Avgas in it. (Better yet, you may wish to “pickle” the aircraft. For more information, take a look at Steve Ells’ 2015 article “Flying, Interrupted: Modern Engine Preservation” in the archives at PiperFlyer.org.) 

Aviation carburetors are some of the most reliable inventions ever made. Their simple design and quality construction offer years of trouble-free service as long as they are flown regularly and proper steps are taken to ensure a clean fuel supply.

Know your FAR/AIM and check with your mechanic before starting any work. Always get instruction from an A&P prior to attempting any aircraft maintenance tasks.

Jacqueline Shipe grew up in an aviation home; her dad was a flight instructor. She soloed at age 16 and went on to get her CFII and ATP certificate. Shipe also attended Kentucky Tech and obtained an airframe and powerplant license. She has worked as a mechanic for the airlines and on a variety of General Aviation planes. She’s also logged over 5,000 hours of flight instruction time. Send question or comments to .


Avstar Fuel Systems Inc.
Marvel-Schebler Aircraft Carburetors, LLC
Accumulating Knowledge: De-Ice Boots

Accumulating Knowledge: De-Ice Boots

A brief history of pneumatic boots, their operation and proper care.

AS Jimmy Doolittle was demonstrating the technique of blind flying in 1929, work was being done by B.F. Goodrich and the National Advisory Committee for Aeronautics (NACA) to address airframe icing. 

William C. Geer, Ph.D., a retired chemist from the B.F. Goodrich Company, became interested in the problem of airframe icing when it caused a number of crashes of airmail planes. With IMC flight in its infancy at the time, airframe icing was seen as a barrier to progress.

In the early 1930s, work by Geer and B.F. Goodrich focused on rubber coatings to inhibit the development of ice. How to get rid of the ice that did accumulate, despite the rubber boot and the concoction that was smeared on them to prevent the buildup, led to the idea of having inflatable tubes to knock off the ice. The de-ice boot was born.


Structure, activation and various types

The boots themselves are generally constructed with five or more spanwise tubes. These are created by layers of rubber laid up in such a way as to create the channels which expand when system pressure is applied to them. The inflation pressure is typically around 18 psi.

Once the concept of inflating tubes in the boots was proven to be effective, complete de-ice systems were soon developed. In addition to the boots themselves, a timer, valves and a pressure source were necessary. 

The researchers also discovered that in normal, non-icing flight, the aerodynamic pressure differentials around the wing could allow the boots to expand somewhat without activation of the system. The solution was to apply a small amount of suction to the boots during the times when the boots were not being operated by the pilot.

Modern systems use a pressure pump driven by the engine(s) which in normal operation discharges through a venturi, which supplies the suction to the boots to keep them tight to the airfoils when the boots are not in use. 

When activated, electrically controlled valves switch the pressure from the venturi to inflate some or all of the boots. The timer will keep the boots inflated for several seconds before switching back to the suction mode.

Many common installations will inflate all the boots simultaneously. One example is the Piper Aztec. 

With the certification requirements for approval for Flight Into Known Icing (FIKI)—which applies only with aircraft certified after 1973 or if the manufacturers chose to obtain FIKI certification—additional boots were added inboard of the engines, and boots were added to cover all tail surfaces. 

With these expanded systems, it is common for the timing system to inflate the wing boots and the tail boots separately. An example is the later Piper Navajo series after FIKI certification. This is likely due to the requirement that the de-ice system still function after the failure of one of the pneumatic pumps.

De-icing systems are available for aftermarket installation. As each aircraft will utilize differently sized boots, and testing is required to make sure that any installation does not hinder the flight characteristics of the aircraft, most aftermarket installations are by STC. B/E Aerospace and Goodrich hold many of these STCs, but not all.

Proper operation 

There is a certain amount of controversy over the proper operation of pneumatic de-ice boots. The common wisdom—which has come down from the early days of icing flight—is to wait until one-quarter to one-half of an inch of ice has accumulated before popping the boots. The concept has been enshrined in numerous aircraft Approved Flight Manuals and POHs. 

