Par 203 Page 13 specified for field repair, the blade must either be retired or sent to a repair station for evaluation before further flight.
f. Inspection of Composite (Including Wooden) Blades. For composite blades, a thorough visual inspection is recommended together with a coin tap inspection of each composite blade, including the metal erosion shield on the leading edge (follow the manufacturer’s instructions for coin tap test limits so as not to damage the blade). Repair damage, delaminations, or debonds within the limits specified. If blade damage is beyond that specified by the manufacturer for minor repair, the blade must either be retired or sent to a repair station for evaluation before further service.
g. Inspection for Lightning Strike on Composite Blades. Any composite blade suspected of lightning strike should be inspected and may require overhaul. Lightning strikes usually enter a composite blade through the metal erosion shield. If the blade has a metallic lightning screen or tip lightning strip, the lightning strike may enter the screen or through the tip instead of the erosion shield. If a lightning strike is present, a darkened area and possible pitting, usually in the proximity of the tip, will be noticeable. If a lightning strike is suspected or detected, follow the manufacturer’s service instructions before further flight.
204. INSPECTION LEVELS.
a. Three Levels of Inspection. To ensure airworthiness, propeller inspection may be accomplished at three levels:
(1) Preflight or walk-around inspection, detailed inspection (such as a 100-hour or annual inspection), and teardown inspection (or overhaul).
(2) These levels correspond to three of the four inspection levels carried out on airframes. Regardless of the level of inspection, the inspection should follow a standard procedure to be carried out in a similar fashion every time it is accomplished. The major elements of any inspection include the following:
(a) Basis for the Inspection. Purpose, documents needed, inspection frequency, and what is needed to accomplish the inspection.
(b) Preparation. Cleaning, surface preparation, equipment and calibration requirements, etc.
(c) Implementation. Define what the inspection is and how it is to be carried out, defect criteria, post-cleaning, etc.
(d) Evaluation. Accept/reject criteria, reporting.
b. Preflight/Walk-Around Inspection. The propeller portion of the walk-around is an important element of the process of airworthiness maintenance. It should not be merely a superficial look but a studied review of the condition of everything that might give trouble during the forthcoming flight.
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(1) Blade. The blade and its surface should be carefully inspected for conditions affecting airworthiness as detailed below.
(a) Surface Damage. Look for surface damage on both sides of the blades, such as dents, nicks, scratches, and corrosion. Surface imperfections can also be felt by running your fingernail along the blade leading edge. Damage should be repaired before flight. Whenever a noticeable dent, nick, corrosion pit, or bump is observed, an appropriately rated mechanic should blend it out. The mechanic should remove all corrosion products and determine that the section thickness has not been reduced below allowable limits. Allowable thickness limits should be obtained from the manufacturer’s maintenance manual or other FAA acceptable propeller inspection criteria.
(b) Erosion. Examine the blade for evidence of erosion. If metallic blades appear to show erosion beyond limits, the propeller should be removed from service and evaluated by an appropriately rated repair station. Check the condition of the paint on blades and spinners that have protective paint. Paint protects the surface of the blade from erosion, and the blade should be repaired before the paint wears through and the blade structure begins to erode. Do not apply excessive paint and do not paint propeller components unless it is in accordance with manufacturer’s instructions since improper painting may affect propeller balance, operation, and static electricity discharge or have other unintended consequences.
(c) Composite or Wood Delaminations. Although not susceptible to corrosion like metal propellers, wood or composite propellers have special problems that can lead to an unairworthy condition. Wood or composite propellers are susceptible to internal damage from small stone strikes that can create delamination or microcracks and permit intrusion of moisture.
Moisture will cause expansion of existing cracks and delaminations. When moisture freezes within the blade, it causes delamination. When inspecting wood or composite propeller blades, look for cracks or delamination on the blade surface and at blade edges. In wooden propellers, check the gluelines for debonding and look for warp and loss of protective coating (paint or varnish). If drain holes are present, it is imperative that they be inspected since they may become clogged with insects and debris. Clogged drain holes can cause moisture retention.
(d) Straightness. Sight down the edges to find any deformation.
