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Maintenance & Technical (165)

Proper Entry Procedure: Fitting & Adjusting the Piper PA-28 Entry Door

Proper Entry Procedure: Fitting & Adjusting the Piper PA-28 Entry Door

Before you can properly seal the door, you must ensure it is fitting properly. Here is a step-by-step guide for removing, checking and adjusting the door.

The only way to properly adjust a door on a Piper PA-28 series aircraft is with the door seal removed. Only then will you know if the door is fitting properly. The following procedures should be accomplished before installing a new seal, and they can only be done by or under the supervision of an A&P mechanic.

Remove the door

Remove the screw, step bushing and washer attaching the doorstop to the doorsill plate. Remove the cotter pins, clevis pins and washers from door hinges. Set the door aside on a blanket or other protective covering.

Remove the old seal

There is no easy way to remove the old seal and adhesive, but I’ve found that using an electric heat gun (such as those used for paint removal) aids this process considerably. First, locate the seal joint and with the heat gun apply heat to the seal and carefully begin to lift the seal from the edge of the door. Continue applying heat in the apex of the seal as you lift it from the edge of the door until you have it completely separated from the door.

Remove the door seal adhesive

Removing old adhesive can be performed using one of two methods. One way is to use a small (three-inch) brass brush on a drill motor and literally peel the adhesive off the door. This method does require that the door edge be repainted. (An aerosol such as Krylon paint may be used to repaint the edge of the door, and it stands up well over the years.)

The second method is to dissolve the adhesive with a product called Goof Off. Goof Off, touted as “The Miracle Remover,” will not affect paint or Plexiglas.

I’ve found that applying heat to the old adhesive and then wiping the area using a rag saturated with Goof Off will remove the residue. Use caution to prevent combustion. Ensure any Goof Off liquid remaining on the door has fully dried before reapplying heat from the heat gun.

When the door edge is cleaned up, you are ready proceed with the adjustment.

Check for wear in the hinge

Over the years, the eyebolts and clevis pins (door hinge system) can become worn to a point where the door will sag and not close or seal properly. It’s imperative that these parts be checked for wear before proceeding.


A good way to check the parts for wear is to slightly open the door and see if you can raise up on the door. There should be no movement (or very little movement) of the door vertically.

If you can raise up on the door, the eyebolts and clevis pins are worn out and should be replaced. (The tolerance when new is only three thousands of an inch.)

These items are not expensive, yet they are critical to properly closing and sealing the door. Aircraft Door Seals sells eyebolts and clevis pin sets. The new eyebolts and clevis pins come with complete instructions for installation and can be replaced in less than five minutes.


Reinstall the door

Place the door into position over the eyebolts and install the washers and clevis pins in the door hinges. (Do not reconnect doorstop to the doorsill on the fuselage at this time.) Close the door and secure the upper latch.

With the door closed and latched, verify the front edge of the door is flush with the fuselage. Many times the door will not be flush; instead, it will actually be fitting inside of the fuselage anywhere from 1/8 inch to 3/16 inch. It must be flush with the fuselage before you proceed.

If you find the door is not fitting flush, this may be corrected by the installation of spacers (washers) under each eyebolt (or as required) which will move the upper or lower portion of the door and enhance the door’s fit. The washers you’ll need are AN960-516 (thick) and AN960-516L (thin)—typically, just one or two under each eyebolt will correct the fit.

To remove the eyebolt, you must remove the door. Just inside the cabin in front of the door opening (behind the interior trim), you will find a 5/16-24 nut for the upper and lower eyebolt. Slide a half-inch box wrench behind the upholstery, placing it over the nut.

Using a crescent wrench on the eyebolt, unscrew the eyebolt (counterclockwise) and remove it. It is helpful to have an assistant place the washers on the eyebolts so you do not have to move the wrench and nut. Install one or more washers as required on the eyebolt(s) and reinstall. Do not over-tighten—just snug is sufficient.

Reinstall the door and verify the front edge of the door fits flush with the fuselage. If not, repeat this procedure using thick and/or thin spacers until it does fit flush.

Note: Many times the factory installation leaves a little to be desired. With the door fully closed, inspect the clearance between the edge of the door and outer periphery of the fuselage door opening. Many times I have found the edge of the door skin actually hitting the fuselage, especially at the front edge. You should have a minimum of 1/16 inch clearance. If not, file the edge of the door until it has the proper clearance.

Adjust the door

If the door does not fit flush with the fuselage around the entire opening, start with the adjustment of the main latch by loosening the two flat head screws and move the striker plate (in or out) as required. Re-tighten the two screws. Repeat this as necessary until the door fits flush. The door should have a 1/16 inch to 1/8 inch clearance around the entire edge of the door and fuselage.


On early Piper models (pre-1968) I’ve found the latch clevis pin to be bent, which will prevent the door from latching properly. If it is bent, it must be replaced. Aircraft Door Seals stocks this clevis pin.

To provide the proper vertical adjustment of the door, insert the necessary washer combinations between the cabin door hinge(s), clevis pins and the fuselage eyebolts. Also verify that the fittings riveted to the door have not been bent. The fittings forming the portion that fits over the eyebolts should be straight.

Adjust the upper door safety latch

To adjust the door upper (hook) latch, remove the two screws from the latch plate on the top of the fuselage door opening. Remove the plate and rotate the loop clockwise or counterclockwise (a small amount of WD-40 on the threads will help) to make necessary adjustments.


Replace the latch plate and secure with the two attachment screws. Check the fit of the door.

Many times the upper latch hook can become bent and actually hit the upper portion of the door opening (fuselage). The upper hook should be centered in the upper opening. If not, using vise grips, clamp the hook at the point where there is a slight bend in the hook and slightly bend the hook until it is centered in the opening. Caution: When bending the hook, support the hook with your thumb in the area where you are bending. This will prevent the latch from being damaged.


Check the fit and make final adjustments

When the door is properly adjusted, there should be approximately a 1/16 to 1/8 inch gap around the outer periphery of the door between the door edge and the fuselage.

Insert the cotter key(s) in the clevis pins and bend the cotter key ends around the clevis pins and trim off the excess cotter key length as required.

It is not uncommon for the forward top edge of the door to not fit totally flush with the top edge of the fuselage. This condition is due to the variables in the assembly process of the door. Many times I have found it necessary to adjust the fit of this portion of the door by slightly bending the door upper edge.

This procedure will not damage the door and has been done by the factory for years, but it must not be done with the door installed. It is best done with the door lying flat on a blanket and manually massaging the upper portion of the door with your knee until you are satisfied with the fit.

The entry door has been cleaned up, fitted and adjusted, but you’re not done yet. Follow the manufacturer’s instructions to the letter for successful installation of the new door seal.

Dick Russ is a multi-thousand-hour commercial, multi-engine and instrument-rated pilot. He’s also a flight test engineer and an A&P/IA who has restored many Pipers. In addition to his career as a freelance writer and aviation business owner, he was senior engineer on the Shuttle Enterprise Approach and Landing Test Program at Edwards AFB. Russ holds three patents on aviation components. Send questions or comments to . 


Aircraft Door Seals
The Straight Dope on Fabric-Covered Airplanes

The Straight Dope on Fabric-Covered Airplanes

Fabric-covered planes in good condition are available, but you need to know what to look for.

Aircraft have been covered in cloth since the Wright Brothers took flight, and the material had to be as light as possible yet strong enough to withstand the demands of flight. 

The standard material used in the early days was cotton or linen. Vintage aircraft typically had wood wings and steel tubing used in the fuselage. 

The materials

The use of cotton or linen cloth is still approved; however, it is rarely used today because synthetic materials and improved processes are available. 

Synthetic materials and associated application processes not only reduce the amount of labor required, but also provide longer life, resistance to rot and fungus, and are safer in the case of fire (during material application, and while in flight). 

Polyester cloth specific to aviation applications is almost exclusively used in the recovering (or initial covering) of an aircraft today. Fiberglass cloth has been used as well, and other synthetic materials have been experimented with and/or are in development. 

The most important difference between newer synthetic materials and the original cotton and linen cloth is the fact that cotton is more difficult to work with. In addition, cotton is subject to attacks by fungus, mildew, chemicals (such as acid rain) and is susceptible to damage from rodents and sunlight. 

While synthetic fabric is deteriorated by sunlight too, it has better resistance to the effects of ultraviolet light. Synthetic fabric is also resistant to fungus attack, and while it can be damaged by chemicals, it is more resistant to damage than cotton. 

Cotton and the compatible nitrocellulose dope used to stiffen the fabric in the recovering process are flammable. Nitrate-based dope is extremely flammable even after it dries, and is seldom used today.

Synthetic fabrics sometimes call for cellulose acetate butyrate dope according to the STC, but oftentimes a material that is less flammable and more suitable to the synthetic fabric process is used. 

A significant factor regarding polyester cloth is that the tautness of the fabric is controlled by heating the fabric with a temperature-regulated device similar to a clothes iron. Application of dope or sealant materials will not appreciably shrink polyester, as is the case with cotton fabric. 

Aviation-specific synthetic fabric can be much stronger than cotton fabric. This is a key issue in the pull testing (strength) of the raw fabric to determine continued airworthiness years after the initial fabric application process has been completed. Fabric is considered airworthy until the strength degrades to less than 70 percent of the original design strength. 

The FAA testing specification has always been in reference to the original material the aircraft was designed and certified with. Aircraft produced under the CAR 3 rules were approved with cotton or linen cloth of different grades depending on wing loading and maximum airspeed limitation. For example, aircraft could be certified with grade A cotton, intermediate cloth or glider cloth, depending on the never exceed speeds and wing loading, and then could be later recovered with a fabric of a higher rating. 

The process

Working with cotton or linen requires special techniques and processes for a good-looking and airworthy cover job. When recovering an aircraft, the structure has to be carefully inspected and all defects repaired; then it can be primed and protected prior applying the fabric. 

The fabric has to be cut and sewn to the shape of the wing or fuselage and cemented or tacked into position. After the fabric is installed and secured to the frame, it’s permanently attached to the wing ribs with a special lacing cord using a designated knot. 


The spacing of the rib stitches varies according to the VNE (never exceed) speed of the aircraft and if the area is in the propeller slipstream or not. Some aircraft use screws or fabric clips in place of the rib stitching. 

After the rib stitching, the next procedure is the application of cloth tape to cover the stitching and the installation of inspection rings, grommets and patches in various locations to protect the underlying fabric. 


A plasticized liquid lacquer (i.e., dope) is applied to the fabric in several applications initially by brush and then by spray gun to form an airtight and waterproof bond that also tightens and stiffens the fabric materials. 

