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A Step-by-Step Guide to Overhauls

A Step-by-Step Guide to Overhauls

 

Most engines are “sent out” to specialty shops for overhaul. Peek behind the doors at Triad Aviation as author Jacqueline Shipe guides you through engine overhaul procedures.

The single biggest repair expense most airplane owners will ever face is an engine overhaul. Overhaul costs increase every year along with parts prices. The engine overhaul process has become somewhat of a specialized procedure. Most mechanics won’t consider overhauling an engine themselves. The engine is typically removed and sent out for overhaul. 

 

When is an overhaul necessary?

The first step in the overhaul process is determining that an engine does in fact need an overhaul. Mere time since the last overhaul doesn’t always equate to needing to overhaul an engine. Part 135 operators must legally comply with engine manufacturers’ recommended times between overhauls. However, the only legal requirement for everyone else is engine condition. 

An engine that is run regularly (at least once a week) with cylinders that have good compressions with no exhaust valve leakage is a good candidate to keep running. Regular oil changes must consistently demonstrate that no excessive metal is being produced by the engine. Such an engine can safely and legally go beyond the manufacturer’s recommended time between overhauls (TBO). 

Cylinder issues can be resolved by replacing the affected cylinder, or by completing a “top” overhaul and replacing all the cylinders.

So, what might indicate it’s time for an overhaul? Excessive amounts of metal that have been determined to be coming from the bottom end parts (camshaft, lifter bodies, gears or crankshaft bearings) is one sign. If an engine has crankcase cracks that are outside allowable limits, it’s time. If an engine has problems producing its rated power even though cylinder compressions are good and fuel and ignition systems are within limits and working properly, an overhaul is likely needed in the near future.  (For more, see “Is Your Engine Worn Out?” by Steve Ells in the October 2017 issue of Piper Flyer. —Ed.)

 
The overhaul process

An overhaul always includes a complete disassembly of the engine, thorough cleaning and inspection of parts, repair of parts as needed and disposal of defective parts. 

Major items such as the crankshaft, crankcase and connecting rods are subject to special inspections. 

Parts that are subjects of Airworthiness Directives or Service Bulletins are typically replaced or repaired in accordance with the steps outlined in the AD or bulletin. 

Parts are measured for excessive wear and proper clearances. The allowable dimensions and clearances are given in the manufacturer’s overhaul manual in two separate columns; one for manufacture (new) limits and one for service limits. The service limits are larger and allow for looser fits than manufacture limits. Some shops rebuild engines based on manufacture limits, while others use service limits. 

 
Crankshaft

The crankshaft is arguably the most important component in an aircraft engine. It absorbs the force generated by the reciprocating strokes of the pistons and rods and transforms it into rotational force for the propeller. The crankshaft is continuously subjected to loads and stresses from engine operation and the rotating propeller. Cracks or defects on a crankshaft can cause sudden engine failure or excessive, premature wear on the bearings. As a result, the crankshaft is probably the most inspected, measured and scrutinized part in the entire engine during the overhaul.

After engine disassembly, the crankshaft is cleaned and degreased in a chemical vat, dried and inspected. Most shops have a Magnaflux machine to inspect the crankshaft for cracks. 

The crankshaft is clamped between two copper-plated pads and an electric current is sent through the crankshaft to magnetize it. 

The crankshaft is then coated with a fluorescent solution containing magnetic particles. If there is a fracture in the crankshaft, the magnetic particles will align along the edges of the fracture. The fluorescent solution makes cracks easy to see under a black light. 

Once the magnetic particle inspection is complete, the crankshaft is cleaned again, and each journal is polished. Some shops have a machine that spins the crankshaft while a polishing rag is held stationary on one journal at a time with a special tool. Other shops use a machine with a circular cloth that is spun around each journal. The polishing process removes light scoring and surface corrosion as well as providing a clean journal surface so that good measurements can be obtained of each journal. 

Excessive scoring or pits caused by corrosion that cannot be removed by polishing the crankshaft can usually be removed by grinding off a specified amount of material. The manufacturer sets the sizes to which the crank can be reground, and it varies based on the engine model. Most Lycoming crankshafts can be ground to three-thousandths, six-thousandths or ten-thousandths of an inch undersize. Continental usually allows five-thousandths or ten-thousandths undersize. 