(e) Looseness. Feel the blades and move them to find unusual changes in looseness and unusual play. Blade-to-blade differences indicate that an internal problem may exist. Some propeller blades are designed to be loose. In this case, look for blade-to-blade differences to indicate unusual play.
(f) Deicing System. Check the deice or anti-ice boots, if installed, for good adhesion to the propeller. Examine the boots and exposed wiring for breaks and burn through (see Figure 2-3).
Par 204 Page 15 FIGURE 2-3.
Heater Edge Debond (
g) Sheath Cracks and Debonds. Check the leading edge erosion sheath of wood or composite blades, if installed, for cracks and debonds. If damaged, repair before further flight or follow the manufacturer’s recommendations for permissible limits on cracked or debonded leading edge erosion sheaths (see Figure 2-4).
FIGURE 2-4.
Sheath Crack (
2) Oil and Grease Leakage. Look for oil or lubricant leaks in unusual places, like the outside surfaces and seals. Oil or grease leakage may be due to a seal failure or a crack. The source of the oil or grease leak should be determined before flight. During maintenance, wipe the surfaces of the propeller after this inspection, not before, since oil leaking from a crack may assist in detecting it. Some manufacturers fill hubs with red oil that gives a positive warning of a crack in the hub.
(3) Spinner and Bulkhead. Externally check the spinner and bulkhead for security and for missing fasteners, damage, and cracks. Cracks typically originate from the attachment screws. Cracks should be serviced in accordance with the manufacturer’s manuals. Check for looseness of the bulkhead. This could be an indication that the mounting bolts are loose.
(4) General Condition. Look for loose wires, grease fittings, and debris.
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(5) Control System. The control system (governor) of a controllable pitch propeller should be checked to determine whether the system is operating properly and not leaking.
(6) Maintenance Records. Note any indications in the logbook for future reference to determine whether a condition is getting worse.
c. Detailed Inspection (Such as at a 100-Hour or Annual Inspection). A more rigorous visual inspection can detect some conditions affecting airworthiness of propellers, particularly from external sources of damage, such as erosion, deicing boot damage, and blade tracking.
Many locations encountering damage are hidden. For these areas, conducting a worthwhile inspection will require some disassembly and special equipment. Such an inspection is a detailed inspection and may be accomplished in the field by appropriately rated maintenance technicians.
Generally speaking, a detailed inspection requires removal of the spinner and certain other components as indicated below. Propellers suspected of more severe damage may require removal from the aircraft and the next level of inspection: a teardown inspection.
(1) Inspection Interval. Most manufacturers require a periodic inspection at fixed intervals, usually between 1 and 2 years or 100 to 300 flight-hours, whichever is sooner. A detailed propeller inspection is done at each aircraft annual inspection or as required in the aircraft maintenance procedures. This inspection reduces the possibility that a propeller will be neglected and subjected to unobserved corrosion and other damage.
(2) Fixed-Pitch Propeller. The detailed inspection uses a procedure similar to that used in the walk-around inspection, but more time and closer scrutiny is required. Inspect the propeller section by section (if it is a large propeller, consider marking off sections to ensure complete coverage). Use the best possible light and use a 10X magnifier for any questionable areas. Look for cracks and corrosion pitting that may have escaped earlier scrutiny. The magnifier is a great help in determining whether a scratch is, in fact, a crack. All applicable propeller ADs must be checked for compliance. The following detailed inspections are recommended.
(a) Propeller Blades. Visually inspect for excessive wear and erosion, damage, nicks, cracks, corrosion, lightning strikes, and ground strikes.
1 Check for distortion, security to the engine, and tip tracking. Check the adequacy of protective coatings, paint, and plating.
2 Check the leading edge erosion shield for damage, debonding, and cracks.
3 Check composite and wood blades for delaminations, surface cracks, and exposed composite.
(b) Spinner and Bulkhead Assembly. Detailed inspection procedures require removal of the propeller spinner for examination of the hub area. Inspect the spinner and spinner bulkhead for cracks and repairs, such as stopdrill holes and weldments. Spinner bulkhead attachment tabs frequently succumb to fatigue and should be checked. It is good practice to remove the spinner after each 100 flight-hours to observe the propeller-to-engine attachment. If repairs are present, they should conform to the manufacturer’s maintenance documents.