The proper fit of cotton or linen fabric prior to doping is important, as extremely taut fabric caused by multiple applications of dope will shrink and distort or damage the underlying structure requiring removal, repairs and reapplication of the fabric. 

Proper health precautions must be followed when applying doping agents, especially when applying urethane in a spray form as it is extremely toxic. 

Multiple applications of various mixtures of dope are applied generally by spray gun. Mixtures may include dope with silver metallic compounds for resistance to light, dope with fungicide for resistance to fungus, and pigmented dope for the final color applications. 

Purchase considerations

An aircraft covered with polyester fabric—if it is applied according to STC, properly maintained and kept in a hangar—can have an almost indefinite life. However, when considering the purchase of a fabric-covered airplane, it is important to seek a mechanic that is familiar with this type of aircraft and knows what to look for. 

With the cost of a complete recover job for a simple airplane such as a Piper J-3 Cub or Piper PA-18 Super Cub in the $30,000 to $40,000 range, you must be certain of the condition of not only the fabric, but what lies underneath. 

As with most airplane purchases, it is always good to look for an aircraft that is in excellent condition and pay the asking price rather than look for the bargain. That bargain plane could require recovering that would make the final cost exceed the value of the aircraft. 

Prior to contracting with a mechanic to do a pre-purchase inspection, there are areas which you can check yourself just to see if the fabric-covered aircraft is in a condition that you would consider purchasing it. 

Keep in mind that vintage tailwheel aircraft probably have had a few ground loops, with airframe and/or engine damage and major repairs. Damage history is almost a given—but what this means for the purchaser is that the repairs must have been done correctly and that the aircraft flies like it should. 

The first order of business is to check the aircraft records, including any FAA Form 337 documents, to get an idea of the history of the repairs done to the airframe and engine. 

After checking the aircraft records, including compliance with all ADs, it would be wise to make up a written list of items to check on a pre-purchase walkaround. Make notes of anything you have a question about. 

Start with the condition of the fabric, and what the finish looks like. Check for cracked and missing paint or dope that would allow sunlight to directly access the fabric. Look for ringworm in the fabric; this indicates that the paint job is failing and will cause the cloth to deteriorate in a short time if exposed to direct sunlight. 

Check for patches, noting any especially large patch areas—these would require a logbook entry, or possibly a 337 form indicating a major repair. If there is no logbook entry indicating a repair was made where a large patch is located, be suspicious. There could have been major damage to the airframe structure that was repaired improperly, or not at all. 


Wrinkles or sags in the fabric most likely point to structural damage. For example, a dent in the metal leading edge of a wing would cause a sag or wrinkle in the fabric that would be visible from the outside. 


Blisters or rough areas under the fabric along lower longerons are an indication of rust in the steel tubing. Other areas could also have blisters or rough spots, such as the horizontal stabilizer, elevator or rudder; water is often trapped in these areas and eventually causes rust or corrosion. 


A fabric-covered aircraft should have sufficient drain holes or grommets installed—not only to allow moisture to escape, but also allow air to circulate and expel any moisture created by condensation. 


With the owner’s permission, pull a few inspection plates off from under the wings, especially in the area where the lift struts attach to the spar. Use a flashlight to take a good look at the wooden spar around the bolt holes, checking for obvious defects such as cracks or splits in the wood.


Move the strut at the upper end and see if there is any evidence of movement between the spar and the lift strut attachment fitting. Whether the spar is wood or metal, any movement is not good and could cause the spar to crack in this location, which would be an expensive repair or replacement. 

While the inspection plates are off, take a look up through the wing. Sunlight is the number-one enemy of fabric, and any daylight showing through the upper wing surface means a reduction in the useful life of the fabric. A very dull indication of light is okay, but if you can see a shadow of a person’s hand blocking the sunlight, then there probably isn’t enough light-resistant silver or pigmented dope remaining on the fabric. 

While the inspection plates are off, take a look at the rib stitching to see if the lacing cord is intact. Rodents have been known to get into a wing and chew the lacing cords, requiring expensive repairs. Rodents and birds can destroy an aircraft, especially if the structure is compromised by droppings or if drain holes are plugged with debris. 


While on the subject of wing ribs, note that over the years, several aircraft accidents (and at least one fatality) have occurred as a result of missing rib nails that secure the rib to a wooden spar. 


Another problem with wings and ribs is that of dissimilar metal corrosion when steel clips are used to secure fabric to the individual aluminum wing ribs. 

Since tailwheel equipped aircraft are sometimes involved in ground loops, check the wingtips for damage. Look at the fabric to see that there are no scrapes or tears, and check the wingtip for cracks or damage by looking up and out toward the tip through an inspection hole near the wingtip. 

Take a look at the lower rudder area and tail post for signs of damage such as loose fabric, wrinkles or sags, and possibly bent tubing from a hard landing on the tail. 


Get up on a stepladder and check the center section and inboard wing fabric directly in the propeller slipstream. This area sees a lot of vibration and heavy airstream deflection from the propeller, which induces wear/chafing and weakening of the fabric. 

The use of a suction cup on the fabric—attempting to pull up on the fabric in this area—is a simple test to see if the fabric is weak and/or not secure, requiring repair or replacement. 

Final thoughts

When evaluating a fabric-covered aircraft, you really need to take enough time to go over the paperwork and the aircraft completely. Repairs to structure or a complete recover job are considered major repairs; they are expensive, and legally must be done by (or supervised by) an experienced and licensed mechanic with inspector status to complete the FAA Form 337. 

Recovering a Type Certificated aircraft is a job I recommend you leave to the experts. Errors in the fabric replacement process are easily made—and these can be difficult and costly to correct. Mistakes may even require starting the job over. 

Replacement of aircraft fabric is a big job because it never is just a plain recover job—there may be repairs required along with the preparation involved, such as completely disassembling the fuselage frame and sand blasting the fuselage, inspecting for damage and rust, and applying dope proof primer. 


Woodwork requires proper preparation with cleaning, sanding and application of special dope proof sealer. 

Multiple repairs to the structure, to include welding prior to the recover process, are more common than one may anticipate. These repairs can become overwhelming unless the job is properly planned and executed by an experienced person. 

How much a recover job costs depends on the process used, how many repairs are required prior to covering, and if you are able to assist in the process. 

Poly-Fiber publishes an estimated cost of materials for recovering a Cub as approximately $5,500, and estimates the time required as a month. These figures are probably optimistic, especially if you don’t have experience or close supervision. 

When considering the purchase of a fabric-covered aircraft, look for a well-maintained aircraft with a quality fabric cover job. A quality job should last 20 years or more, depending on environmental conditions and exposure to sunlight. Any bargain-priced fabric-covered plane will most likely cost more to own. 

Vintage tailwheel aircraft can be a joy to own and fly. Enjoy the experience, buy the best—and leave the recover job to someone else!  

Michael Berry is a former aircraft repair shop owner. He is also a multi-engine rated ATP (757/727), A&P/IA, airplane owner, turbojet flight engineer and Part 121 air carrier captain. Berry has over 15,000 pilot hours. Send questions or comments to .


Consolidated Aircraft Coatings (Poly-Fiber)
Place, Peel, Press & Spray

Place, Peel, Press & Spray

The pros and cons of using decals and stencils to apply aircraft graphics.

You’ve just repainted your airplane and it’s beautiful. There’s just one detail keeping you grounded: applying the required N-numbers and placards. 

According to the dictionary, a placard is a “sign for public display.” So, what’s the best method to publicly display your aircraft information? Decals or paint? 

Dried-out decals

I’ve never liked decals, particularly after seeing so many cracked and peeling from airplanes sitting in the sun on the flight line, but they do have some advantages. 

First, almost anyone with patience can apply them. If you mess up, peel it off and try again. Another benefit is that they can be easily removed. If there’s a chance you may change your N-number, decals are the way to go. 


There’s one caveat, though: the paint around decals can fade, so after a few years it may not look so great if you make a change. (Kind of like a girl with a suntan from one bikini who then wears another with a different cut. You can tell where the sun’s been, and where it hasn’t.)

Unless you have the cash to hire a professional painter, or you’re an artist yourself, you’ll want to use decals for any complex designs. Unusual or personalized images can be sent to an aviation graphics company for printing on decal material.


Soap is for the kitchen dishes

When applying a decal to an airplane, many people suggest using soapy water—often a mixture using common dish soap—to make a slippery surface for the decal to float upon. This allows for fine adjustment of the decal’s position and for air bubbles trapped underneath to be squeezed out. 

I don’t think soap is good for adhesion, and I have to wonder if the reason many decals are peeling and cracking is that they’ve been degraded by a sunbaked soap film. If you use my method, you shouldn’t have any bubble problems and you won’t need to make last-second, soapy adjustments to your decal’s position.

My method to apply decals

1. Use a small piece of masking tape to place the decal where you want it, with the backing material against the aircraft. The decal material is made of three layers, a heavy backing, the decal in the middle and a light protective paper. You’ll be able to see through the light paper side to ensure its orientation.

2. Step back and look at the position carefully. Compare what you see in front of you to your photos or design plan. Is the decal straight? Does it match the “line” of your plane? It might look better if it matches the airplane’s lines versus being dead-straight.

3. Adjust the decal until you are absolutely sure that’s where you want it, then completely tape down the top edge.

4. Flip the decal up, making a hinge of the tape. Crease the tape so it moves easily.

5. Have a soft, clean cloth within reach and start peeling the backing off the decal from the top, next to the tape hinge. Press down that topmost edge of the decal and use the cloth to smooth the decal as you slowly peel the backing material off. The cloth will help it go down smoothly without any bubbles. Work slowly, and pull the backing material off in a straight line.

6. When the decal is fully applied, remove the protective paper and masking tape, and rub the decal gently with the cloth.

A second pair of hands helps during any decal application, and is essential for large decals—one person pulls off the backing, while the other smoothes the decal. 

For really large decals—such as 12-inch N-numbers, which could be five or six feet long—it’s best to cut the decal into manageable pieces. After taping down the top edge (step three, above), cut vertically between the numbers. The decal will look like a row of teeth. Then apply one number at a time.

Placarded information

Aviation graphics companies sell sets of decals for all the placarded information needed inside and outside your particular aircraft, such as “No Step,” “Avgas Only” and “Fasten Seat Belts.” Do I really need a “No Smoking” decal on my instrument panel? Evidently, it’s required. 

Use the same tape-hinge method described earlier for these small decals.