Once the crankshaft has been ground down to limits (referred to in the field as “ten under”), any further scoring or pitting defects in the journals will most likely result in the crankshaft being scrapped at the next overhaul. Reground crankshafts require oversize bearings to maintain proper clearances.

When all the machining and polishing processes are complete, the diameters of the main bearing journals and connecting rod bearing journals are measured with a micrometer at several points around the circumference of each journal. The smallest measured diameter is used to determine if each journal is within limits. 

The inside diameters of the connecting rod and crankcase main bearings are measured by installing the bearings and temporarily installing the bolts and nuts, securing the case halves and connecting rod halves together. A telescoping gauge is then used to measure the inside diameter of the bearings. Clearances are obtained by subtracting the journal diameter from the bearing internal diameter. Clearances must fall within the limits set by the manufacturer.

The crankshaft is also measured for straightness (or run-out) using a dial indicator. The crankshaft is placed in a holder that supports the crankshaft while still allowing it to rotate. A dial indicator reading is then usually taken on the rear main journal as well as the crankshaft flange. The readings must not exceed allowable limits. 

It is a fairly rare occurrence when a crankshaft is rejected. Aircraft crankshafts are constructed with high-quality metals at manufacture and, barring misuse or a prop strike, generally pass inspections through multiple overhauls.

If the crankshaft needs to be replaced for any reason, it adds a significant amount to the cost of an overhaul. Some shops try to help owners by finding a serviceable used crankshaft, which is usually one-half to one-third the cost of a new crankshaft. 

 
Crankcase

The crankcase provides the housing to hold all the internal components (crankshaft, camshaft, rods) as well as providing a place to attach the cylinders, accessory case and oil sump. The crankcase is made of cast aluminum and must be strong enough to absorb all the opposing forces of the engine as it is in operation. 

Crankcases receive a thorough cleaning and inspection at overhaul. 

Some shops use abrasive media to clean the case and some use a chemical vat. Chemical-only cleaning processes are preferred because residue from blast material is difficult to remove from all the creases and recesses in the case. Any leftover media causes scratching and scoring once the engine is placed back in operation. 

Crankcases are inspected for cracks using a dye penetrant inspection. The case is saturated in fluorescent colored penetrant, then rinsed. The penetrant seeps into cracks making them easily seen once the case is sprayed with developer or examined under a black light.

Some cases are more prone to cracking than others. As an example, Lycoming “narrow deck” cases crack far more often than the thicker “wide deck” cases. Narrow deck cases utilize cylinders that have a thinner hold-down flange. The cylinder base nuts are Allen head (internal wrenching) types; while the wide deck cases have cylinders with thicker hold-down flange with standard six-sided nuts. Cracks can sometimes be welded and repaired depending on their location. 

Cases can have fretting damage or small areas of corrosion where the case halves are joined, especially near through-bolts. Cases with damage are generally sent to specialized machine shops such as DivCo or Crankcase Services to have the mating surfaces machined smooth. Some shops “line bore” the center bearing areas so that the crankshaft main bearings are perfectly straight and aligned with the each other. 

Regardless of whether the case is simply cleaned and inspected or sent out for further machine work, the mating surfaces of the case halves must be smooth and perfectly flat to ensure a proper seal once they are assembled. A silk thread is used to seal the case halves along with a special non-hardening compound designed to hold the thread in place as the case halves are assembled. Any irregularities in the mating surfaces will result in case leaks. 

Crankcases, like crankshafts, are expensive to replace and can add significantly to the cost of an overhaul if replacement is required. 

 
Connecting rods

Connecting rods are Magnafluxed, cleaned and dimensionally checked at overhaul. Connecting rod bearings along with the bolts and nuts that secure the rod halves are always replaced at overhaul. Connecting rod bushings are not always replaced, depending on the wear and condition on the bushings. 

The rods are checked with special dowel tools to be sure they aren’t bent or twisted. The connecting rod is turned sideways and held in a vertical plane. One dowel slides through the connecting rod bushing and the other through the crankshaft bearing. After they are inserted, the ends of the dowels are laid on perfectly-matched metal blocks. The four ends of each dowel pin should lay perfectly flat if the rod is not twisted at all. 

The dowels are left in place and a special gauge is attached to the end of the crankshaft bearing dowel. This gauge telescopes and it is extended until it touches the end of the shorter connecting rod bushing dowel.

After this measurement is made, the gauge is removed and placed on the opposite end of the crankshaft bearing dowel. If the rod is square and not bent, the gauge will line up and touch the short dowel on the opposite side without being extended or shortened.

