AC 20-37E Date: 9/9/05
ADVISORY CIRCULAR
AIRCRAFT PROPELLER MAINTENANCE
Flight Standards Service Washington, D.C.
Initiated By: AFS-350
Subject: AIRCRAFT PROPELLER
MAINTENANCE Date: 9/9/05 Initiated By: AFS-350 AC No: 20-37E Change:
1. PURPOSE. This advisory circular (AC) provides information and describes maintenance procedures for owners, operators, and Federal Aviation Administration (FAA)-certificated maintenance personnel during the service life of aircraft propellers. It further recommends minimum requirements for propeller field maintenance and provides a checklist for propeller annual inspection.
2. CANCELLATION. This AC cancels AC 20-37D, Aircraft Propeller Maintenance, dated 8/15/89.
3. PRINCIPAL CHANGES. This AC has been updated to provide more current guidance for inspection, maintenance, and field repair of aircraft propellers. Propellers of all types are covered, including propellers of composite, wood, and metal. Inspection and maintenance methods contained within should be used where guidance is otherwise unavailable.
4. RELATED READING MATERIAL. Manufacturers’ instructions for continued airworthiness, service bulletins, maintenance records, and airworthiness directives are the primary documents containing information relating to the care and maintenance of propellers. In addition, AC 43.13-1, Acceptable Methods, Techniques, and Practices - Aircraft Inspection and Repair, current edition, and AC 43-4, Corrosion Control for Aircraft, contain many references to propeller inspection and maintenance, current edition.
NOTE: These ACs may be dow nloaded free of charge from the follow ing FAA public Web site: http://www.faa.gov/regulations_policies/.
5. DISCUSSION. When properly maintained, propellers are designed to operate safely in a high-stress environment for extended periods of time. However, despite the design factors engineered into propellers, failures continue to occur. FAA data on propeller failures indicate that failures occur across the entire spectrum of aircraft engine-propeller combinations. The propeller maintenance information contained in this AC provides maintenance personnel with information and techniques to reduce these failures and increase propeller service life. Because of the age of some propeller systems, original guidance and instructions for maintenance, service, and overhaul may not have been updated (e.g., if the manufacturer is no longer in
business) with improved techniques. Accordingly, this AC is intended to provide supplementary guidance on propeller maintenance and service methods when such guidance is lacking from the manufacturer or is not consistent with current practice.
/s/ John M. Allen (for) James J. Ballough Director, Flight Standards Service Page ii Par 5
CONTENTS
Page CHAPTER 1. DESIGN DAMAGE AND FAILURES
100. Propeller Major Repair and Overhaul1
101. Propeller Design and Causes of Failure1
102. Types of Propeller Damage1
Figure 1-1. Hub Surface Corrosion2
Figure 1-2. Polished Blade Surface Corrosion2
Figure 1-3. Pitting2
Figure 1-4. Pitting on a Shot Peened Surface2
Figure 1-5. Exfoliation on the Blade Leading Edge3
Figure 1-6. Blade Nick4
Figure 1-7. Erosion on a Metal Blade4
Figure 1-8. Hub Crack4
Figure 1-9. Blade Crack from a Nick4
Figure 1-10. Lightning Strike on a Metal Blade Tip5
Figure 1-11. Strong Lightning Strike on a Composite Blade Tip5
Figure 1-12. Lightning Strike on a Composite Surface5
Figure 1-13. Propeller Blade Struck by a Foreign Object6
CHAPTER 2. MAINTENANCE AND OVERHAUL
200. Requirements for Maintenance and Overhaul7
201. Cleaning8
Figure 2-1. Grease Leak Residues9
202. Inspection Methods9
203. Types of Inspection10
Figure 2-2. Propeller Tracking (Wood Block or Cowling Fixture Shown)12
204. Inspection Levels13
Figure 2-3. Heater Edge Debond15
Figure 2-4. Sheath Crack15
205. Limitations20
206. Procedures for Maintenance21
Table 2-1. Blade Leading Edge Repair24
Figure 2-5. Techniques for Blade Repair25
Figure 2-6. Composite Blade Erosion26
CHAPTER 3. ACCESSORIES AND BALANCING
300. Propeller Balancing27
Figure 3-1. Balance Tape that was not Removed29
301. Tachometer Inspection29
302. Governors, Feathering, and Reversing Mechanisms29
303. Propeller De-Icers30
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CHAPTER 4. INSTALLATION
400. Propeller Installation31
APPENDIX 1. GLOSSARY OF COMMON PROPELLER TERMS (3 Pages)1
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CHAPTER 1. DESIGN DAMAGE AND FAILURES
100. PROPELLER MAJOR REPAIRS AND ALTERATIONS. Major repairs or alterations are only permitted within the context of this document or when a propeller manufacturer’s data approves that major repair or alteration. Only an appropriately rated repair station may accomplish major repairs or alterations. Refer to Title 14 of the Code of Federal Regulations (14 CFR) part 43, Appendix A, for identification of major alterations and repairs to propellers.