Stencils and paint

Painted graphics look better, especially after years under the sun, but painting also takes far more effort (or money, if you want someone else to make the effort). 

First, you must apply stencils and protect nearby areas of the aircraft from overspray, then mix up and spray toxic paints, remove the stencils and protective materials, and clean the spray equipment. The stencils, paint and rental of a spray gun cost far more than decals. 

Painted markings are also almost impossible to change—you’d have to repaint the background color first—so be sure before you start spraying. 


If it’s so much effort, why paint? 

It looks really good and stands up to the elements when done right.

Large stencils can cost hundreds of dollars—quite a bit more than masking tape—but modern stencil materials give a much sharper edge and don’t allow paint to bleed underneath. 

I think it’s worth the cost. This is especially true when painting on fabric covered aircraft with ribs and stitches to cover. The difference between figures sprayed through computer-cut stencils and those done using masking tape is obvious, at least to me.

Some painters still prefer to use tape to mask out N-numbers and insignias for painting, citing the cost and difficulty of handling large stencils. But, in my opinion, that’s a tradeoff of quality for cost. You have to be a real artist to create straight, properly aligned letters with a roll of masking tape, and there’s inevitably a few spots where the paint bleeds.


If you do use masking tape, run your thumbnail over the edges to make sure they are pressed down completely. And, no matter how well you think the tape is adhering, paint will bleed under it every time if you apply the paint too heavily at first. A very light, almost dry coat will seal the edges and prevent bleeding when you apply a heavy coat to finish the job.

My method for applying stencils is the same as it is for decals. The only difference is the additional step of removing the middle layer—the one cut in the shape you want to paint—so the paint can reach the surface.

Really large stencils might take three people to apply: two to pull off the backing material and one to smooth the stencil.

Take a second look

I ordered two identical stencils for my aircraft. You would think the company would set up the type (in this case, a large N-number), hit “print 2,” and they’d be spit from the stencil-cutting machine exactly alike. Well, they weren’t. 

The painter placed the stencils on my aircraft according to my instructions and sprayed. Only after he peeled them off did we see that the spacing of the letters was incorrect on one. The letters were too close together.

My pilot friends say, “no one will ever notice,” and maybe that’s true—but I noticed it immediately. I could have fixed it by increasing the spacing myself, if only I’d seen it earlier. The lesson here is: stand back and take a long look before slinging paint. 

I’m sure there are decal people, stencil people, masking tape people… and everyone has their own opinion. You can make your own choice. That’s part of the fun of having your own plane, isn’t it?

What methods have you found work best on your airplane? Visit the forums at PiperFlyer.org to share your successes, upload your photos, and get more ideas.

Dennis K. Johnson is a writer and a New York City-based travel photographer. He flies sailplanes whenever possible and is the owner of N105T, a newly restored Piper Super Cub Special. Send questions or comments to .

Papa’s Got a Brand New… Fuel

Papa’s Got a Brand New… Fuel

Swift Fuels’ 94 Octane Unleaded Avgas

Earlier this month I burned 25 gallons of Swift Fuels’ 94UL unleaded Avgas in the 180 hp Lycoming O-360 in my 1960 Piper Comanche, Papa. 

Swift Fuels of Lafayette, Ind. has submitted its 102 octane unleaded (102UL) Avgas to the FAA for testing in the Piston Aircraft Fuels Initiative (PAFI) program, but it also announced in mid-2015 that it was producing a 94 octane unleaded (94UL) Avgas. 

In the last year and a half, 94UL hasn’t gained much traction even though it’s approved for operation in a wide range of GA engine and airframes. 

94UL is produced to ASTM Standard D7547, the specification for hydrocarbon unleaded aviation gasoline. This lead-free Avgas was developed at the request of the military in 1994 for use in its drone fleet. 94UL is a stable fuel with a “tank life” of two years. 

I am looking forward to the day when Avgas will be free of tetraethyl lead (TEL), and when I saw that Swift offered a lead-free Avgas that I could legally use, I wanted to try it. What I found was very interesting.

By the end of my flight testing I hadn’t seen one iota of discernible difference in any engine parameter—EGT, CHT, manifold pressure, rpm or oil temperature—between the 94UL and 100LL Avgas. 


Data collection

The data I’ve captured is by no means an exhaustive test. I haven’t done an extreme heat or extreme cold temperature starting test. I haven’t done a high altitude (18,000 feet MSL) operational test. I haven’t done an in-flight restarting test. Nor have I done a fuel system compatibility test. 

But thanks to the data collection feature of my Electronics International CGR-30P and 30C engine monitor, I could collect and plot the engine data gathered during the three test flights using EGView from EG Trends. 

I also asked Joe Godrey and Savvy Analysis to check my plots. He verified my findings.


Preparing for the tests 

There is one 30-gallon bladder-style fuel tank in each wing of my airplane. The fuel selector valve has three positions: left to the engine, right to the engine, and off. There’s no both position. 

After flying the right tank empty and sumping the remaining unusable fuel out through the system low point drain, I paid Rabbit Aviation Services at the San Carlos Airport (KSQL) $118.37 to pump 26.6 gallons of 94UL into the right-wing tank. 

I also topped off the left tank with 8.4 gallons of 100LL ($38.22). That crunches down to 100LL at $4.55 a gallon and 94UL at $4.45. (Vendors set the pump prices; when buying from Rabbit there’s minimal direct cost savings.) The fuelers at Rabbit asked if my airplane was approved for auto gas or 94UL Avgas before dispatching the 94UL truck. 

Initial observations

94UL smells different than Avgas and is clear. I checked the two fuels for weight. The 94UL is lighter at 5.79 pounds/gallon than the 100LL at 5.94 pounds. 

I flew three one-plus hour flights, switching back and forth between the left and right tanks. 

I switched during a full power climb; I switched with the mixture leaned to peak EGT on the first cylinder to peak; and I switched during my normal cruise power and mixture settings while level at 5,500 feet MSL. I also switched on descent and while idling before flight and after landing. 

In addition to collecting the engine parameters digitally, I also watched for any EGT difference in the seconds following the switches. I never saw the numbers change.


Users’ reports

John Poppy at the Portage Municipal Airport (C47) in Portage, Wis., a popular fueling stop near AirVenture, said he’s heard “zero negative feedback” about 94UL. 

Poppy has a 1,000 gallon tank and says he pays two cents a gallon for shipping for the five-hour drive from the Swift production plant in Lafayette, Ind. Poppy sells 94UL for $3.35 a gallon—59 cents per gallon less than his 100LL. 

Poppy told me that one customer who flies a Cessna 182 has been using it for over a year while commuting to another state. According to Poppy, the customer’s mechanic asked if he had taken his engine apart and cleaned it after pulling the cylinders for a top overhaul. 

Rich Volker of RV Airshows burns it in the 600 hp Pratt and Whitney R-1340 that powers the Harvard Mk IV he flies in his airshow routine. Volker told me he flies his routines at full power and in his opinion, his engine can’t tell the difference. 

Dennis Wyman runs the engine shop at G&N Aircraft in Griffin, Ind. Wyman told me that his experience is that running 94UL results in less deposits on pistons and valves. In his experience, the switch between the two fuels is transparent. 

The only change Wyman has seen is that the combustion chamber of an engine that uses 94UL looks slightly darker than a 100LL chamber. Can you use 94UL?

You can use 94UL is your airplane fits into one of the following options:

• Airframe/engine combinations that have an Auto Fuel STC (e.g., an STC from Petersen Aviation);

• Airframe/engine combinations OEM-approved for auto fuel (e.g., ultralights, LSAs and experimental aircraft);

• Airframe/engine combinations Type Certificated to operate on Grade 80 (listed as Grade 80/87 in ASTM D910) or Grade UL91 (ASTM D7547) Avgas; (Note: If the fuel data plate on the engine lists 80/87 as the fuel, you can legally use 94UL without an STC. This includes Piper singles such as PA-18, -20, -22 and 150 hp PA-28s.) 

• Airframe/engine combinations Type Certificated to operate on minimum 80 octane or lower (e.g., 73 or 65 octane) Avgas; or

• Airframe/engine combinations with an Avgas STC purchased from Swift Fuels.

The engine data plate on my Lycoming O-360-A1A specifies 91/96 octane fuel, yet my Piper PA-24 Comanche had never been approved for an auto fuel STC. My only avenue to use 94UL was buying an Avgas STC from Swift. 

Where can you get 94UL?

Per the user map on the Swift Fuels website, there’s only one public source for 94UL west of the Mississippi River, and it’s in California. 

There are also 14 that are cited as “private users.” The 18 other public outlets for 94UL include three in Florida, one in South Carolina, one in Ohio, one in Missouri, four in Indiana and eight in Wisconsin. (Note: If you would like find out more about setting up a 94UL station, contact the folks at Swift. They have a team that will tell you how to get started.)

One of the potential roadblocks between availability and pumping 94UL at your airport is tankage. Most airports now have two tanks—one for jet fuel and one for 100LL. One option for adding a third is installing a box station from U-Fuel in Elk Mound, Wis. 

U-Fuel offers a split tank—94UL on one side and 100LL on the other. It appears that split models have the same footprint as existing single-fuel models. 


94UL is here now; PAFI fuel is a few years away

Since most privately owned and operated airplanes in the GA fleet can safely burn 94UL, and since Swift sells it for less than today’s 100LL, Swift’s 94UL seems like a winner. 

No one knows when the new unleaded 100 octane Avgas will be produced—it’s still being tested in the Piston Aviation Fuels Initiative (PAFI) program. 

The PAFI program is scheduled to complete the fuels testing in 2018, but there could well be a time lapse between the approval date and the production and delivery to your local airport. 

Based on my testing and my belief that TEL creates a wide range of problems in our air-cooled engines, I would be burning unleaded aviation fuel today if there was a pump with a Swift 94UL placard close by. 

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


Engine monitors and cluster gauge replacements
Electronics International – PFA supporter


EGView software – data analysis tool
EG Trends Inc.


Engine rebuilding, engine overhaul and engine sales
G&N Aircraft, Inc.


Auto fuel STCs
Petersen Aviation, Inc.


94UL fuel service (West Coast)
Rabbit Aviation Services, Inc.


Savvy Analysis – engine monitor data organizer
Savvy Aircraft Maintenance Management, Inc.


94 octane unleaded Avgas, Avgas STC
Swift Fuels


Aviation fuel stations


Further reading
FAA PAFI program
Q&A: Pitot Static Checks for a Cherokee 180 & Apache Stabilator Torque Tube Inspection

Q&A: Pitot Static Checks for a Cherokee 180 & Apache Stabilator Torque Tube Inspection

Q: Hi Steve,

I need more information on what my mechanic calls “pitot static checks.” I ask because he said I need them every two years—but then said only one is needed if I don’t fly IFR.