Par 204 Page 17 1 Visually inspect for cracks, wear and abrasion, loose or missing attachment bolts or screws, large dents, excessive screw hole wear, and loose or missing retaining hardware.
2 Check the cowl-to-spinner fit for damage and interference.
(c) Propeller Assembly.
1 Examine propeller attach bolts for looseness, missing safety wires, and cracks. Cracked or broken bolts may be the result of overtorquing. Correct torquing procedures are given in the manufacturer’s service instructions. The use of dry or lubricated bolt threads for torquing purposes is not standardized and may change from one engine to another using the same propeller. Cracks may be present in the hub area between or adjacent to bolt holes and along the hub pilot bore. Cracks in these areas cannot be repaired and require immediate removal of the propeller.
(aa) Visually inspect for cracks, corrosion, nicks, and damage.
(bb) Check the attaching bolt for improper torquing and improper safety.
(cc) Check the condition of fasteners, tie wires, clips, etc.
(dd) Check the adequacy of protective coatings, paint, and plating.
2 Conduct a functional check including revolutions per minute (RPM) control, RPM limits, idle setting, responsiveness, and vibration.
(3) Controllable Pitch/Constant Speed. A complete detailed inspection of controllable pitch propellers requires removal of the spinner for examination and servicing of the propeller hub and blade clamp area. All inspection and servicing of the pitch control mechanism should follow the recommendations of the propeller, engine, and airframe manufacturer. All propeller ADs must be checked for compliance.
(a) Propeller Blades.
1 Visually inspect for excessive wear and erosion, damage, nicks, cracks, corrosion, lightning strikes, and ground strikes.
2 Check for distortion, looseness, and pitch tip travel.
3 Check the adequacy of protective coatings, paint, and plating.
4 Check the leading edge erosion shield for damage, debonding, and cracks.
5 Check composite blades for delaminations, surface cracks, and exposed composite.
6 Perform tip-tracking procedures.
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(b) Spinner and Bulkhead Assembly. Detailed inspection procedures require removal of the propeller spinner for examination of the hub and blade retention area. Inspect the spinner and spinner bulkhead for cracks and repairs, such as stopdrill holes and weldments.
Spinner bulkhead attachment tabs and bolts frequently succumb to fatigue and should be checked. It is good practice to remove the spinner every 100 hours to observe the hub and blade retention area. If repairs are present, they should conform to the manufacturer’s maintenance documents.
1 Visually inspect for cracks, wear and abrasion, loose or missing attachment bolts or screws, large dents, excessive screw hole wear, and loose or missing retaining hardware.
2 Check the cowl-to-spinner fit for damage and interference that could affect propeller operation.
3 Check the security of balance weights.
(c) Propeller Assembly. Propeller attach bolts should be examined for looseness, missing safety wires, and cracks. Cracked or broken bolts are usually the result of overtorquing.
Correct torquing procedures are given in the manufacturer’s service instructions. The use of dry or lubricated bolt threads is not standardized and may change from one engine to another that use the same propeller.
1 Visually inspect for cracks, corrosion, nicks, damage, and oil and grease leaks.
2 Check the attaching bolt for improper torquing and improper safety ties and tabs.
3 Check the condition of fasteners, tie wires, clips, etc.
4 Check adequacy of protective coatings, paint, and plating.
5 Conduct a functional check including RPM control, RPM limits, idle setting, ability to feather, responsiveness, and vibration.
6 Check oil or grease for water and contamination with corrosion by products.
7 Examine the hub area for oil and grease leaks, missing lubrication fittings, and missing lubrication covers.
(d) Hub, Piston, Cylinder, Dome, and Blade Clamps. The hub and associated clamps, as well as pitch change mechanisms, should be inspected for corrosion from all sources, including rain, snow, and bird droppings that may have entered through spinner openings. On propeller models with blade clamps, when servicing the propeller thrust bearings through lubrication fittings in the blade clamps, the rear lubrication fitting on each clamp must be removed to avoid extruding grease past the bearing grease seal and damaging the seal. Look for water or discolored grease exiting the rear lubrication fitting. If clear or discolored water is present, it may be a sign of corrosion in the hub. Protective covers should be pressed over the
Par 204 Page 19 ends of all lubrication fittings. Check the blade seals for leakage. Propeller domes, pistons, and cylinders should be checked for leaks at the seals and on the gas fill valve (if so equipped).