101. PROPELLER DESIGN AND CAUSES OF FAILURE.
a. A propeller is one of the most highly stressed components on an aircraft. During normal operation, 10 to 25 tons of centrifugal force pull the blades from the hub while the blades are bending and flexing due to thrust and torque loads and engine, aerodynamic and gyroscopic vibratory loads. A properly maintained propeller is designed to perform normally under these loads, but when propeller components are damaged by corrosion, stone nicks, ground strikes, etc., an additional unintended stress concentration is imposed and the design margin of safety may not be adequate. The result is excessive stress and the propeller may fail.
b. Additional causes of overstress conditions are exposure to overspeed conditions, other object strikes, unauthorized alterations, engine problems, worn engine vibration dampers, lightning strike, etc. Most mechanical damage takes the form of sharp-edged nicks and scratches created by the displacement of material from the blade surface and corrosion that forms pits and other defects in the blade surface. This small-scale damage tends to concentrate stress in the affected area and eventually, these high-stress areas may develop cracks. As a crack propagates, the stress becomes increasingly concentrated, increasing the crack growth rate. The growing crack may result in blade failure.
102. TYPES OF PROPELLER DAMAGE. Many types of damage cause propellers to fail or become unairworthy. FAA data on propeller failures indicates that the majority of failures occur in the blade at the tip region, usually within several inches from the tip and often due to a crack initiator such as a pit, nick, or gouge. However, a blade failure can occur along any portion of a blade, including the mid-blade, shank, and hub, particularly when nicks, scratches, corrosion, and cracks are present. Therefore, during propeller inspection and routine maintenance, it is important to inspect the entire blade. The severity of the damage determines the type of repairs required. Additional guidance on damage is given in manufacturers’ service documents, Chapter 2, paragraph 205, of this AC, and AC 43.13-1, current edition. The following paragraphs describe some of the types of damage that may be found in propellers.
a. Corrosion. One of the principal causes of loss of airworthiness in propellers is corrosion.
External corrosion on metal blades, hubs, and other components poses a serious problem.
Internal corrosion may exist where moisture may collect in internal cavities such as hubs, blade clamps, and pitch control mechanisms. This threatens propeller structural integrity and performance without being noticed. The overhaul calendar time periods for propellers are established so that the propeller can be disassembled to inspect internal surfaces. Moreover, corrosion acts continuously, regardless of the actual time in operation. Corrosion on metal propeller components can be classified into three distinct types.
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(1) Surface Corrosion. The loss of surface metal due to chemical or electro-chemical action with visible oxidation products usually having a contrasting color and texture to the base metal. Surface corrosion, as shown in Figures 1-1 and 1-2, generally results when the corrosion protection on a metal surface has been removed by erosion or by polishing. Therefore, removing paint and corrosion protection, such as when polishing blades, is not recommended.
FIGURE 1-1.
Hub Surface Corrosion FIGURE 1-2.
Polished Blade Surface Corrosion
(2) Pitting. Pits consist of visible corrosion cavities extending inward from the metal surface. They can grow on the surface, under decals, or under improperly installed de-ice boots.