 
Camshaft and lifters

The camshaft and lifter bodies are generally replaced or sent out to be reground to remove any light scoring marks or surface deformities. The camshaft lobes go through a carburizing process to harden them at manufacture. The depth of the carburized layer of metal is not very deep (about fifteen-thousandths of an inch) and it is possible for machine shops to accidentally grind below that layer. The camshaft lobe would wear down rapidly once placed in use if that happened. Additionally, the lobes are not only elliptically shaped, but they have a slight taper across the top of the lobe to ensure that the lifter body spins as it contacts the lobe. It takes very precise machine work when grinding the lobe to maintain its original shape and the taper across the top. Camshafts should only be sent to high-quality, experienced machine shops like Aircraft Specialties for machining work.  

Camshafts are not terribly expensive when purchased new (compared to major parts like crankshafts or cases). Typically, the cost of buying a new camshaft and all the lifters is only a few hundred dollars more than having the old ones reground. 

(For more on camshafts and lifters, see Jacqueline Shipe’s July 2017 article in Piper Flyer. —Ed.)
Accessory case, oil sump, gears

The accessory case and oil sump are typically cleaned, inspected and reused. The Lycoming oil sumps that have intake pipes routed through the sump are reswedged around the intake pipe end to ensure there are no leaks down the road. This involves using a special tool which swells the pipe back out a little so that it forms a better seal when it is inserted into the sump opening.

The accessory case is inspected with dye penetrant and cleaned. The gears in the accessory case are cleaned, Magnafluxed and reused. 

 
Cylinders

Individual cylinder assemblies can be overhauled, but by the time the valves, guides and seats are replaced, the cost is almost equal to the cost of a new cylinder. Most overhaul facilities that I’m familiar with install new cylinders rather than overhauling the old ones. 

The cylinder must absorb the heat and pressure of combustion every time it completes a cycle while in operation. Metal fatigues over time and with a relatively low cost difference between new and overhauled cylinders, new cylinders are the best choice for long-lasting operation. They also typically come with their own warranties, so shops like them. 

It’s important to note that there is no logbook tracking for individual cylinder assemblies. Times in operation are kept of engines, but not of the individual engine parts. Therefore, it is impossible to really know how much operating time cylinders have on them when purchasing overhauled cylinders outright. The times that are on the existing installed cylinders on an engine can be difficult to trace unless they were new at the time of installation. 

 
Fuel system

The fuel injection system or carburetor is generally sent out for overhaul at a specialty shop or replaced with a new unit. Very few overhaul facilities overhaul the fuel system components in-house. Even Lycoming gets all the fuel injection system components and carburetors for both their new and rebuilt engines from Avstar Fuel Systems in Florida. 

 
Accessories and other items

All other accessories are typically sent to specialty shops for an overhaul or are replaced with new. Magnetos, ignition harnesses and vacuum pumps are generally replaced with new units. Alternators and starters are generally rebuilt. 

Oil coolers should always be sent out for specialized porting and cleaning to be sure all metal particles and sludge buildup is completely removed. The oil passages through the coolers make several 180-degree turns. Small metal particles and contaminants build up in the coolers around the curves and it is impossible to remove all the debris with just a simple flushing. Oftentimes, new oil coolers are fairly inexpensive, and it is easier and cheaper to simply replace them rather than overhaul them. 

All hoses should be replaced at overhaul. Hoses deteriorate with age and exposure to heat, and should be replaced periodically. New hose installations also help prevent contaminating the freshly overhauled engine with any sludge or debris remaining in the hose. 

It’s also a good idea to replace all the SCAT hoses. Most of the tubing (like the aluminum oil return lines) is cleaned, inspected and reused.

 

Choosing an overhaul facility

Engine overhauls are extremely expensive. When it’s time to overhaul an engine, choosing a high-quality facility to do the job is important. The best way to choose where to send an engine is usually by personal referral. Ask other owners what shop(s) they have used and what the long-term results have been. Owners or operators that have put three to five hundred hours on an engine usually know by that time whether the overhaul was a good one. Low cylinder compressions, oil leaks or other problems are signs that the overhaul may not have been the best. 

Most Part 91 owners only have to face an engine overhaul once. The process can be stressful to go through. Owners who do lots of research ahead of time, understand the process and ask lots of questions can help to avoid major problems down the road. 

