In May 2008 the FAA published five new SAIBs or Suggested Airworthiness Information Bulletins on propeller maintenance. While these bulletins do not carry the weight of an AD, they do come about as a result of discussions with field organizations such as aircraft type clubs as well as maintenance shops who identify problems they see as recurring or actually becoming worse.
Such problems now seem to be somewhat a result of our aging aircraft fleet and frequent deferred or minimal maintenance to get past an inspection or minimal overhaul compliance. Some of the following SAIBs result from owners doing the least they can to get by maintenance wise, which sometimes has unanticipated costly consequences down the road.
This SAIB discusses the effects of the lack of or deferred engine damper maintenance on propeller life and the increased potential for propeller failures. It also discusses potential causes of damper damage/wear. We have discussed engine crankshaft dampers in past LPM articles, but briefly these engine dampers are generally found in aircraft engines of over 200 HP.
Dampers are moveable weights attached to the crankshaft by pins and bushings that allow limited movement. The dampening effects are designed to reduce peak vibratory stress occurring at specific frequencies in certain parts of the engine or the propeller or both.
Propellers installed on dampened engines are vibrationally tested and certificated based on the test results. The tests are performed on engines with a specific damper configuration and the propeller approval is granted for only those specific engine and propeller combinations.
Any variations in the crankshaft damper configuration can have significant adverse effects on the propeller vibratory stress characteristics. Such variations can occur from excessive wear or damage in the pins and bushings of the crankshaft damper assemblies, improperly installed crankshaft dampers, as well as situations where a propeller was
Used on an incompatible, or untested, engine/propeller combination—something that occurs with much greater frequency than one may imagine. This is an item that is supposed to be checked at every annual, but often is overlooked.
Damper Damage Causes per the FAA
Prolonged engine operation without an oil change is one issue. That means you should change the oil even with no hours on the engine during the service period. Lycoming calls for four months and TCM allows six between oil changes.
Excessive time-between-overhaul (TBO) is another increasing problem. Operators have extended engine life by doing “top overhauls” without splitting the engine case, which is necessary for damper pin/bushing inspection or replacement.
Non-compliance with inspection or replacement requirements of damper pins and bushings during overhaul is an increasing problem. The proper inspection of the damper and associated components is simply skipped in some field overhauls as a cost saving.
Improper (abrupt) handling of throttles during engine run-up to full static RPM or decrease from full to idle power during flight causes damage. An example is to chop the power and descend at high speed where the prop is allowed to “drive” the engine. This is where the term “detuning” the crank dampers comes from and what causes it is the slamming of the dampers against the pins and changing the dampening frequency through damage to the mounting pins.
A prop strike or sudden stoppage of a rotating engine without doing an engine teardown and inspection will often come back to haunt an operator (or the next unwitting owner). Both TCM and Lycoming have very specific teardown guidance service bulletins, and essentially state that if the engine was running when the prop hit something, then it’s likely a teardown will be needed to check all the internal areas that may be damaged such as the dampers and crank gears. The torque forces are much higher than most people think even at low RPM.
Simple “dialing of the crank” is a woefully inadequate procedure, yet a common answer to what is done following a prop strike report. There are case histories in the incident database of cranks departing the engine in flight, only to delve into the engine history to see a prop strike that was not properly inspected and dealt with some months earlier.
Watch the Signs
Perform proper and timely maintenance of the aircraft, propeller, and engine to avoid a significant increase in propeller vibratory stress. Do not ignore vibration or changes in vibration patterns as this can be a sign of a more serious problem.
During engine overhaul, follow manufacturer’s recommendations for replacement of crankshaft damper pins and bushings. Use of new parts is important to ensure long term, safe operation. Some harmful vibrations may not be felt by the pilot, so the only cure is to perform called for maintenance and parts replacements. The manufacturers provide these lists via service instructions such as Lycoming SB 240T.
Prop installations on experimentals can be downright dangerous if a kit maker’s guidance is ignored. A specific example cited is the use of Hartzell ( )8468( ) series blades installed on Lycoming ( )( )-360 series engines. (There are numerous variations in blades in this series.)
Two recent accidents resulted from this combination. This blade designuses a thinner airfoil than that used on other Lycoming 360 applications. Also, the accident-related blades incorporated a modification that reduced the blade diameter to less than other approved designs. These differences result in exposure to high vibratory stress loadings that exceed the structural levels allowed for safe operation.
In both accidents, the high propeller blade vibratory stress loading is considered a contributing factor in the failures. These accidents show that specific engine and propeller combinations are an important design consideration that requires careful research regardless of the propeller manufacturer.
The best advice is to consult with the kit vendor (when applicable) and the engine and prop makers. Prop repair facilities or aftermarket vendors are not the best choice for such decisions of appropriate application of a propeller.