I am partway through my private pilot training and am using my dad’s Piper Cherokee 180. He said I could fly it as much as I want if I pay for the maintenance and upkeep.

I think it’s great that I get to fly the same airplane every lesson. I started out renting at a flight school and I personally didn’t like when I had to flip-flop between different airplanes. I think it made it harder for me to concentrate fully on the flying part.

But I’m afraid Dad hasn’t kept up with the maintenance on his Cherokee. For instance, the last pitot static check I found in the logbooks was over 10 years ago. I know who was doing his annuals and decided to go to a nearby well-established shop for the first annual I’m paying for.

So far they haven’t found any big-ticket items (whew!) but there have been plenty of catch-up items. I’m okay with that, because I’m going to be loading my family in this airplane and I want to be able to feel like it’s ready.

That’s my story. Now, the pitot static test?

—Learning Larry

A: Dear Larry,

Welcome to the world of flying. I feel like you’ve already made some good decisions regarding your training and the importance of having confidence in the maintenance work done on your airplane.

Unfortunately, there have been and continue to be “soft” annuals performed on a small number of airplanes every year. I’m glad you have resolved to take the steps required to get your dad’s airplane completely airworthy.

The pitot static system check you’re asking about is two separate checks. Both checks are spelled out in FAR Part 91, “General Operating and Flight Rules.”

The first rule is sometimes referred to as the IFR rule. It ensures the altimeter is working correctly and that the automatic altitude reporting system in your airplane is working and within tolerances. If you never fly IFR, you don’t have to keep this one current.

This rule, under FAR 91.411, “Altimeter system and altitude reporting equipment tests and inspections,” says that no one can operate in controlled airspace while operating under IFR unless, within the preceding 24 months, “each static pressure system, each altimeter instrument, and each automatic pressure altitude reporting system has been tested and inspected and found to comply with appendices E and F of part 43 of this chapter.”

I’ll explain a little more about this mandate—but it’s important to realize that even if you’re flying in clear weather, this inspection must be current if you’re on an IFR flight plan.

In fact, I think it’s a good idea to get in the habit of filing IFR from time to time on all except local flights because it helps keep procedures sharp and maintains a pilot’s awareness of how the “system” works.

The second rule, under FAR 91.413, is the transponder rule. 91.413, “ATC transponder tests and inspections,” states: “No persons may use an ATC transponder that is specified in 91.215(a), 121.345(c), or Sec. 135.143(c) of this chapter unless, within the preceding 24 calendar months, the ATC transponder has been tested and inspected and found to comply with appendix F of part 43 of this chapter.”

While the transponder test is required for all aircraft, there’s quite a bit of national airspace where a transponder is not required. This airspace is spelled out in FAR 91.215, but realistically, keeping your transponder check up-to-date assures that your system (and the airplane) is legal to fly almost anywhere in the country. Easier to just “get ‘er done.”

Many maintenance shops can perform both 91.411 and 91.413 tests, provided they have FAA approval in the form of a repair station license for these tests.

Avionics shops, manufacturers of the airplane, as well as a few other places also have this equipment.

Some folks grouse a little bit about the costs—which range from $200 to $300 for both certifications—but it is important to realize that the equipment needed for certifying your system also should be recertified on a regular basis, and that costs the shop some bucks, too.

I hope that answers your questions.

Happy flying.


Q: Hi Steve,

My mechanic wants to remove the horizontal tail feathers off my old Apache for what he says is an inspection from corrosion of the tube.

What’s he talking about?

—Apache Al

A: Hi Al,

Your mechanic is talking about Piper Service Bulletin No. 1160. It was issued in 2005 and calls for an inspection of the stabilator torque tube for internal and external corrosion.

The torque tube is a steel tube that ro-tates on large roller bearings that are supported in two-piece housings securely bolted to the aftmost bulkhead in the fuselage.

Since the left and right stabilator “tail feathers” are not normally removed during yearly maintenance, and since corroded torque tubes have been found, I feel that this is an important inspection.

I had to remove the tail feathers on my Comanche to comply with AD 2012-17-06 that related to an inspection for cracks in the stabilator horn. I did the inspection called for in SB 1160 at that time. AD 2012-17-06 does not apply to your Aztec.

Since my Comanche had spent much of its life near Phoenix where corrosion and rust rarely occur, I didn’t have any problem pulling my “feathers.”

However, I did do my best to soak the tube with AeroKroil before and during removal. The key to removing my feathers was to go slow and continue to apply Kroil.

I twisted the feathers slightly at first, and then more and more on the tube, and eventually they slid off.

SB1160 provides both a minimum outside diameter for the torque tube (2.3113 inches) and a minimum tube wall thickness (0.161 inches).

If there’s any deviation due to rust, the tube must be replaced. The part number for the tube for your Aztec is 16067-00. I just checked with Piper and was told that part number 16067-00 is no longer available.

If your torque tube is airworthy, make sure to apply a protective coating after the inspection. If it isn’t, a used serviceable torque tube assembly may be available through a salvage yard.

According to Tom Pentecost at DSA Flightline Group, owner of Piper Parts Plus (P3), the replacement kit (p/n 652-579) listed in Table 1 on page three of the service bulletin is still available with a lead time of 100 days.


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


Penetrating oil


Piper replacement kit 652-579
Piper Parts Plus (P3) – PFA supporter


Further reading
FAR 91.411 and FAR 91.413


Piper Service Bulletin No. 1160
PiperFlyer.org/forum under “Magazine Extras”
Researching the Regs: Owner Produced Parts

Researching the Regs: Owner Produced Parts

An in-depth look at FAR 21.9 and Advisory Circular No. 23-27 by aviation legal consultant and A&P/IA Kristin Winter

The FAA keeps an iron grip on the supply of approved replacement parts for Type Certificated aircraft. Replacement parts generally must come from the airframe, engine or propeller manufacturer, or from an approved source that has been issued Parts Manufacturing Approval, commonly referred to as a PMA. 

There are some other limited exceptions for what the FAA refers to as “standard” parts, such as nuts, bolts and other hardware manufactured under an industry standard such as AN (Army-Navy) or MS (Military Standard), or parts manufactured by a repair station. 

There is one major exception to the FAA’s tight grip, and that is the owner produced part. Owner produced parts are commonly used by the airlines, which often have a large fleet of the same or similar types of aircraft. 

Like the owner of a General Aviation aircraft, an airline often wants to avoid the high cost of commonly used parts from the original equipment manufacturer (OEM), so it will reverse engineer and produce batches of parts that are
then used in its fleet. An example might be landing gear bearings that wear
out frequently. 

The availability of owner produced parts appears to go back for decades, though the origin is unclear. As it is of biggest benefit to airline operators, there is a major constituency to make sure that owner produced parts remain an available solution for all aircraft owners and operators.

FAA standards and definitions

The FAA sets out the limitations on replacement parts in FAR 21.9. 

Paragraph (a)(5) provides that one type of approved part is one that is “[p]roduced by an owner or operator for maintaining or altering that owner or operator’s product.” 

This simple statement leaves lots of questions unanswered. One common question is: must the owner or operator physically produce the part themselves? Most GA aircraft owners may not be equipped or sufficiently skilled to make a part in their basement. Fortunately, the friendly FAA has provided an interpretation. 

When the FAA renumbered and revised Part 21 in 2009, it specifically made mention in the Federal Register that the interpretation memorandum issued on Aug. 5, 1993 was still operative. 

The answer to the first common question as to whether the owner/operator must physically produce the part is clearly no. The FAA memorandum states that “An owner would be considered a producer of a part if the owner participated in controlling the design, manufacture or quality of the part.” 

The memorandum goes on to provide five nonexclusive indicia that an owner “participated” in the production of the part (italics added):

1. The person provided the manufacturer with design or performance data from which to manufacture the part. (This may occur, for instance, where a person provided a part to a manufacturer and asked that the part be duplicated.)

2. The person provided the manufacturer with materials from which to manufacture the part.

3. The person provided the manufacturer with fabrication processes or assembly methods to be used in the manufacture of the part.

4. The person provided the manufacturer with quality control procedures to be used in the manufacture of the part.

5. The person supervised the manufacturer of the part.

Responsibility of owners, responsibility of mechanics

So what does this mean to our aircraft owner faced with the unavailability of a critical part, or who is suffering from cardiac arrest at the cost and time delay of obtaining the part from the original equipment manufacturer? 

It is important to keep in mind that it is the owner or operator’s obligation to produce a part that is airworthy, meaning that the part conforms to type design and is safe to install in the aircraft. 

The installing mechanic’s responsibility is only to make a reasonable assessment that it is an airworthy part and to install it properly. (It will likely help the mechanic feel comfortable if they are provided with a copy of the drawing, the specifications, and/or have been part of the process from the beginning.)


Ways an owner can participate

Two options for owner participation jump out of the memorandum on first blush. 

First, the owner can provide the part to an appropriate manufacturer (such as a machine shop) and ask them to duplicate it. 

The other option is if an owner supervises the production, which might involve working with the mechanic to fabricate the part. Supervision would not likely require an owner to stand there every moment, but to be reasonably available to provide supervision or answer questions. 

In practice, owner supervision might be a little difficult given the likely disparity of knowledge between the owner and the mechanic. However, if the owner is willing to certify in the logs that he or she supervised the production of the part, it is unlikely to be challenged.

FAR 21.9 put into practice 

So let’s look at some practical applications. Not long ago, I determined that it was time to replace the flap tracks on my Twin Comanche. I had spoken with another owner who had the same problem, and we agreed to pool our resources to obtain some owner produced parts. 

I obtained an exemplar track and sent it to a metallurgical lab for analysis to determine the proper material. The lab charged a bit over $200 for the testing and provided a formal report. The other owner produced a drawing of the part. 

Armed with material and drawing, we had a couple of ship sets made. The cost was about $75 each, and the sets were created in five working days (turnaround can vary depending on how busy the shop is), compared to a cost of over $300 each from Piper and a wait of unknown duration. 

We clearly met the first example of conditions that qualified me as participating in controlling the design of the part. (See photo, top of page 26.)

Some considerations 

One of the most useful options for the aircraft owner is the specific acknowledgement by the FAA legal memorandum that an owner may provide the part and ask that it be duplicated. 

There is one complexity here in that many machine shops can duplicate a part, but are not equipped to determine what material it was made from. That might mean that the owner will need to resort to the metallurgical testing lab as we did with the flap tracks. 