The NTSB has been at war against this mindset for a couple of decades. Based on its research, the NTSB, and to some degree the FAA, have been advocating turning the pneumatic wing de-icing system on at the first sign of icing.

Older-style boots (those dating before the 1960s), may have been prone to a condition called “ice bridging.” This is where ice would build to the point that it formed a bridge over the top of the boots. (Ernie Gann reported on this phenomenon on a DC-2 in his semi-autobiographical book “Fate is the Hunter.”) 

However, the NTSB is adamant that modern boots, (i.e., any that have been installed in the last 50 years or so) will not form an ice bridge. 

Against this, northern pilots do occasionally claim to have seen ice bridging occur in newer aircraft. 

After half a dozen years flying in the ice of the Great Lakes and in Southeast Alaska, I can say that I have never seen ice bridging—but that does not mean I am completely sold on the NTSB’s recommendation to activate the boots at the first sign of ice on the wings.

The other consideration is whether the boots leave less residual ice if they are operated continuously, or if they are cycled after a small buildup. 

I believe that this issue is a bit more nuanced than the NTSB is willing to recognize. When to operate the boots encompasses numerous factors, in my experience. 

It is a fact that the ice sheds better at higher speeds as the force of the airflow on the ice increases exponentially with airspeed. It is also beyond argument that some airfoils are much more sensitive to an accretion of ice than are others. Outside air temperature, the type of ice, and the condition of the boots all affect the ability of the boots to shed the ice. 

The NTSB guidelines seem to assume that all wing de-ice systems can be turned on and that they will then cycle at intervals. This is not the case on most aircraft where you have to individually activate each cycle. When flying single-pilot IFR, I rarely have time to sit and punch the wing de-ice button every few seconds. 

I tend to wait until I have a noticeable accumulation of ice before popping the boots. I don’t wait for one-quarter of an inch, and certainly not a full one-half inch; I generally try to activate them during a high speed descent, and again if I get into air that is at freezing or above. I also try to make a final activation after breaking out of the ice, or on final approach if I haven’t gotten out of the ice. (This is the author’s personal procedure in her own aircraft, and is for readers’ information only. Piper Flyer urges all pilots to read NTSB and FAA recommendations. —Ed.) 

Care for pneumatic boots

A set of de-ice boots is very expensive, so they should be cleaned and protection should be applied. 

There are three manufacturers of boot cleaning and sealant products. Goodrich Corp. makes ShineMaster for cleaning and AgeMaster for protecting boots. Jet Stream Aviation Products makes Pbs Boot Prep and Pbs Boot Sealant. Real Clean Aviation Products makes a similar de-ice boot care system called Real Shine. 

In my experience there is one product that needs to be on your shelf, and that is B.F.G’s Icex II. Even after cleaning and sealing, the application of a slippery coating before charging off in the ice is a very good thing. It makes a huge difference in the ability to shed ice. Icex II is expensive, but you won’t have regrets about the cost when the ice is being shed off the wings cleanly. Besides, a quart can last a couple of years for most pilots.

Kristin Winter has been an airport rat for almost four decades. She holds an ATP-SE/ME rating and is a CFIAIM, AGI, IGI. In addition, Winter is an A&P/IA. She has over 8,000 hours, of which about 1,000 are in the Twin Comanche and another 1,000 in the Navajo series. She owns and operates a 1969 C model Twinkie affectionately known as Maggie. She uses Maggie in furtherance of her aviation legal and consulting practice; she also assists would-be Comanche, Twin Comanche, and other Piper owners with training and pre-purchase consulting. Send questions or comments to .


De-icing equipment
– PFA supporters

B/E Aerospace, Inc.
Goodrich Deicer Service Center

De-ice boot care
– PFA supporter

ShineMaster, AgeMaster, Icex II

Goodrich Corp.
(UTC Aerospace Systems)

Other de-ice boot treatments and protectants

Jet Stream Aviation Products, Inc.
Real Clean Aircraft Detailing Products

Further reading

AMT Airframe Handbook

Volume 2, Chapter 15:
Ice and Rain Protection. 

U.S. Department of Transportation Federal Aviation Administration Flight Standards Service, 2012.

Available at PiperFlyer.org/forum under “Magazine Extras”

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