 

 

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


RESOURCES >>>>>


ENGINE OVERHAULS
Airmark Overhaul
airmarkoverhaul.com

Granite Air Center
graniteair.com

Poplar Grove Airmotive
poplargroveairmotive.com/engine-shop


RAM Aircraft
ramaircraft.com


Triad Aviation
hhtriad.com

CRANKCASE INSPECTION / REPAIR
Aircraft Specialties Services
aircraft-specialties.com


Crankcase Services, Inc.
crankcaseservices.com


DivCo, Inc.
divcoinc.com


FUEL SYSTEM OVERHAUL
Avstar Fuel Systems, Inc.
avstardirect.com

 
Aircraft Accessories of Oklahoma
aircraftaccessoriesofok.com

Airplane Maintenance for the DIYer: First Steps

Airplane Maintenance for the DIYer: First Steps

Owning an airplane is usually the result of years of hard work and planning. For many, it is a fun and rewarding experience—the fulfillment of a lifelong dream. 

Although airplane ownership is a big source of joy, it can also be an expensive and responsibility-filled endeavor. In fact, cost is the number-one concern that most pilots have when it comes to owning a plane. 

The initial purchase price of a plane is only one part of the equation. Insurance, fuel, storage, maintenance, avionics upgrades and any updates to the paint or interior can add up to be far more than the initial purchase price over a period of time.  

One way to lower the operating costs is to be actively involved in your plane’s maintenance. In addition to cleaning the plane, there is a surprisingly long list of maintenance actions that an owner may legally perform on his or her aircraft, provided it is not operated under FAR Parts 121, 129 or 135. 

The benefits of DIY maintenance

There are several benefits for owners who decide to do a lot of their own maintenance. Long-term, it does save on labor costs, although initially there are some expenses for tools and supplies. 

In addition to the cost savings, working on a plane gives a person the opportunity to get a better understanding of how different systems operate and how things on their aircraft are put together. This translates into a better understanding of the readings on the gauges in the cockpit and may allow the pilot to detect potential problems more quickly. 

Owner-performed maintenance also helps a pilot know how to operate the plane in a prudent manner that is easy on mechanical items. 

Owners also typically aren’t as pressed for time as mechanics working in a shop. This means that they can take the time to address cosmetic issues as well as maintenance issues. Little things like repainting removed items, fixing cracks in plastic or fiberglass trim pieces, or replacing rusted panel screws with stainless ones not only makes a plane look better, it adds to the resale value.

Preventive maintenance 

FAR 43 Appendix A, section (c) lists the maintenance tasks that an owner with a private pilot certificate is allowed to do and legally sign off. These all fall under the category of preventive maintenance, and the list is pretty extensive. 

A few of the items listed include tire changes, landing gear strut servicing, greasing wheel bearings, oil changes, fuel strainer cleaning, replacing or servicing the battery, and (with the exception of the control surfaces) even repainting a plane. 

Although these tasks are legal to perform, some of them are a little complicated, and the consequences if a mistake is made are high. Specialized tools and maintenance manuals are required for a number of the procedures. 

It is best for owners who decide to tackle some of these maintenance tasks themselves to pay a mechanic to show them the ropes for the first time. It is also a good idea for any owner to buy the latest revision of the parts and service manual for the specific make and year model of the plane he or she owns. 

Even folks that aren’t interested in maintaining their planes themselves can still benefit from a parts and service manual so they may look up part numbers, compare parts prices and have the information available in case the mechanic they work with doesn’t have it. (Mechanics have extensive libraries, but it is nice to supply them with complete paper copies that are easy to access.)

 

The necessary tools

In addition to the manuals, there are a few tools that are required for preventive maintenance. Most folks already have a general tool set for home use. The same items needed for tinkering on a car are needed for a plane: socket and wrench sets, screwdrivers, etc. 

A good ratcheting screwdriver that has separate bits works well for removing panels. The DeWalt brand Phillips drywall bits are great for removing stuck screws because the end is rounded so that more of the bit sinks into the screw head, making it easier to break the screw loose and less likely to round out the head. 

Screw guns really speed things up, but aren’t a necessity. A 7/8-inch socket made just for aviation spark plugs is nice to have also, and can be purchased from almost any aircraft parts distributor. 

Safety wire pliers and a can of .032 inch safety wire are handy to keep around. The oil filters and most of the bolts that require safety wire utilize this size. The pliers vary in price—from over $200 for high-quality ones, to around 20 bucks for a cheaper set. 