Consideration must also be given to whether the part had any protective coating which should be duplicated. This could mean having the completed part anodized or cadmium plated. 

All of this may make it uneconomical to use the owner-produced exception if one is simply trying to avoid purchasing an overpriced bushing from the manufacturer. 

This is an area where an active type club that maintains a database of parts that have been owner produced—and possibly test results, and even CNC programs—can be most helpful. There appears to be no requirement that participation in the design requires an owner to reinvent the drawing, material specs, etc.

Gray areas remain

There are some gray areas still, even with the FAA’s memorandum. Take, for instance, a retractable gear single with a loose bushing where the nosegear pivots. The boss in the mount is worn slightly oversized so that bushing is no longer a press fit. 

To use a new OEM bushing would require removing the engine, removing the mount and sending the mount out for repair. The cost could easily exceed $5,000. 

A repair involving an oversized bushing might be a cost effective solution, provided your mechanic is comfortable making that repair. 

As the owner, you send the bushing to the lab and sketch out a drawing with the dimensions that have the bushing .001 inch wider than the factory. 

With the material specification in hand and the dimensions, you may then have a machine shop fabricate one. I have done this with great success and the only way I could tell a new OEM bushing from the oversized one was to get out a micrometer. (See photo, bottom left.) 

Success here will require a mechanic comfortable with the oversized bushing being an acceptable minor repair, so it is important to discuss this with your mechanic before embarking on such a repair. 

It is not unreasonable for the mechanic to ask the owner to make a logbook entry that they provided a part produced under FAR 21.9(a)(5) and provide the information used for its manufacture.

Why not use a commercially available part?

Any discussion of owner produced parts seems to raise the question about whether an owner can just go and buy the part which is commercially available. 

A good example of this might be a wheel bearing which is frequently a standard Timken bearing. From Piper, that bearing is $110.24; the parts catalog even identifies it as a Timken 13889 bearing. One can likely get the same item from the local auto parts store or a bearing supply company for $25 to $30—but is that legal?

As an owner produced part, the interpretation seems to suggest that it is not, though to my knowledge that hasn’t been clearly addressed—especially in the context of Piper identifying the actual vendor part number. 

If the aircraft owner got out the calipers and confirmed that the auto parts bearing was the same size, number of rollers, etc., arguably that would qualify as participating in the quality control of the part. This is another gray area. 

But there is one more option. Fortunately with this example, McFarlane Aviation has already obtained a PMA for the bearing, so for $40 instead of $100, one can obtain a bearing with a PMA stamp. 


Substitute parts under Advisory Circular No. 23-27

Advisory Circular (AC) 23-27 provides information on using substitute parts for small, unpressurized aircraft Type Certificated before 1980. That includes most standard Piper airplanes. 

Note that the operative deadline is not when the aircraft was produced, but when it was certificated—so even a late model Piper Archer is going to qualify, as it was Type Certificated before 1980.

While a bit confusing in its applicability, this AC appears to provide an approval for parts substitution if such would be considered a minor repair or minor alteration and to provide the basis for a field approval if a major repair or major alteration is required. 

Directly applicable to our example is the provision that states that “You may substitute parts where a direct substitute for a part/material can be found under manufacturer part number, military specification, or other recognized standard, such as the SAE.” 

For most aircraft owners, AC 23-27 can provide a route for substituting an industry standard part for an OEM part which may no longer be available in a timely manner or for a reasonable price. Mil-Spec switches, SAE alternator belts, batteries, etc. can often be used under the guidance of this Advisory Circular without resorting to an owner produced part.

For those of us who cannot afford a new or nearly new aircraft and who don’t have the luxury of just dropping off our plane at the local OEM service center with the keys and the Visa card, FAR 21.9 and AC 23-27 can be helpful in keeping aircraft maintenance cost effective, while still meeting the regulatory requirements.

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 .


Further reading
FAA Memorandum, Aug. 5, 1993
“Definition of ‘Owner Produced Part,’ FAR 21.303(b)(2)”
Advisory Circular No. 23-27,
May 18, 2009
“Parts and Materials Substitution for Vintage Aircraft”

Both documents are available at PiperFlyer.org/Forum under “Magazine Extras”

PMA wheel bearings
McFarlane Aviation Products
– PFA supporter
Lock Haven Low Wings Comanche Aileron ADs 36

Lock Haven Low Wings Comanche Aileron ADs 36

This detailed report from a Comanche expert spells out exactly how to check for compliance with two Airworthiness Directives. 

The Comanche’s ailerons have been a source of trouble from the beginning. It was not a good bit of detail design. Poor loads analysis led to component cracking in two locations.

One issue was the outer hinge bracket concentrated stress in the aileron spar and caused cracks. The other was the nose ribs used to attach the aileron counterweight to the aileron spar were cracking. These issues apply to both single Comanches and Twin Comanches.

The aileron issues date from the early days of the Comanches. The first Piper service information regarding the ailerons was issued in 1959. Additional bulletins came out in the late 1960s. A pair of Airworthiness Directives provided a final resolution in the late 1970s. 

Piper did step up nearly 40 years ago and offered a kit which resolved the issues. Unfortunately, not all aircraft have had the kits installed and thus require additional inspections. 

Both of those ADs are current and recurring at 100-hour intervals until a kit is installed. The kits resolved the issues with the Comanche ailerons. Any further problems are rare or nonexistent for modified aircraft. 

It is very common to find Comanches that have not had one or both of the kits installed, hence removing the ailerons every 100 hours is required. I have often found that the logbooks claim the ADs have been complied with, but do not specify the method of compliance. 

Vague logbook entries can lead less-careful IAs to terminate the 100-hour inspections thinking that the correct kit was installed, when in fact it had not been. I will discuss each AD and explain how to make a visual inspection to determine whether the ADs were in fact properly terminated by installation of the correct kits.

Outer aileron hinge bracket AD 77-08-01 and Piper Service Letter 787

At the end of 1976, Piper issued Service Letter 787, alerting the fleet to the possibility of cracks developing in the aileron spar at the outer aileron hinge bracket. SL 787 recommended a recurring 100-hour visual inspection which required the ailerons to be removed. 

SL 787 also provided for a terminating action by installation of Piper Part No. 760-914, Aileron Outboard Hinge Bracket Replacement. 

The FAA mandated these aileron inspections with the issuance of AD 77-08-01. This AD also approved the installation of kit 760-914 as a terminating action.


Complying with AD 77-08-01

You can tell if your aircraft has kit 760-914 installed by visual inspection of the outboard hinge bracket. The replacement bracket has a much larger base/reinforcement to spread the load over more of the aileron spar. In addition, the new bracket is made from steel instead of aluminum. CherryMAX rivets will have been used to attach the hinge bracket to the aileron. 

If you know what to look for, the difference between new and old brackets is immediately obvious. Until then, a magnet is a quick way to check if your aircraft has the new steel bracket. 

Most aircraft I see have had this kit installed. It was easy to accomplish, and doubtless rather inexpensive back in the 1970s. 

The 760-914 kits are very difficult to find these days. Piper doesn’t stock them. Piper’s price is about $600 per kit and two kits are necessary to do both ailerons. Piper’s delivery time is about four months. Installation should be about an hour per aileron.

(These brackets are difficult to find! If you read last month’s “The View from Here,” you know that as a Piper Flyer member, you have access to PFA’s parts locating service. Log in to PiperFlyer.org; from the “Members” menu, click on “Parts Locating” and fill out the form. We’ll get on it as quickly as we can. You can also email . —Ed.) 

Aileron nose ribs 

AD 79-20-10 and Piper Service Letter 850 

The aileron nose ribs attach to the counterweight that extends inside the wing. The shaft bolts to the nose ribs, with a lead mass at the other end. This puts a lot of stress on the nose ribs. 

Problems with the aileron nose ribs go back to 1959. Cracks started appearing very quickly. It took four versions of nose ribs before Piper got it right, and each was further reinforced from the previous version. 

Piper issued Service Bulletin 173 which mandated installing a second version of the nose rib with some additional reinforcement. In about 1968, Piper issued a Service Spares Letter and associated kit, Part No. 757-162. This kit further upgraded the nose ribs to the third version of the rib: Part No. 20234-31.

Several years later, the FAA issued AD 74-10-03 which mandated the installation of kit 757-162 or a recurring inspection. Subsequent history showed that cracks could develop even in the third version of the nose rib.

In 1979, Piper issued a Service Letter to Comanche owners, warning of further cracks in the aileron nose ribs and possibly the spar itself. Service Letter 850 sets forth recurring 100-hour inspections, which can be discontinued when kit 763-893 has been installed.

The 763-893 kit contained the fourth version of the nose rib, Part No. 20234-42. The FAA also issued another Airworthiness Directive, AD 79-20-10. 

AD 79-20-10 superseded AD 74-10-03 and included all aircraft—even the ones that had the previous kit (757-162) installed. AD 79-20-10 merely mandates compliance with Service Letter 850 and provides the same terminating action.


Complying with AD 79-20-10

Compliance with the nose rib AD is somewhat challenging to confirm without removing the aileron. However, the aileron nose rib can be inspected for compliance with a flexible videoscope. 

By going in through the inboard hinge and snaking the scope outboard in the aileron, one should be able to see enough of the nose rib to make a positive identification. An example of what can been seen is shown on Page 40. 

This is the only sure way of verifying that the Part No. 20234-42 nose ribs have been installed and that removal of the ailerons every 100 hours is no longer required. 

In my experience, most aircraft have not had this kit installed. I am skeptical as to whether the rest are receiving the mandated 100-hour inspection for cracks, as there is some confusion in the field. 

Often, IAs see that a kit was installed in the late 1960s or early 1970s and assume that terminates the AD. This has even confused some longtime Comanche-savvy mechanics; one in particular insisted that version three of the nose ribs (i.e., installation of Part No. 20234-31) terminated the AD. Piper confirmed to me by email that only version four nose ribs comply with SB 850; and hence terminate the inspection requirement of AD 79-20-10.

An IA must also be careful if an aileron has been replaced. I have seen cases where the logs stated that the 20234-42 kit had been installed in both ailerons, but I found them installed only in one. As it turned out, the aileron had been replaced with one from a salvage aircraft—and no one thought to retrieve the -42 nose rib from the removed aileron and install it on the replacement aileron.

Kit 763-893 installation

The Piper instructions for installing the kit require removal of the aileron and removing numerous rivets. This allows the skin to be peeled back and permits a mechanic to get to the back side of the spar to buck the rivets for the new nose ribs. 