Cracks may be present in the hub and retention area. Cracks in these areas cannot be repaired and require immediate removal of the propeller from further service.
(e) Pitch-Change Mechanisms. Fiber block pitch-change mechanisms should be inspected for deterioration, fit, and the security of the pitch-clamp forks.
1 Check all connections and cable routings between propeller governor and cockpit control.
2 Inspect for propeller operation, insecure mounting, binding, and restricted travel.
(f) Feathering Mechanisms. Certain models of full-feathering propellers use spring-loaded pins to retain the feathered blade position. Spring and pin units should be cleaned, inspected, and relubricated as per the manufacturer’s recommendations and applicable ADs.
Certain propeller models use an air charge for feathering. This should be checked for proper air pressure.
(g) Counterweights. Pitch change counterweights on blades or blade clamps should be inspected for security, cracks, corrosion, and to ensure that adequate counterweight clearance exists within the spinner.
(h) Deicing and Anti-Icing Devices.
1 Check the condition of rubber boots for damage, erosion, and attachment.
2 Check the condition of the slip ring and deice brushes for defects, excessive wear, and proper orientation.
3 Check the condition of the components, wire harness, and alcohol feed mechanism.
4 Conduct a functional check.
d. Teardown Inspections. Teardown inspections may only be conducted at an appropriately rated repair station. The entire propeller is disassembled and all components are inspected for wear, cracks, etc., in accordance with the manufacturer’s specifications. Teardown inspections are needed when the walk-around or detailed inspection uncovers problems that require further attention or when required by ADs or the manufacturer’s service limitations.
Teardown inspections are not considered within the scope of field inspections and repairs and may only be performed in accordance with the requirements of the propeller manufacturer.
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205. LIMITATIONS. Operational and service personnel should be familiar with the following limitations during any inspection.
a. Corrosion. Other than small areas of light surface corrosion with no evidence of pitting (if allowable corrosion is defined by the manufacturer), the presence of corrosion may require propeller removal and reconditioning by an appropriately rated repair station.
Intergranular corrosion may be present when the corrosion protective coatings (paint, anodize, etc.) have been lost. Corrosion pitting should be removed as described in the propeller manufacturer’s service documents and applicable ADs.
b. Unauthorized Straightening of Blades. A bent propeller cannot be straightened without special processing in an appropriately rated repair station because bending may harden the aluminum and lead to catastrophic blade failure. Careful visual inspection of the leading edges and the flat-faced portion of the blade may sometimes detect unapproved repairs. Any deformation of the flat portion, such as bows or kinks, may indicate previous unauthorized straightening of the blade. Sighting along the leading edge of a propeller blade for any signs of bending can also provide evidence of unapproved blade straightening. Blades should also be examined for any discoloration that would indicate unauthorized heating. Aluminum blades that have been heated for any repair must be removed from service since only cold (room temperature) straightening by an approved repair station is authorized. All blades showing evidence of unapproved repairs require removal of the entire propeller and proper assessment by a repair station. Field service personnel should never straighten bent propellers to facilitate shipping when the propellers are being sent to a repair station for inspection and repair. This procedure can conceal important information related to the severity of the damage.
c. Blade Shortening. Propeller tip damage will sometimes lead field maintenance personnel to consider removing damaged material from the blade tips. However, propellers are often “tuned” to the aircraft engine and airframe resonant frequency by being manufactured with a particular diameter to minimize vibration. Shortening of the blades without reference to approved data could create an unairworthy condition unless the manufacturer specifically permits shortening of the blades on a particular propeller. With certain limitations, specific minor repairs may be accomplished. The manufacturer’s service documents or AC 43.13-1, current edition, shows the criteria for determining whether a minor repair of a blade tip represents blade shortening. When conditions indicate, inspect the blade tips for evidence of shortening and, if necessary, measure the propeller diameter to determine if an unauthorized repair has altered it.
d. Minor Blade Damage Limits. Aircraft maintenance personnel should limit all blade repairs (except those performed at an appropriately rated repair facility) to those allowed by the manufacturer.
e. Blade Polishing. The FAA receives frequent inquiries from airplane owners and maintenance personnel asking whether it is acceptable to polish propeller blades. It is almost always not acceptable. Corrosion protection such as paint and anodize should not be removed from the surface of a propeller blade. Propeller blades must be maintained to the type design. If the original design had corrosion protection and the instructions for continued airworthiness call for corrosion protection, then the corrosion protection should be maintained to those instructions.