The better quality pliers are designed so that the teeth won’t gouge into the wire and weaken it as the pliers are clamped down. Safety wire can be twisted by hand; it’s just a little more difficult to do if you’re working in a tight place. 

 

Jacks and a tail weight

The biggest equipment investment that an owner who is really serious about maintaining his or her plane might want to consider investing in is a set of jacks. 

Low-wing planes and planes with retractable landing gear all need to be completely raised on jacks periodically for gear servicing or tire changes. Jacks can range in price from around $300 to a couple thousand dollars per jack, depending on the style and quality. 

The better ones have a long metal tube that slides over the hydraulic piston. This tube has holes drilled in slight increments along the length to allow a safety pin to be installed to prevent the jack from accidentally lowering if it loses hydraulic pressure. This type of jack is the safest, but it is a little more expensive than others. 

In addition to two jacks, a tail weight will be needed. These are fairly easy to make with some steel tubing and an old galvanized tub filled with concrete. The jack manufacturers also sell tail weight kits that are easy to assemble and fairly inexpensive; one just has to be sure the weight is heavy enough to counterbalance the heavy nose as the plane is lifted. 

Any time a plane is jacked, use caution to ensure it is being raised evenly on both sides. If the work is being done outside, make sure the wind is not forecast to get too high. Significant damage can occur to a plane if it falls off a jack.

Logbook entries

After purchasing the proper tools and manuals, and with a little guidance, a person is well on his or her way to performing a variety of preventive maintenance items. 

Once a particular task has been completed, a logbook endorsement should be made stating the date, tachometer time, a description of what was done and the reference material that was used for completing the task. It is good to also include the part numbers for installed items. 

For example: “September 15, 2014; 2245 tach time; removed and replaced landing light bulb part number GE4554 in accordance with Piper Cherokee service manual; operational check good.” The signature and the pilot certificate number of the person completing the work is what returns the airplane to service. 

Once a pilot gets started working on their plane, he or she may find it almost as rewarding an experience as flying it. The benefits for owners that learn to do a lot of their own maintenance can be well worth the initial investment in tools and materials.

Note: In future issues of Piper Flyer, Jacqueline Shipe will be discussing specific preventive maintenance items step-by-step. 

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 logged over 5,000 hours of flight instruction time. Send question or comments to .

Propeller Vibration and  Dynamic Balancing

Propeller Vibration and Dynamic Balancing

Feeling a bit shaky when you’re flying (and tired when you land)? It might not be your nerves; but rather the side effects of excessive vibration. Vibration can originate from an aircraft’s engine, propeller or spinner, and if unchecked, can lead to further mechanical problems. Vibration can also cause pilot and passenger fatigue. Luckily, computerized dynamic propeller balancing is a cost-effective route to help dampen those pesky vibrations. 

Aircraft propellers are manufactured to be durable, strong and able to absorb the stresses of flight. Depending on the model, propeller blades weigh anywhere from a few ounces to several pounds. The rotational speeds and aerodynamic loads imposed in flight on a propeller make it susceptible to vibration if it has even the slightest imbalance. 
Excessive vibration can come from an aircraft’s engine, propeller, spinner or a combination of all three.

Why vibration matters
No matter what its source, excessive vibration can lead to a number of problems. The engine’s vibration isolators are designed to filter out most of the vibration so that it is not transmitted to the airframe, but they don’t eliminate all of it.
An out-of-balance propeller that causes the engine to vibrate in its mount will wear out the vibration isolators. Cracks in the airframe can form as a result of excessive shaking. Cracks can also form on the cowling itself, and on the spinner or spinner bulkhead.
Vibration can cause cracked or loose exhaust connections. As an illustration of the importance of balance when it comes to exhaust systems, the makers of PowerFlow exhausts actually require a dynamic propeller balance for their exhaust systems to qualify for an extended warranty.
That’s not all. Vibration is hard on engine components and can cause premature engine wear. On top of creating mechanical issues, vibration is also source of pilot stress and fatigue.