There is an approved Alternative Method of Compliance that uses rivnuts and screws instead of rivets, which eliminates the need to unstitch most of the upper skin of the aileron. This is a big time-saver. (A copy of the AMOC can be found at PiperFlyer.org. —Ed.) 

As with the aileron kit, the nose rib kits have become very hard to find. Piper still lists them but doesn’t stock them. The last time I checked, the delivery time was listed at over four months and the cost was over $1,200. One kit covers both ailerons, unlike the kit for the outer hinge bracket. 

Occasionally, a nose rib kit comes up for sale as new old stock, but those have become rarer and rarer. The installation takes a fair amount of time; and the high labor cost is likely the reason that a large percentage of Comanches have not had the nose rib kit installed.

I installed kit 763-893 on my aircraft several years ago. I have done a few others since. The first time I installed the kit, it took me about 12 hours to do the first aileron, and six hours to do the second one, using the AMOC. Obviously, there is a learning curve here. If I had to pay shop labor rates, the return on investment would have been questionable. 

It takes only about an hour to remove each aileron, do the visual inspection and reinstall. However, only doing the inspection and not installing the kit raises the potential of a much more expensive repair later. The inspection finds cracks. Cracks mean that the repair might be more expensive than installing the kit in the first place.

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 is a recognized authority on Piper Comanche aircraft. Currently she is serving as Director of Operations for a commuter airline in Southeast Alaska. Send questions or comments to .


AD 77-08-01 Aileron Spar Cracks

AD 79-20-10 Mandating Compliance with Piper Service Letter No. 850 rgl.faa.gov/Regulatory_and_Guidance_Library/rgAD.nsf/AOCADSearch/B81EE4072873C7688625699E004AF817?OpenDocument

PA-28 & PA-32 Wing Spar Cracks: What You Should Know

PA-28 & PA-32 Wing Spar Cracks: What You Should Know

STEVE ELLS delves into the history of Cherokee wing spar cracks and investigates inspection methods. 

By now, every Cherokee owner, from the earliest PA-28-180 to the most recent Arrow, has heard about the inflight wing separation that occurred April 4, 2018, to a Piper Arrow owned by Embry-Riddle Aeronautical University in Daytona Beach, Florida. 

A very experienced flight instructor/check airman and a student perished when the left wing of a 1997 Arrow with slightly more than 7,000 airframe hours broke off. 

The NTSB issued Preliminary Report No. ERA18FA120 following the April 4, 2018, wing separation. The report said, “The airplane entered the traffic pattern and performed a touch-and-go landing.” ATC issued a transponder code and the pilot asked for approval to turn crosswind. ATC told the pilot to continue his upwind heading. 

The next sentence in the NTSB report stated, “Radar data indicate the aircraft climbed to 900 feet MSL at a groundspeed of 80 knots and a heading of 240 degrees before radar contact was lost. According to multiple witness, all within 2,500 feet of the accident site, they saw the airplane flying normally, then watched as the left wing separated from the aircraft.”

Wing separation history

Unfortunately for Piper Aircraft, the FAA, and PA-28 and PA-32 owners, this is not the first wing separation in PA-28/-32 series aircraft.

The first recorded separation took place 31 years ago when the left wing of a PA-28-181 Archer II broke off. 

Following that accident, Jim Burnett, Chairman of the National Transportation Safety Board, sent this letter to Donald Engen, Director of the Federal Aviation Administration:

On March 30, 1987, a Piper PA-28-181, N8191V, crashed following an in-flight separation at the wing root attachment while in cruise flight at low altitude near Marlin, Texas. The airplane, which was owned and operated by Griffin Pipeline Patrol Company, was patrolling a pipeline right of way at the time of the accident.

The pilot, the sole occupant, received fatal injuries.

Although the investigation is continuing, preliminary examination by the Safety Board’s Materials Laboratory disclosed features indicative of fatigue cracking in the separated left wing main spar. Fatigue cracking initiated at two locations just outboard of the outermost forward attachment bolt hole in the lower T-shaped spar cap of the main spar. Fatigue propagation was upward through the thickness and chordwise completely through the forward leg of the lower spar cap (about 1.3 inches). A small area of fatigue cracking also was noted in the forward web fracture piece adjacent to the forward outboard attachment bolt hole. 

Examination of the left wing at the accident site disclosed evidence of an approximate 10-inch-long crack that had been stop-drilled in the upper wing skin. The crack was located forward of the main spar at the wing root and was oriented chordwise parallel to the fuselage.

The accident airplane had been flown 7,488 hours since new. Following the separation, the operator then inspected a second PA-28-181 with 7,878 hours and found upper wing skin cracks that the maintenance department had stop-drilled. When the wings were removed, a visual inspection of the spar caps at the outboard attachment hole showed “crack indications” in the same area. 

The Burnett letter also included this: “Representatives of Piper Aircraft Corporation (Piper) indicate that other Piper models have wing spar structures similar to that of the PA-28.”

The NTSB issued three recommendations to the FAA: 

1. Issue an airworthiness directive to require an immediate inspection of the main wing spars and upper wing skin at the wing root of Piper PA-28 airplane with over a specified number of service hours for evidence of cracking. Particular attention should be placed on inspecting the bottom surface of the lower spar cap adjacent to the outboard forward attachment bolt hole at the wing root attachment, as well as along the upper wing skin adjacent to the fuselage just forward of the main spar. (Class I, Urgent Action) (A-87-40)

2. Based on the inspection described in Safety Recommendation A-87-40, establish a recurrent periodic inspection of the wing root area for cracks by an approved method to identify those cracks before they become critical. (Class I, Urgent Action) (A-87-41)

3. Conduct a Directed Safety Investigation to inspect the lower spar cap and upper skin on other Piper model airplanes that have a similarly configured wing spar structure to that of the model PA-28 airplane. (Class I, Urgent Action) (A-87-42)AD 87-08-08 and Service Letter 997

The FAA published Airworthiness Directive 87-08-08, issued May 5, 1987, only 36 days after the wing separation.

Piper followed with Service Letter 997, issued May 14, 1987, which detailed the importance of proper wing removal procedures.

AD 87-08-08 applied to all PA-28 models, including the PA-28-201T Turbo Dakota. It also included PA-32-260 and PA-32-300 aircraft. Additionally, the AD applied to PA-28R retractable gear versions of the PA-28. The only model excluded was the PA-28-236 Dakota.

Aircraft with less than 5,000 hours total time in service (TIS) had to comply before reaching 5,050 hours; aircraft with more than 5,000 hours TIS had to comply within the next 50 hours of flight time.

The AD required that both wings be removed. One shop owner I spoke with told me than his two-man experienced crew could support the fuselage and remove both wings in 16 man-hours. His shop already had the fixtures to support the fuselage and wings. A shop doing the inspection for the first time would need to build these tools.

Compliance required a visual inspection—using a magnifying glass of at least 10 power—“for cracks in the lower spar cap from the wing skin line outboard of the outboard row of wing attach bolt holes to an area midway between the second and third row or bolt holes from the outboard row.” 

The AD also mandated the use of non-destructive crack detection tools such as the dye penetrant method and eddy current testing to aid in the search for cracks. 

If no cracks were found, the wings could be reinstalled. 

If even the tiniest crack was found, the airplane was deemed to be unairworthy until a new spar or a wing with no spar cracks was installed. 

The upper wing skins were also inspected for cracks. If found, the skin cracks had to be repaired using repair methods acceptable to the Administrator.

AD 87-08-08 did not require further inspections. Apparently, the author(s) of the AD didn’t plan for any future cracking. That assumption was incorrect. 

In an effort to determine the extent of the cracking in the fleet, AD 87-08-08 also mandated that within five days of the completion of each inspection, that all inspection results be sent to the National Safety Data Branch of the FAA in Oklahoma City, Oklahoma.

AD 87-08-08 was rescinded Sept. 28, 1987, less than six months after it was issued. PA-28 and PA-32 owners and operators no longer had to pull the wings to inspect for cracks.

Piper Service Bulletin 886 

On June 8, 1988, Piper Aircraft issued Service Bulletin (SB) No. 886 entitled “Wing Spar Inspection.” (This and a related Service Bulletin are available under “Magazine Extras” on the forums at PiperFlyer.org. —Ed.)

SB 886 divided the PA-28 and PA-32 airplanes into two groups.1

Group I applies to all PA-28-140 through PA-28-181 Archer II; and PA-28R-180 and PA-28R-200 Arrow II aircraft.

Group II applies to all PA-28-235 airplanes; PA-32-260 and PA-32-300; as well as all PA-28R-201 Arrow III, PA-28R-201T Turbo Arrow III and PA-28RT-201 Arrow IV and PA-28RT-201T Turbo Arrow IV aircraft. 


It also includes this sentence: “To date, over five hundred (500) inspections have been accomplished. Only two (2) negative findings were reported on a pair of PA-32s operating in a severe environment and with considerable damage histories.”

The SB directs owners and operators to determine which “usage class” applies to their airplane.

This Service Bulletin provides instructions for:

1. determining the aircraft’s “usage class;”

2. determining the initial and recurring inspection times; and

3. accomplishing the wing spar inspection(s).

Determining aircraft usage class

The usage classes that Piper provides in SB 886 are Normal Usage (Class A), Severe Usage (Class B), Extreme Usage (Class C) and Unknown Usage (Class D).

The SB defines “normal flight training operations” as Normal Usage (Class A). 

Severe Usage is defined as “aircraft which have engaged in severe usage, involving contour or terrain following operations, (such as power/pipeline patrol, fish/game spotting, aerial application, aerial advertising, police patrol, livestock management or other activities) where a significant part of the total flight time has been spent at below one thousand (1,000) feet AGL.”

Extreme Usage is defined as aircraft that have been significantly damaged, such as damage which “required major repair or replacement of wing(s), landing gear or engine mount.”

Unknown Usage is defined as “aircraft and/or wings of unknown or undetermined operational or maintenance history.”

The SB warns owners: “However, if there is any doubt as to the aircraft’s operating history, it is recommended that the initial inspection be conducted in accordance with the UNKNOWN USAGE CLASS ‘D’ Compliance Time.”

Determining inspection compliance times

Once the Usage Class has been defined, it is used to “determine the applicable initial or repetitive wing spar inspection compliance time from TABLE 1.”

The compliance times differ somewhat between Group I and Group II aircraft. Based on engineering studies completed by Piper, all Group I aircraft in the Normal Usage category must have an initial crack inspection at 62,900 hours total TIS. The aircraft must also have repetitive inspections thereafter every 6,000 hours TIS.