There are also reports of prop repair stations being asked to overhaul props for experimentals with blades no longer considered acceptable for certificated aircraft. If it’s out of spec for a certified aircraft it certainly is no less reckless on an experimental.
Propeller maintenance (especially constant speed types) is often not performed to established standards in spite of the fact that the consequences of a prop failure are usually very severe. Prop repair facilities report seeing constant speed props that have over 20 or more years in service and never been off the plane.
Particularly constant speed props can have severe corrosion that is hidden unless it’s torn down and inspected. Moreover, a slick paint job on the outside can cover dangerous nick damage or corrosion. See the manufacturers instructions for continued airworthiness and AC 20-37E, which we discussed in some detail last year in LPM for more specifics. The FAA reports of prop failures due to corrosion almost invariably have a history of ignoring all the recommended maintenance intervals and inspections.
To maintain the airworthiness of propellers, it is important that maintenance and overhauls are properly performed by trained personnel, and that maintenance and overhauls are performed in accordance with current approved technical data, (i.e., manufacturers’ service documents).
Propellers should be inspected and serviced at the specified intervals or something more reasonable than 20-year intervals. Even fixed-pitch props need to be inspected periodically.
When in service, propellers are operated within design specifications and all limitations for that propeller model installation are observed. See Chapter 8 of AC 43.13-1B for lots of details on what is acceptable for damage and proper repair procedures of nicks. It’s a lot more than just filing them out. There are acceptable dimension limits as well for length and blade thickness, too.
The subject of RPM restrictions and placards is often given short shrift. There are actually two situations that lead to possible failures. Some aircraft have restricted operational areas on the tach where prolonged operation is not authorized. The other area of concern is the RPM redline limits.
Both of these situations can be violated inadvertently by an inaccurate tach, which is all too common with mechanical tachs, especially with 35 year-old tachs being commonplace as well as cheap offshore replacement tachs of questionable “adjustability” or calibration right out of the box. They may not even be the proper tach for the application, as different tach hour settings are required (such as 2566) so that hours are more accurately recorded.
Other factors to consider are that instrument panel placards for RPM restrictions might be incorrect or missing. Or, if a propeller and/or engine was replaced or modified, the propeller RPM restrictions or placards might not be correctly updated. Last, non-compliance with Airworthiness Directives that require changes to RPM restrictions are not reported.
The FAA requirement for accuracy is 4 percent, but it’s hardly ever checked at an annual as it’s not part of the standard checklist, therefore errors are rampant, and operators may be damaging their engines or promoting propeller failure and not even realize it.
Relatively cheap electronic tach checkers are available but still not very worthwhile due to less than stellar accuracy. For good accuracy, go with the True Tach II. It’s easy to use and accurate.
Moreover, a new dimension has been added with aftermarket tuned exhaust systems that allow excess RPM to be obtained in fixed pitch props that was previously not available due to engine back pressure. What this means is that, in spite of the ability to push an engine to greater RPMs than in the past, it is neither good judgement, nor approved to exceed the limits established in the POH.
The aftermarket suppliers specifically point this out in their literature. However, reports of field tests constantly brag about being able to exceed climb or even redline RPM by a few hundred—a situation that manufacturer service data calls for an engine teardown inspection depending on the length of time of the over-rev condition. Are a few more knots worth a premature overhaul or the extra gas?
Check the aircraft records for replacement or modification of the tachometer, and changes to the propeller model, engine model, or installation changes. Verify that the proper RPM restrictions are accurately marked on the tachometer and instrument panel placard.
Check the accuracy of the tachometer to ensure that the readings are accurate. Check the accuracy of mechanical tachometers at intervals not to exceed 60 months. Contact the propeller manufacturer for corrective action if the propeller was operated in a restricted range.
Also, read the engine maker’s service bulletin for actions required if the engine was operated in a restricted range or over-speed by even a few hundred RPM for prolonged periods. (Anything over 10 percent past redline is considered the RPM over-speed threshold.)
The purpose of this SAIB is to better define a propeller general visual inspection for both operators and repair facilities. Airplane owners, operators, and propeller repair stations should have FAA accepted guidelines for performing “cosmetic repairs.” These recommendations are provided below.
Service issues, such as oil leaks or minor blade damage, typically involve repair of the specific problem without repair or rework of the entire propeller. During repair of a given problem, propellers are sometimes given additional cosmetic repairs, for example, repainting of blades and/or replacement of decals.
If the blade is not given a thorough rework and inspection prior to repainting, there is concern that the paint may hide flaws such as nicks, corrosion, or other material surface flaws. Airplane owners, operators, and propeller repair stations should have FAA accepted guidelines for performing “cosmetic repairs.”
To that end, the following is the latest FAA recommendation for a general visual inspection, which has alternatively been called a “propeller search inspection. Perform a visual inspection of all observable component parts for evidence of wear, damage, corrosion, grease leaks, or oil leaks.