Where’s it coming from?
The frequency at which a vibration is occurring gives a clue as to whether the vibration is being caused by the propeller itself or the engine.
Vibrations are counted by the number of vibrations per revolution (of the propeller). They are referred to as “two-per,” “half-per” and so on.  A one-per vibration occurs on every revolution of the propeller and is indicative of the propeller itself, or in rare cases the engine crankshaft. A half-per vibration occurs on every other revolution of the propeller and is usually caused by a cylinder malfunction. A small vibration occurring at a frequency greater than once per revolution (two-per or more) is usually indicative of bearing wear or a malfunction in an accessory such as the alternator.

Causes of vibration: engine and mounts
As noted above, vibration can originate from sources other than the propeller. The engine and engine mounts can be a culprit.
Compression imbalances or a cylinder with excessively low compression can cause vibration. Extreme wear in crankshaft counterweights can also cause vibration.
Worn engine vibration isolators permit excessive vibration, and can allow the front of the engine to sag downward.
A cracked engine mount can cause a great amount of vibration.

Causes of vibration: spinner
Spinners that have a heavy spot due to a manufacturing defect or repair can cause a slight imbalance which produces vibration. Laying the spinner on a flat table and slightly rolling it can sometimes detect a heavy spot on the spinner. If the spinner comes to rest with the same spot on the bottom each time it probably has a heavy spot.  A spinner with a heavy spot can make dynamically balancing a propeller difficult.
The forward tip of the spinner should be aligned with the center of rotation of the propeller. If the nose of the spinner appears to wobble when observed by an onlooker outside of the plane as the engine is run, the spinner should be realigned by loosening the mounting screws and retightening them as the spinner is held firmly in place.
Cracked or broken spinner bulkheads can also cause the spinner to wobble. It is a good idea to inspect them closely if any defect is noted.

Causes of vibration: propeller
Vibration originating from the propeller is usually caused by a mass imbalance. A mass imbalance is when the center of gravity of the propeller is not in the same location as the center of rotation of the propeller. It is usually caused by the removal of material on blades to repair nicks or from differing degrees of blade erosion. Luckily, this can often be remedied by balancing the propeller and checking for correct blade track and indexing.

Static propeller balancing
Aircraft propellers are statically balanced at the time of manufacture and at propeller shops. Static balancing is the process of checking the weight of the hub and blades for even distribution. This ensures that the propeller is not subjected to any turning or bending force due to a heavy area on one of the blades or hub.
During a static balance, the propeller is mounted on a mandrel resting on low-friction bearings so that the propeller is free to spin, with a minimal amount of force needed to move it.


When the propeller is turned slightly, it should remain in the new position it is placed in without backing up or continuing to turn. The process is similar to balancing a wheel assembly.
If an imbalance exists in the propeller or hub, the heavy area will cause the propeller to rotate so that the heavy spot ends up on the bottom. Some shops mount the propeller in a horizontal plane on the top of a shaft that has an indicator rigidly suspended from the bottom of the mounting shaft.


As the propeller is turned, if the indicator on the bottom of the shaft leans to one side rather than maintaining a vertical position, the propeller has a heavy spot.
Weights can be added or subtracted to the hub to statically balance most controllable-pitch propellers.


Fixed-pitch propellers are statically balanced by removing an allowable amount of material from the heavy blade.
Static balance is initially adjusted at propeller assembly and fine-tuned after the propeller is completely assembled and painted. Propellers with de-icing (“hot props”) are adjusted after all anti-ice boots are installed.

Propeller blade track
Once a propeller has been statically balanced and installed on the aircraft, the track of each blade should be checked. The blade track refers to the path that each blade tip travels. On a perfect propeller, the tracks will be identical.
The track is checked by placing a solid object next to a propeller blade near the end so that the propeller blade is free to rotate past it, and marking exactly where each blade tip passes the object. There shouldn’t be more than 1/16 inch in difference between the tracks.
The airplane needs to be chocked so that it can’t move and the propeller should be pushed in slightly against the engine as each blade is checked to remove the endplay from the thrust bearing in the engine. A blade that is out of track will cause an aerodynamic imbalance because its angle of attack will differ from the other blade or blades. Also, differing blade tracks can indicate that the propeller has been damaged in some way.

Propeller indexing
The propeller index refers to the location on the crankshaft flange where the propeller is installed. Engine and airframe manufacturers designate where the propeller should be installed on the crankshaft flange with the No. 1 cylinder’s piston on top dead center of the compression stroke. Typically, on most small, two-blade, fixed-pitch propeller aircraft, the propeller is installed with the top blade aligned with the bolt-hole preceding the vertical position as viewed facing the propeller. This corresponds roughly with the 2 o’clock and 8 o’clock position.
There’s no reason to guess as to what indexing is correct. The maintenance manual for each aircraft model gives the specifications on where to install the propeller on the flange. Propellers installed in the incorrect location on the flange can cause vibration.