All Group II aircraft in the Normal Usage category must have the initial inspection at 30,600 hours TIS, and the repetitive inspections every 3,000 hours TIS. 

Group I aircraft in the Severe Usage category must comply with the initial inspection requirement at 3,700 hours TIS with repetitive inspections every 1,600 hours TIS thereafter.

Group II requirements are initial at 1,800 hours TIS and repetitive inspections every 800 hours thereafter. 

Interpreting this complex bulletin

The catch here is that the Piper SB says that aircraft in the Unknown Usage category should have the wings pulled to complete the initial inspection within 50 hours TIS unless the crack inspection required in AD 87-08-08 has been completed. The repetitive inspection intervals are then based on which Usage Category applies.

What this means for owners is that if the usage history prior to their purchase of the airplane is unknown, this SB, which Piper considers mandatory, requires that the wings be pulled, and the crack inspection completed right now. 

I have purchased the ownership and major repair records from the FAA Aircraft Registry office—available to all owners on CD for nothing more than a phone call and less than $20—for all the airplanes I’ve owned. (For a link to the online request form, see Resources. —Ed.)

But these records only hint at how each airplane was flown prior to my ownership. Unless past ownership and flight conditions are known, an airplane is automatically in the “Unknown Usage” category. 

The smoking guns

In 1987, William Johnson was an A&P mechanic and station manager for Yute Air in Dillingham, Alaska. Johnson, in addition to holding an A&P, also holds an Inspection Authorization (IA), and is an Airline Transport Pilot (ATP) with over 20,000 Alaska flying hours. 

During my annual trip to the Alaska Airman’s meeting at Anchorage International Airport (PANC) in early May 2018, Johnson told me that every PA-32 that he and his crew inspected following the 1987 AD had cracks in the suspect area. 

“I sent in over 40 Malfunction and Defect (M&D) reports to the FAA about cracks,” he explained during a recent phone conversation.

At the time of the AD, Yute Air was flying 4 PA-32-300 Cherokee Six 300s. In addition to changing the spars on the company Cherokees, Williams also changed spars on other western Alaska Cherokees. 

Johnson told me that Ray Boyce of the FAA and a representative from Piper Aircraft visited him in Dillingham to view the results of his inspections. Comments from the FAA engineer seem to back up Johnson’s concerns that this will be a big problem.

Inspection methods

Since it takes about 16 man-hours to remove the wings prior to the inspection, an estimate of the costs for wing removal, inspection and reattachment, assuming a $75/hour shop rate, would probably start at $3,000. This unexpected cost could be daunting to many owners.

What if there’s a way to do the inspection without removing the wings? 

I wondered if an inspection panel could be cut that would expose the inspection area, so I asked this question of Paul New of Tennessee Aircraft. New is very experienced with structural repairs of Piper and Cessna aircraft. 

New said there’s no way to cut an inspection hole to perform the inspection.

However, in an Investigative Update to the original NTSB Preliminary report (ERA18FA120) on the April 4 wing separation, a second Embry-Riddle flight training PA-28R-201 Arrow (Serial No. 2844135) was inspected using an eddy current inspection (ECI) method. Wing cracks were found. The second airplane had 7,661 hours TIS. 

It was further reported that the cracked wings were reinstalled and subsequently inspected using a new ECI inspection procedure developed by Piper Aircraft. The new method, utilizing a bolt-hole probe inspection technique, was able to confirm the location and size of the previously identified cracks.

Based on the reported success of the bolt-hole ECI inspection, this method may be the fastest and least expensive method to inspect for wing cracks. Additionally, eddy current inspections are much more effective than dye penetrant inspections.

In every AD, there’s a paragraph saying that users are encouraged to submit alternate methods of compliance (AMOC) to solve the condition cited as causing the AD. The question now is whether there is a company that could create an eddy current inspection—or other definitive inspection technique that doesn’t require wing removal—for approval as an AMOC. Due to the equipment required and the possibility of stressing the wing structure during removal and installation, removal of the wing is the least desirable inspection mode.


Liability concerns

In another classic case of “be careful what you pray for,” and, “behind every survivor is a lawyer,” any change to an airplane structure runs into a liability glitch. 

In 1987, Cessna had stopped the manufacture of piston-powered airplanes as a statement to Congress; calling for the need for legal protection from enormous losses in civil courts. A common practice in aviation civil lawsuits to “name everyone who has deep pockets and has ever touched the airplane” meant Cessna was always listed as a defendant—even though it had not touched the subject airplane for decades, in some cases. 

Piper Aircraft did not stop production, but the effects of high product liability costs contributed to the company closing its Vero Beach plant in 1990 and declaring bankruptcy in 1991. 

The entire industry breathed a huge sigh of relief and celebrated Jan. 25, 1994, when Congress enacted Senate Bill 1458, also called the General Aviation Revitalization Act (GARA) of 1994. The bill was enacted to “amend the Federal Aviation Act of 1958 to establish time limitations on certain civil actions against aircraft manufacturers, and for other purposes.”

Cessna resumed production in 1996—a full nine years after it stopped production of piston-powered GA aircraft. 

The most important provision of GARA was the implementation of an 18-year window of responsibility by light airplane manufacturers. This “Statute of Repose” excused Cessna (and Piper) from involvement in civil suits in accidents involving airplanes that had left the factory more than 18 years before.

However, Paragraph 2 of the Act is worded that if Cessna (or Piper, or any other light airplane manufacturer) creates a significant change and mandates the installation of that change on an airplane outside the 18-year window, the company is again subject to civil lawsuits during a new 18-year window. 

In my opinion, installation of a new wing spar or installation of an airworthy used wing does not constitute a change that would reopen the statute window.

The takeaway

So that’s where Piper and the owners and operators of its PA-28 and PA-32 aircraft stand today. According to Piper SB 886 and 978A, if owners and operators can’t determine the usage history, a crack inspection should be performed immediately. 

If cracks are found, and the aircraft is returned to airworthy status by installing a used wing, that used wing must be inspected for cracks prior to installation. 

Given the gravity of a failure, and that 7,000 hours was the tipping point for the two spar failures, and that a well-maintained airplane was found to have cracks at around 7,000 hours, I strongly suggest that Piper Flyer owners start to budget for this inspection. I expect a new AD will be issued mandating inspection for cracks. Hopefully, a low-cost inspection will eliminate the need for wing removal and reinstallation to determine if cracks exist.  

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 .

1 Service Bulletin 978A was issued Aug. 6, 1999 and includes serial numbers not manufactured in when SB 886 was issued. A note on page 2 of Piper Service Bulletin No. 978A reads, “This Service Bulletin is similar to Service Bulletin 886 issued June 8, 1988 with the identical purpose and has been released to add models and serial numbers not manufactured at the time of the original Service Bulletin.”


NTSB Safety Recommendation
(A-87-40, -41, -42)


Piper Aircraft Service Letter No. 997, “Wing Removal and Reinstallation”
Piper Aircraft Service Bulletin No. 886, “Wing Spar Inspection”
Piper Aircraft Service Bulletin No. 978A, “Wing Spar Inspection”
PiperFlyer.org/forum under “Magazine Extras”


Request Copies of Aircraft Records


Piper Wing Spars Explained - Video
Lock Haven Low Wings Type Certificate Data Sheets (TCDS) for Dummies

Lock Haven Low Wings Type Certificate Data Sheets (TCDS) for Dummies

Are you familiar with the wealth of information contained in your airplane’s TCDS? You should be! 

What, you ask, is a Type Certificate Data Sheet (TCDS)? If you are an owner, you need to know. If you are thinking about becoming an owner, you also need to know. The TCDS is something that IAs must to be aware of, but do not always properly utilize. Every certificated airplane has one, save for some antiques. 

The TCDS sets forth some of the critical parameters that the aircraft must meet in order to be considered airworthy. Before about 1960, the data sheets were called “Aircraft Specifications,” but the format is basically the same. The only exception to compliance with the TCDS is if the aircraft has had one of the TCDS particulars modified by a Supplemental Type Certificate (STC) or a field approval. 

What does “airworthy” mean?

Let’s take a quick detour and discuss what is meant by the term “airworthy.” You will not find it in FAR Part 1, which contains definitions for terms used in the FARs. In 2005, the FAA added the definition in FAR 3.5(a), which is likely broadly applicable, even though it could be read as being limited to that section:

“14 CFR §3.5(a) Definitions. The following terms will have the stated meanings when used in this section: Airworthy means the aircraft conforms to its type design and is in a condition for safe operation.”

The interpretation that this 3.5(a) definition should be read as the operational definition of “airworthy” for all purposes is supported by ICAO’s Annex 8 and several NTSB decisions which essentially give the same two-part definition. (ICAO, the International Civil Aviation Organization, first adopted Airworthiness of Aircraft standards (Annex 8) in 1949. —Ed.)

So, to be airworthy, the plane must conform to its type design as set forth by the manufacturer or as properly modified with an STC or a field approval, and it must be safe for operation. This article addresses the first requirement. 

Repairs must be made to certain specifications for an aircraft to adhere to the type design and thus remain airworthy. Changes that constitute a major repair need to be done with approved data. “Approved data” can be in accordance with FAA literature such as Advisory Circular 43.13-1B, the manufacturer’s structural repair manual or another FAA-approved source.

Diving into a TCDS

The starting point for determining whether an aircraft conforms to its type design is the TCDS. This article is limited to examining this important document. A TCDS can be just a few pages or it can be a very lengthy document. 

As an example, we will look at the TCDS for the Piper PA-28 series. (See Resources for a link to the FAA Regulatory and Guidance Library, where you can access the TCDS database. —Ed.)

A TCDS is generally divided into four parts. The first part is the specifications for each different model. The second is titled “Data pertinent to all models.” The third section is: “Equipment.” The fourth section is the notes.

All the PA-28 series share the same TCDS. There are 22 individual models covered by TCDS 2A13. The format for the specifications of each model is pretty standard. (See Part 01, Page 23.) The heading tells you Piper’s designation for the model—in this case, PA-28R-200—and the serial number range covered. This happens to be the Arrow II. 

Because it differed sufficiently from the previous years, the Arrow II got a new model specification. The changes made from the previous model are also listed in the header. 

Of particular interest are the engine and propeller options available. Most owners and many mechanics don’t realize that you can change between TCDS-listed propellers or engines without an STC or field approval. If the option is listed in the TCDS, then it is already approved—and the most that might be necessary is to obtain the installation drawing, if different, from Piper.