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206. PROCEDURES FOR MAINTENANCE. Maintenance begins with the operator and includes inspection together with regular care. Many maintenance tasks that may be accomplished at a minimum cost can extend the life of the propeller and reduce or prevent costly repairs. The following is a listing of what operators of aircraft CAN and CANNOT DO.
a. Operators Can Do the Following:
(1) Perform a visual preflight inspection of the blades for nicks, scratches, dents, erosion, corrosion, cracks, etc. Apparent damage found should be referred to an appropriately rated mechanic. A crack or bend is cause for removal of the propeller.
(2) Check the propeller spinner attaching screws for security and check the spinner for damage.
(3) Check the propeller for evidence of oil or grease leakage.
(4) Clean propeller blades periodically using fresh water, a nonalkaline cleaner, and a soft cloth or brush. Dry with a soft cloth.
(5) Ensure that the tachometer is appropriately marked for operational limitations of the propeller and that the tachometer accuracy is checked at periodic inspection intervals.
(6) Make sure that the applicable installation, information, and warning decals are on the propeller. These decals may include warnings against pushing or pulling on the propeller, the model number, the correct bolt torque, dynamic balancing information, and any other manufacturer’s identification.
(7) Each propeller should have its own maintenance record.
(8) The operator should recondition or overhaul the propeller when it reaches the manufacturer’s recommended service time limits.
(9) For safety and glare reduction for conventional single-engine tractor type aircraft, keep the blade backs painted flat black and the propeller tips painted with the appropriate colors to ensure good visibility. Repaint blades equally so that the balance of the propeller is not disturbed. Pusher props may have unique paint color requirements for good visibility for ground personnel. Refer to the propeller manufacturer’s maintenance documents.
(10) Leave two-bladed metal propellers in the one o’clock position to minimize bird droppings and water being retained in the spinner. Wood propellers should be stored horizontal to prevent moisture accumulation in one blade, which would cause unbalance.
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b. Operators Cannot Do the Following:
(1) Do not operate any aircraft after a propeller has been subjected to an impact without a thorough inspection by an appropriately rated person or repair station.
(2) Never straighten a damaged propeller. Even partial straightening of blades to ease shipment to a repair station may result in hidden damage not being detected and an unairworthy propeller being returned to service.
(3) Never repair any blade defect by welding, heating, or peening. Propeller manufacturers do not permit this because it can induce premature blade failure.
(4) Do not fill any damaged areas of metal blades with bulk-filler materials, such as epoxy or auto body fillers. This prevents areas of potential cracking from being inspected.
Additionally, filling a damaged area will not correct the stress risers caused by the dent or those caused by the loading that introduced the dent.
(5) Do not paint over areas of corrosion on blades. Corroded areas should be removed in accordance with approved procedures prior to applying the approved protective finish.
(6) Do not run up engines in areas containing loose rocks, gravel, or debris. Avoid quartering rear winds during ground run-up because this activity can cause damaging stresses.
(7) Do not push or pull on propeller blades when moving the aircraft by hand. Towbars are specifically designed for this operation.
(8) Do not polish blades unless specifically permitted by the manufacturer’s instructions.
(9) Never install a propeller on an aircraft unless it is a model approved by the aircraft TCDS or an appropriate Supplemental Type Certificate (STC). The service history must be properly documented, and a preinstallation inspection must indicate that the propeller is airworthy.
c. Minor Repairs of Aluminum Blades. Limited minor repairs may be made on propellers by appropriately rated maintenance technicians either on the aircraft or when the propeller is removed. Minor dents, cuts, scars, scratches, and nicks may be removed if their removal does not weaken the blade, substantially change weight or balance, or otherwise impair its performance. The following paragraphs give guidance on the methods for accomplishing minor repairs. Before attempting to repair a propeller blade, determine whether the propeller manufacturer has published damage limits that govern repair procedures applicable to that part.