Dynamic propeller balancing
Dynamic propeller balancing is the process of checking for vibration while the propeller is in motion. The propeller is installed on the engine and the engine is run through its complete rpm range.
A dynamic balance is performed using a vibration-detecting sensor mounted to the top of the engine, and a photo sensor mounted so that it has a clear view of the rear of the propeller blades. The sensor detects a reflective piece of tape placed on the rear of one of the blades each time it passes through the sensor’s beam.)


The vibration sensor is an accelerometer containing a crystal which detects the direction and amount of force of each vibration. The sensor is calibrated and reads the force in inches per second (IPS). This information, along with the location of the reflective tape provided by the photo sensor, is transmitted to the analyzer.
The analyzer attached to both sensors gathers information about the amount and frequency of any vibration, accurately records rpm and calculates the amount and location of weight to be added to correct an imbalance.


After the engine and propeller run, the specified amount of weight is placed in the location given by the analyzer. The weight is added according to the propeller balance equipment manufacturer’s instructions. Usually, the propeller is rotated by hand so that the reflective tape is lined up with the photo sensor. The number of degrees shown on the screen of the analyzer marks the spot needing the weight. A measurement is made from the accelerometer in the direction of propeller rotation, and the spot is marked.
On airplanes with Lycoming engines, the weight is usually added to one or more of the holes on the outer section of the starter ring. On airplanes with Continental engines, the weight is usually added by drilling a hole in the spinner backing plate. An AN3 bolt or #10 structural screw with a locknut and stacks of large-area washers are used to add weight.

Washers installed for prop balancing

A maximum of six washers is allowed per screw.
It may take several runs and additions and subtractions of weight to eliminate an imbalance, or at least bring it into a reasonable range. 

Sometimes, especially with Lycoming engines, the weight can’t be placed at the location pinpointed by the analyzer because a hole isn’t available at that exact spot. In that case, the weight should be halved and installed at two different holes on each side of the target location.
The installed weight(s) should be checked for adequate clearance from the starter and other components by pulling the propeller around by hand and making sure the weight doesn’t come close to contacting anything as it rotates with the propeller.
Vibration levels are labeled according to a standard scale. Vibration levels of 0 to 0.07 IPS are considered good. 0.07 to 0.15 IPS is considered fair. 0.15 to 0.25 is considered slightly rough. 0.25 to 0.5 is moderately rough. 0.5 to 1.0 is very rough, and 1.0 to 1.25 is considered dangerous. 1.20 is the maximum allowable FAA limit for a dynamic imbalance.

Dynamic balancing errors
The person performing a dynamic balance should use high-quality, calibrated equipment. Erroneous sensor readings will cause weight to be added in incorrect locations.
The propeller and spinner should be clean before the balance procedure is started. Spinners should be removed and cleaned on the inside as well, especially on propellers that require grease.
Controllable-pitch propellers should be greased and serviced properly with nitrogen as required before the balance procedure is started.
Finally, the weather conditions should be favorable. Accurate readings are best obtained when the engine and propeller is run in calm air. The plane should be pointed into whatever wind is blowing, not only to aid in engine cooling, but also because a tailwind or crosswind can affect the readings.

Troubleshooting other problems during a dynamic balance
The propeller balancing equipment can also be used as a troubleshooting aid when an engine does not seem to be developing its normal power.
The photo tachometer gives an accurate rpm indication at full-throttle static rpm.  If the engine is not making its full rated power, further investigation is warranted. A lower-than-normal output is indicative of an engine defect or excessive wear.
Most airplanes use mechanical tachometers. These tachometers are seldom accurate and can read high or low when compared to actual engine rpm.
The photo tachometer is more accurate for troubleshooting use. It will also help determine whether a mechanical tach is reading incorrectly; and if so, by how much.

Conclusion
Most airplanes have at least a slight propeller imbalance, even if it hasn’t become bad enough to be noticed by the pilot. It is always best to correct vibration problems early because they tend to grow in magnitude as wear occurs.
The benefits of propeller balancing greatly offset the cost. Reducing vibration helps reduce wear and fatigue, extending the service life of many components not only on the engine, but on the airframe itself.

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 .


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