The next section contains the data that applies to all models of the PA-28. (See Part 02, Page 24.) Of most significance is the certification basis, which in the case of the PA-28, is different for each model, notwithstanding that the list is in the section that supposedly applies to all models. 

In addition to listing the regulations forming the basis of certification, it will also give the amendment date. For the PA-28, most are a mixture of CAR 3 and FAR Part 23. 

While all PA-28 aircraft are basically CAR 3 aircraft—even ones built today—the more recent the model, the more additional FAR Part 23 requirements have been added. As the models were produced over the years and especially when changes were made, the FAA would often impose additional requirements. That is how a CAR 3 aircraft might have some FAR Part 23 sections required as well. 

Many owners and pilots are not aware that there are two bodies of certification regulations that apply to common GA aircraft. Even many mechanics are really only aware of FAR Part 23, and may incorrectly assume that it applies to all aircraft. I have seen more than one maintenance bill for fixing something to comply with the wrong regulation. 

A common example is the requirement for the level of fireproofing of seat covering material. The older certification regulation is Civil Air Regulation Part 3, abbreviated CAR 3. In general, CAR 3 does not require burn certifications. Here’s why you must read carefully: the section of FAR 23 that requires burn certification has been added to the basis of some aircraft designs that were originally all CAR 3. 

It is good to know what actually applies to your model and serial number so that you can understand whether the maintenance personnel are applying the correct standards during annual inspection or repairs.

The next section pertains to the equipment. (See Part 03, Page 24.) For older aircraft that use Aircraft Specifications instead of a TCDS (like the PA-24 Comanche series), there is a list of individual pieces of equipment, engine options, etc. This can be detailed and require some level of approval to deviate from what is listed. While usually a PMA part or STC will cover that requirement, occasionally it requires a field approval.

Also in this equipment section is a listing of the AFM/POH required to be in the aircraft. It is worth checking to make sure you have the correct AFM/POH, as it is not uncommon to find them missing and some generic pilot’s guide in its place.

Lastly comes the notes. (See Part 04, Page 24.) The notes can cover a wide range of topics, some germane to a particular model and some more generally applicable. Important ones for the IA and owner are those dealing with required placards. You would probably not be shocked at the number of placards that disappear over the years.

Why is this information important?

For the owner, the TCDS for their aircraft is worth reading through at least once. 

It is also a source of good information when researching a model to buy. For example, sales ads often misstate the model year, believing that model years start January 1. Like in the auto industry, the beginning of a model year can vary, but is usually sometime in the fall. 

Knowing what the basis of airworthiness is for your aircraft better prepares you to understand the requirements to be a more active participant in the care and feeding of the plane.

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 is a recognized authority on Piper Comanche aircraft. Currently she is serving as Director of Operations for a commuter airline in Southeastern Alaska. Send questions or comments to .


Type Certificate Data Sheets for Piper aircraft
under “Databases” on right, select “Type Certificate Data Sheets” and search “By Make (TC Holder)”
Installing Shoulder Harnesses

Installing Shoulder Harnesses

Adding shoulder harnesses in at least the front seats should be a must for any aircraft that does not have them. A&P/IA Kristin Winter reports on the recent installation of a three-point system on an aircraft that previously had only lap belts.

Before the early 1970s, Piper did not provide shoulder harnesses for its aircraft. In fairness, that was only slightly after the auto manufacturers did the same thing. 

Lack of shoulder harnesses have resulted in life-altering brain injuries from accidents in which the front seat occupants could have walked away virtually unharmed. Fortunately, a number of companies have STCs for the retrofitting of shoulder harnesses in Piper aircraft. 

Several retrofit options

The largest manufacturer of restraint systems is AmSafe. It supplies much of the OEM market, be it a GA manufacturer or a commercial aircraft manufacturer like Boeing. 

The only product AmSafe currently offers as a GA retrofit is its seatbelt airbags. (Winter has a set of seatbelt airbags she plans to install in her Twin Comanche, and will report on that project in a future issue of Piper Flyer. —Ed.)

The largest holder of shoulder harness STCs for Piper aircraft is Alpha Aviation in Minnesota. Alpha Aviation has STCs covering early PA-23, PA-24, PA-28, PA-30, PA-32 and PA-39 aircraft. 

B.A.S. in Washington has an STC for the PA-28/32/34. The kit offered by B.A.S. is a four-point shoulder harness/lap belt system. 

Aero Fabricators, a company affiliated with Wag-Aero, has several STCs as well. Wisconsin-based Wag-Aero offers kits for J-3, PA-11, PA-18 and PA-20/22 aircraft, as well as PA-28s. 

Univair in Aurora, Colo. markets AmSafe shoulder harness restraint systems for J-3s, PA-11s, PA-12s and PA-18s.

A brief look at the kits

Most of the STC kits have two sets of components. One is the belts and fittings. 

Some shoulder harnesses have inertia reels, and some include a fixed belt. The latter type generally costs less, but can be a bit less convenient for the pilot when he or she has to loosen the belt in order to reach something in the cockpit. 

The other set of components in a kit is whatever is required to provide the necessary structure in order to mount the shoulder harness with enough strength to provide the necessary protection to the user. 

This typically involves some reinforcements that need to be attached to the fuselage, usually involving riveting. For that reason, this is not a project for an owner alone unless he or she possesses an A&P license and the tools necessary for the job. 

In addition, a shoulder harness installation constitutes a major alteration, and requires that an A&P/IA inspect and sign an FAA Form 337.

Required tools and supplies

The tooling necessary to complete this project will vary a bit depending on what structure is required. Some airplanes may already have the structure installed because shoulder harnesses were an optional item for that particular model year; others may need quite a bit of reinforcing pieces installed in order to provide the necessary support. 

Regardless of the kit, the tools necessary for installing solid rivets—and possibly blind rivets as well—will be a necessity. 

Any practicing A&P is likely to have the necessary tooling, but for an owner that is interested in participating and wants his or her own tools, the major item is a 2X rivet gun. U.S. Industrial Tool, Sioux Tools and Chicago Pneumatic are just a few of several rivet gun manufacturers. 

The most important part of the rivet gun is a good “teasing” trigger that lets the operator control the force and frequency of the blows. The gun also needs a rivet set, which is the part that actually touches the rivet, and a spring retainer to hold it to the rivet gun. 

In addition, a selection of Cleco temporary fasteners and pliers will also be necessary. 

It goes without saying that the ability to drill holes will be key. A #30 drill bit is used for a 1/8-inch rivet and #40 is for a 3/32-inch rivet. There are some excellent YouTube videos done by EAA on the basics of sheet metal work. (See Resources for additional information. —Ed.)

This is a project that any owner with mechanical aptitude can tackle with supervision by an A&P. Doing so will be a great learning experience for those interested in understanding more about what is involved in aircraft maintenance.

A Comanche 250 project

Recently I participated in, inspected and signed the Form 337 on the installation for a 1959 PA-24-250 Comanche which had never had shoulder harnesses installed. The new owner was keen to have the safety advantage of shoulder harnesses. 

The Alpha Aviation kit for the Comanche 250 was very complete and of excellent quality. (See photo 01, page 22.) The kit included all required parts and hardware including restraints for two front seats, an 8130-3 Airworthiness Certificate, an installation manual and a copy of the STC and signed STC authorization. 

First steps

The first step is to gently remove the headliner from the area to provide access to the structure above the rear window. (See photo 02 on page 24, top.) Removing the headliner can be a challenging project and needs to be undertaken carefully to avoid damaging the headliner.

The structural portion of the kit consisted of a stringer and two doubler plates. The two doubler plates are riveted together with a carefully-laid-out pattern, and to this doubler plate assembly is mounted the attachment point for the inertia reel for the shoulder strap. Then a longitudinal stringer and the assembled doubler plate must be fitted and riveted to the airframe above the rear window. 


Measuring and positioning 

Careful measurement is key to making sure that the stringer and doubler are properly positioned. This is done by riveting the bottom of the assembled doubler to the existing stringer that runs above the window, as shown in photo 03 (page 24, bottom). Note that blind fasteners were used here. 

The installation of the assembled doubler sets the position for the new stringer, which runs from the door frame back to the frame at the back bulkhead. 

The photo below shows the assembled doubler and the stringer fitted to the aircraft and held in with spring sheet clamps, referred to colloquially as Clecos. Clecos have been used since before World War II and are indispensable for aircraft sheet metal work. The most common type require a special set of pliers to install and remove them. 


Clecos are color-coded based on the size of the hole they are designed to fill, and are used to pull tight the two sheets of metal. As mentioned earlier, there are several good videos available from EAA that cover Clecos and other basic sheet metal techniques and tools. 

The temporary fastening process

As is good practice, the initial holes were drilled to a smaller size, in this case 3/32 inch, which later were enlarged to 1/8 inch as called for in the instructions. This technique works to clean up any shifting that takes place so that each hole is reasonably precise. 

Once the structure is fitted and all the holes are drilled, the parts are removed in order to clean off the burrs and excess material from around the rivet holes. The parts are then reassembled and held in place with the Clecos.

Anchoring with rivets, and final steps

Solid rivets are generally preferred and more economical, but are not always practical, and the kit from Alpha Aviation provides both solid rivets and blind CherryMax rivets. 

CherryMax rivets are souped-up pop rivets made to an aerospace standard and are designed with a locking collar to fasten the stem into the rivet as the stem forms much of the strength of the fastener. 

CherryMax rivets are used when you can’t get a bucking bar to the back of a solid rivet. They can be seen in photo 04 (below) as they are used to attach the lower side of the assembled doubler to the aircraft’s existing stringer. CherryMax rivets have a number of special pullers that can be used to install them; some are hand-operated and some are pneumatic. 

Once all the rivets are installed, a strong support base has been created to anchor the inertia reel and shoulder harness, as shown in photo 05 (below). 

Reinstallation of the headliner, carefully cutting a hole for the bolt, and subsequently bolting on the inertia reel completes the installation—save for the log entry and completing the Form 337. 


Labor may vary; added safety will not

The labor necessary to install the shoulder harness kit varies with the amount of structure that must be added. It can take anywhere from a handful of hours to a couple of days’ worth of labor, but shoulder harnesses are most needed in the kind of accident that can happen to even the best pilots. 

No restraint will help if you hit a mountain at cruise speed—but landing mishaps, loss of runway control, or even a controlled glide into favorable terrain after an engine failure are more common. These scenarios are where a shoulder harness might make all the difference. 

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

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 .


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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


New PMA fuel cells and fuel cell repair
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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 .


Leakguard butyl inner tubes by Aero Classic
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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.

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