Do not attempt a repair without knowing exactly what minor repairs, if any, are permissible. For example, straightening or reforming a blade is not considered a minor repair. Manufacturers’ service documents and AC 43.13-1, current edition, provide additional repair instructions.
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(1) Tools Required to Complete Minor Metal Blade Repairs Are:
(a) Fine-cut round and flat files.
(b) Ten-power loupe or magnifying glass.
(c) Emery cloth numbers 240, 320, and 600.
(d) Crocus cloth.
(2) Leading or Trailing Edge Damage. Refer to the propeller manufacturer’s maintenance documents for instructions on how to repair this damage. However, if the manufacturer did not publish this information, the following repairs can be made. For nicks, dents, pits, and cuts in the leading or trailing edges of blades, ensure that the bottom of the damage is removed first by rounding out and fairing in the repair only slightly deeper than the damage. Initial removal of material should be done using a fine-cut file. All traces of file marks in the repaired area should be removed with number 240 emery cloth followed by polishing with number 320 emery cloth, then finished with crocus cloth or 600-grit emery cloth, and then visually inspected. An individual edge repair should not exceed a depth of 3/16-inch. The depth of repair should be greater than the depth of damage as given in Table 2-1. When repaired areas do not overlap, more than one repair may be accomplished. The repair length should be 10 times longer than the depth of the repair as shown in Figure 2-5, Techniques for Blade Repair. Refer to the manufacturer’s instructions for repairs aft of the leading edge sections of the blade. For damage exceeding depths shown in Table 2-1, use the specific propeller manufacturer’s repair manual limits.
TABLE 2-1.
Blade Leading Edge Repair When Leading Edge Damage Is: Finished Repair Depth Is:
1/32-inch 1/16-inch 1/16-inch 3/32-inch 3/32-inch 5/32-inch 1/8-inch 3/16-inch
(3) Blade Face Surface Damage. Refer to the propeller manufacturer’s maintenance documents for instructions on how to repair this damage. However, if the manufacturer did not publish this information, the following repairs can be made. For gouges, cuts, and small dents on blade faces, ensure that the bottom of the damage is removed first by rounding out and fairing in the repair to form a saucer-shaped depression only slightly deeper than the damage. The initial repair should be accomplished by filing with a fine-cut file parallel to the damage and finishing with 240 and 320 emery abrasive cloths, as in the manner of damage removal from blade leading edges. Final polishing of the repair should be done with crocus cloth or 600-grit emery cloth. An individual repair should not exceed 1/16-inch in depth, and the surface radius of curvature of the repair must not be less than 3/8-inch. Repair width should be 30 times the repair depth as shown
Par 206 Page 24 in Figure 2-5. More than one repair is permitted when repair areas do not overlap an identical blade radius.
(4) Blade Tip Damage. Refer to the propeller manufacturer’s maintenance documents for instructions on how to repair this damage. However, if the manufacturer did not publish this information, the following repairs can be made. For nicks, dents, pits, and cuts in the tip of blades, repair in accordance with the procedures used for leading and trailing edge repair. Any removal of the blade tip material that reduces the blade radius below the minimum specified for the propeller manufacturer’s model designation and specific installation criteria is not permitted.
(5) Cracks. A crack in a blade may be discovered during the process of repair. Cracks found on a propeller CANNOT be repaired. The presence of a crack indicates that blade failure is virtually certain at any time. Cracks on the leading and trailing edges are especially prone to propagation. Blend outs or repairs should NEVER be attempted on these cracks. Propellers with cracks are unairworthy and MUST be removed from service and clearly identified as unairworthy.
(6) Inspection. Refer to the propeller manufacturer’s maintenance documents for instructions on how to repair this damage. However, if the manufacturer did not publish this information, the following inspection can be performed. Prior to return to service after minor repairs, the reworked area should be inspected with a minimum 10X power lens to ensure that any sharp notches at the bottom of the damage have been removed.