Pitting can appear to be relatively minor - 0.010 inches deep - and still cause major problems since the pits could be a precursor to the initiation of cracks (see Figures 1-3 and 1-4).
FIGURE 1-3.
Pitting FIGURE 1-4.
Pitting on a Shot Peened Surface
(3) Intergranular Corrosion. Occurs in grain boundaries. The presence of intergranular corrosion may be the result of the continued presence of moisture such as under a decal, in a fastener hole, or where the anodize and paint protective barriers have been lost.
Exfoliation is a form of intergranular corrosion that occurs more often in forgings or rolled Page 2 Par 102
sheets, and less often in castings. Exfoliation is sometimes visible as metal flaking and cracks on a blade leading edge (see Figure 1-5).
FIGURE 1-5.
Exfoliation on the Blade Leading Edge
b. Face, Leading Edge, or Twist Misalignment. When propeller blades are bent, twisted, or cocked, they will not be properly aligned with each other in operation. This will cause vibration and may cause a loss of thrust. The level of vibration can be severe and depending on the severity of the misalignment, could lead to catastrophic failure (see Chapter 2, paragraph 203a).
c. Nick. A sharp, notch-like displacement of metal usually found on leading and trailing edges. All nicks are potential crack starters (see Figure 1-6).
d. Erosion. The loss of material from blade surface by the action of small particles such as sand or water and is usually present on the leading edge close to the tip. This damage destroys the blades’ corrosion protection, which might lead to blade failure (see Figure 1-7).
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FIGURE 1-6.
Blade Nick FIGURE 1-7.
Erosion on a Metal Blade
e. Scratches, Gouges, Cuts, and Scoring. These terms describing surface damage are found in Appendix 1, Glossary of Common Propeller Terms.
f. Cracks. When found anywhere in a propeller, cracks are cause for its immediate removal and detailed inspection. Cracks in propellers will grow over time, perhaps very rapidly, and eventually lead to failure (see Figures 1-8 and 1-9).
FIGURE 1-8.
Hub Crack FIGURE 1-9.
Blade Crack from a Nick
g. Dents. Dents can be harmful, depending on their size, location, and configuration. Dents cause local stress risers around their perimeter and at the bottom under the surface. Removing material should repair dents. Filling dents with any material such as auto body compound does nothing to correct the stress riser and is not approved. Failure may occur.
h. Lightning Strike. A lightning strike on a metal blade may be indicated by a small burned and melted area on the blade, a trail of small pits along the blade, or may show no indication at Page 4 Par 102
all (see Figure 1-10). However, the damage from a lightning strike may be severe, affecting the strength of the blade material itself, damaging blade bearings or other internal components.
Lightning always creates residual magnetism in steel parts. Inspection for damage from a reported lightning strike may require specialized equipment, like a gauss meter, to check for magnetism in steel components, and should be accomplished by an appropriately rated propeller repair station. A lightning strike on a composite blade may be indicated by small burnt areas on the composite where the lightning may have attached or exited (see Figure 1-11 or 1-12).
Composite blades may suffer other damage as well. Refer to the propeller manufacturer’s maintenance manual for diagnosis and corrective action.
FIGURE 1-10.
Lightning Strike on a Metal Blade Tip
FIGURE 1-11.
Strong Lightning Strike on a Composite Blade Tip FIGURE 1-12.
Lightning Strike on a Composite Surface
i. Overspeed. A propeller may have been exposed to an overspeed condition and give no indication of the event. However, the event may have severely damaged the propeller due to the dramatic increase in centrifugal loads. If the propeller is suspected of having been operated in an overspeed condition, it should be removed and sent to a propeller repair station to be inspected Par 102 Page 5
for elongation of boltholes, dimension changes, or other signs of stress in accordance with the appropriate manufacturer’s maintenance instructions. Sometimes excessive tolerances in engine or propeller governor settings can permit overspeed conditions into restricted rotational speed ranges without the knowledge of the operator.
j. Foreign Object Strike. A foreign object strike can include a broad spectrum of damage, from no visible damage, to a small nick, to severe ground impact damage. A conservative approach in evaluating the damage is required because of the possibility that there may be hidden damage that is not readily apparent during a superficial, visual inspection (see Figure 1-
13). Refer to the manufacturer’s maintenance instructions for damage limitations.