Any defects found will require further evaluation to determine whether the component remains serviceable until the next scheduled maintenance or overhaul. If vibration, oil, or grease leakage was reported, this may be a sign of a failed seal(s) or a more serious flaw such as a fractured hub or blade (especially if both an oil or grease leak and vibration initiate simultaneously), and a more thorough investigation is appropriate.
If the work being performed does not require complete disassembly, additional disassembly is not required to satisfy Propeller Search Inspection requirements unless evidence of observed defects suggests the need for further investigation.
Perform a logbook review that includes:
1. Determination of the date and flight time-since-last-overhaul.
2. Confirmation that the basic propeller model, blade model, and diameter are approved for the aircraft application involved (use FAA data, e.g., TCDS, or propeller manufacturers’ application guide).
3. Verification of the basic propeller model, blade model, and diameter for the application involved (if there is no propeller logbook or if time-in-service is unknown, a further Propeller Search Inspection might be warranted to determine airworthiness).
4. A properly maintained logbook is important and is required by FAA regulation.
Review and discuss the following items with the aircraft owner/ operator:
1. Determination of the date and flight time-since-last-overhaul (from propeller logbook).
2. Determination of compliance with all applicable FAA airworthiness directives.
Make a logbook entry of all repairs. If the customer either does not authorize, or postpones, correction of conditions found during a search inspection; the repairman should record the issues through entries in both the propeller logbook and customer work order. The aircraft owner/ operator should be notified that this documentation was created.
Examples of defects that might be returnable to service:
1. Minor deterioration of paint or corrosion protection (however, consider that if repair or overhaul may be some years in the future, immediate repair of damaged paint or corrosion may be appropriate).
2. Light wear or scoring typical of normal operation. Refer to the propeller manufacturer’s instructions for continued airworthiness (ICAW) for the appropriate acceptance criteria.
Examples of defects that require maintenance action or further evaluation prior to return to service:
1. Unusual wear of either unexpected severity or in an unexpected location that might be beyond the manufacturer’s service limits.
2. Damage or corrosion of aluminum blades, hubs, or other highly stressed propeller parts.
3. Deteriorated seals or O-rings.
4. Incomplete adhesion or sealing of de-ice boots, erosion shields, or decals.
5. Any suspected crack indication requires confirmation with the appropriate nondestructive test (NDT) inspection.
6. Deteriorated or broken electric de-ice lead wires.
7. If only one blade is removed for repair and found to have either external or internal corrosion, also remove and inspect other blades as they are likely to have similar defects.
Reference Documents—For additional guidance, refer to the appropriate manufacturers’ Propeller Maintenance Manuals and Owner’s Manuals; FAA Advisory Circular 20-37E, Aircraft Propeller Maintenance; and FAA Advisory Circular 43.13-1B, Acceptable Methods, Techniques, and Practices.
For Cosmetic Repairs:
For aluminum propeller blades, prior to painting, rework all damage using the manufacturer’s published field rework procedures in the manufacturers’ manuals or the FAA Advisory Circulars mentioned above. Before painting, inspection and application of a chemical conversion coating and primer paint are required following the manufacturer’s ICA.
For exposed aluminum surfaces, an exposed defect can be inspected while a hidden defect cannot be inspected. A cosmetic repair that creates a hidden defect in an exposed surface is an unacceptable practice. Complete rework and proper repair should be accomplished prior to painting.
Composite propeller blades have different inspection/ acceptance criteria than aluminum propeller blades. Composite propeller blades generally require strict adherence to the manufacturer’s maintenance instructions. Any alternate procedures or materials require FAA approval.
When resealing the edges of de-ice boots or applying decals to aluminum propeller blades, manufacturers commonly require that sealants and decals not be applied to bare aluminum.
For long term corrosion protection, it is important that the aluminum be properly prepared and painted prior to resealing de-ice boots or applying decals. Likewise, sealants and adhesives should not be applied to bare aluminum unless specified by the propeller manufacturer’s maintenance instructions.
Polished propeller blades are rarely an acceptable configuration. Corrosion protection such as paint and anodizing should not be removed from the surface of a propeller blade. (I had a chromed spinner for over 20 years and no one ever said a word. Must have been luck.)
If the original design had corrosion protection and the propeller manufacturer’s ICA call for corrosion protection, then the corrosion protection should be maintained to those instructions. Therefore, do not return polished propeller blades to service without verification of acceptability.
One last data point is that all these SAIBs contain extensive contact list information at both the FAA as well as the propeller makers. We strongly recommend downloading these documents for further study and reference.
We only see ever increasing FAA oversight as well as more standardizing of maintenance practices as the fleet continues to age and odd failures begin to pop up.
This article originally appeared in the July 2013 issue of Light Plane Maintenance.