FIGURE 1-13.
Propeller Blade Struck by a Foreign Object
k. Fire Damage or Heat Damage. On rare occasions, propellers have been exposed to fire or heat damage such as a hangar or engine fire. In the event of such an incident, an inspection is required before further flight. Such parts are normally retired. If there is any indication or suspicion that aluminum propeller parts have been exposed to high temperatures (in excess of 200 °F (93 °C)), then the parts must be assumed to be unairworthy, unless it can be proven that there have been no adverse affects from the incident. Composite propeller blades may have a lower temperature threshold for potential damage. Refer to the manufacturer’s maintenance instructions for this limitation. Confirmation of airworthiness requires complete disassembly and inspection of the propeller by an appropriately rated propeller repair station in accordance with the propeller maintenance manual. Always advise the repair station that the propeller may have been exposed to heat or fire when it is sent in for this type of inspection.
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CHAPTER 2. MAINTENANCE AND OVERHAUL
200. REQUIREMENTS FOR MAINTENANCE AND OVERHAUL.
a. Sources of Propeller Repair Information. Airworthiness Directives (AD), type certificate (TC) data sheets, manufacturers’ manuals, service letters, and bulletins specify methods and limits for propeller maintenance, inspection, repair, and removal from service.
When a manufacturer’s data specifies that major repairs are permitted to a specific model blade or other propeller component, only an appropriately rated repair facility may accomplish those repairs. An FAA-certificated mechanic with at least a powerplant rating can accomplish all other propeller maintenance and minor repair by using the practices and techniques specified by this advisory circular (AC) and in the propeller manufacturer’s service data. Some maintenance and minor repairs in this category are the removal of minor nicks, scratches, small areas of surface corrosion, painting, and minor deicer boot repairs. Because of the complexity of propeller damage and because damage tends to be hidden or not obvious to untrained maintenance personnel, we recommend that propeller damage be referred to experienced repair personnel whenever doubt exists regarding a condition that has been observed. We further recommend that owners/operators follow the manufacturer’s maintenance and overhaul program.
b. Service Personnel Limitations and Responsibilities. Title 14 of the Code of Federal Regulations (14 CFR) part 65, section 65.81, specifically excludes certificated and rated airframe and powerplant mechanics from performing major repairs and/or major alterations on aircraft propellers. Title 14 CFR part 43, Appendix A, defines major alterations and repairs to propellers. However, 14 CFR part 145, section 145.201, provides that an appropriately rated repair station may perform such major repairs or alterations provided the work is done in accordance with technical data approved by the Administrator. Part 145 also specifies the personnel qualifications and other requirements applicable to propeller repair stations. When complying with ADs, service personnel are required to review all applicable manufacturers’ service bulletins (SB), manuals, and other information on the propeller being inspected, overhauled, or repaired, if included by reference.
c. Periodic Reconditioning of Aluminum Fixed-Pitch Propellers. A number of factors will require returning a propeller to a propeller repair station for service, repair, or rework. All propeller manufacturers recommend a periodic reconditioning of aluminum fixed-pitch models at specified service time intervals to prevent blade failure from surface damage that may not be visible. This reconditioning requires the propeller to be returned to a repair station for removal of a thin layer of surface metal to remove surface and subsurface damage such as nicks and corrosion. Fatigue cycles generated by some engine/propeller combinations can require manufacturer-reconditioning intervals as often as every 500 hours of operation.
d. Periodic Overhaul and/or Inspection of Variable Pitch Propellers. Propeller manufacturers recommend a periodic propeller overhaul or teardown inspection. Some propeller makes and models are required by ADs to be inspected, repaired, or partially disassembled for evaluation. In most cases, such a requirement is a major repair or alteration and dictates that the propeller is returned to a propeller repair station.
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