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.
Par 205 Page 20
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.
Par 206 Page 21
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.
Par 206 Page 22
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.
FIGURE 2-5.
Techniques for Blade Repair
BEFORE REPAIR AFTER REPAIR
- 1 inch - 3/8 inch R. Min.
Typical Dent or Gouge Illustration A * Max. Depth - = 1/16 inch 1: 3/8 inch in a Blade Face Maintain original radius - 3/8 inch R Minimum Illustration B Nick or Cut into Blade Leading Edge
- 3/16 inch Max. Depth 3/8 inch R. Min. - 1 inch- Typical Illustration C · 1/16 inch Longitudinal Cut Max. Depth 3/8 inch into Blade Face Illustration C Longitudinal Cut into Blade Face Par 206 Page 25
Par 206 Page 26
d. Minor Maintenance of Composite Blades. Maintenance of composite blades is permitted under some conditions. Follow the manufacturer’s instructions with regard to surface preparation and cleaning. According to manufacturer’s instructions, painting is permitted. The painted surface of the blade should be maintained to prevent erosion damage to the composite material. Replacement of expendable erosion strips or rebonding peeled edges on deice boots is permitted in accordance with the manufacturer’s instructions. Figure 2-6 shows composite blade erosion.
NOTE: Erosion damage to the composite material may require extensive repair at a certificated propeller repair station.
FIGURE 2-6.
Composite Blade Erosion
e. E xpendable Erosion Strips. When replaceable erosion strips are installed on propellers, they should be carefully inspected for wear during walk-around inspections. If the strips lack adhesion or are otherwise damaged, they should be replaced according to the manufacturer’s instructions. The addition of plastic strips without appropriate safeguards can affect propeller balance and otherwise degrade propeller performance. Ensure that all protector strips are approved by the propeller manufacturer or STC before adding them to propellers. The application of an erosion strip on a propeller deice or anti-ice boot may change the ice shedding performance of the boot and such usage must be approved.
Par 300 Page 27 CHAPTER 3. ACCESSORIES AND BALANCING
300. PROPELLER BALANCING. This section describes only the general issues and types of propeller balancing. It is not intended to provide instructions for maintenance personnel to perform static or dynamic propeller balancing.
a. Unbalance.
(1) Propellers can become unbalanced during operation from mishandling, wear, damage, and a variety of other causes. In some instances, the process of moving an aircraft by pushing or pulling on the propeller blades has bent or unseated the blades to an out-of-track condition or damaged the blade preload system. Propeller wear and damage, and the repair of that damage, are also contributors to propeller imbalance. Unauthorized or improper repair of propeller spinners has also been identified as a cause of propeller imbalance.
(2) Unbalance results when the mass of the propeller is not symmetrical around the center of rotation. When the mass is unsymmetrical, a radial force and/or out-of-plane moment couple is formed. Static and common dynamic balance procedures only correct the radial force unbalance by adding an equal force in the opposite direction with balance weights. Only trained, specially equipped, and authorized maintenance personnel should accomplish the dynamic balance procedures.
b. Balancing Difficulties. Propellers that cannot be balanced or propellers that are difficult to balance on the aircraft using approved dynamic balancing procedures may have latent problems that should be investigated. Loose components, loose flange bolts, fractured components, such as hubs or blades, or blades that are out-of-track or angled are some of the more likely problems that will cause balancing difficulties. These are problems that need to be addressed prior to further flight.
c. Balancing Methods. There are two methods of propeller balancing: static balancing and dynamic balancing. Neither method can replace the other because they are used for different purposes.
(1) Static Balancing. A propeller can be statically balanced only by removing it from the aircraft and evaluating the balance on a special fixture. Only appropriately certificated persons or organizations may adjust propeller static balance. Static balance weights are added to or removed from the propeller to correct the measured imbalance, or material from the blades is removed by special grinding techniques.
(2) Dynamic Balancing. Certain models of propellers may be dynamically balanced in place on the aircraft. Dynamic balancing of a propeller is done to provide for the lowest level of vibration in its operating range. Although the propeller is the focal point of the balancing procedure, it is the combination of the engine, engine-mounting system, and the propeller assembly that combine to provide the level of vibration.
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(a) When balance weights are added or removed from the propeller assembly, it is to reduce the level of vibration produced by the engine, engine-mounting system, and the propeller as an operating assembly.
(b) When a propeller that has been balanced as part of a particular engine and the engine-mounting system is removed from that assembly, it cannot be assumed that the dynamic balance would remain the same for another engine or engine-mounting system.
(c) For aircraft or propeller manufacturers that provide procedures for dynamic balancing of the propeller in their maintenance manuals or instructions for continued airworthiness, propeller balancing is not considered a major airframe alteration.
(d) When approved aircraft or propeller manufacturer’s procedures are not available, there are other acceptable dynamic propeller balancing procedures. These include, but are not limited to, the Chadwick-Helmuth Publication No. AW-9511-2, entitled “The Smooth Propeller,” and ACES Publication No. 100-OM-01, entitled “ACES Systems Guide to Propeller Balancing.” Dynamic balancing of propellers using FAA-approved or -accepted dynamic propeller balancing procedures is not considered a major propeller repair unless the propeller static balance weights are altered or when using the Chadwick-Helmuth or ACES type documents on propeller installations of 500 horsepower or more.
(e) Install a placard on the propeller hub or bulkhead stating that the propeller has been dynamically balanced and the assembly of the power train rotating components is an indexed assembly. An entry will be made in the logbook with the date, engine hours, final balance vibration, location of the dynamic balance weights, and signature and certificate number of the maintenance person.
(f) Remove any tape that is installed on the blades to conduct the dynamic balancing procedure upon completion of the balancing procedure. The tape has been known to trap moisture causing corrosion and subsequent blade failure. Dynamic balancing is done on wing and is not to be confused with static balancing. Static balancing is performed by the manufacturer or an appropriately rated repair station (see Figure 3-1).
FIGURE 3-1.
Balance Tape That Was Not Removed
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(g) Maintenance shall be performed in accordance with the performance rules of 14 CFR part 43, § 43.13(a).
(h) The dynamic balancing procedure and the propeller’s return to service must be accomplished by an appropriately rated person in accordance with the previously stated criteria.
301. TACHOMETER INSPECTION. Owing to the exceptionally high stresses that may be generated by particular propeller/engine combinations at certain operating ranges, many propeller and aircraft manufacturers have established revolutions per minute (RPM) restrictions and maximum RPM limits for some models. An improperly operating tachometer can cause an engine to exceed the maximum RPM limits. Since there are no post-manufacture accuracy requirements for engine tachometers, tachometer inaccuracy could be a direct cause of propeller failure, excessive vibration, or unscheduled maintenance. Proper tachometer operation and accuracy should always be checked (using the manufacturer’s procedure, if available) during normal maintenance intervals. One means of checking the tachometer’s accuracy is with a commercial strobe unit through which the rotating propeller is viewed. Strobe unit controls can be adjusted until the propeller image stops in space. A reading on the face of the strobe instrument indicates the propeller’s true RPM.
CAUTION: Be aware there may be multiple false RPM solutions if the instrument sees only one blade passage.
302. GOVERNORS, FEATHERING, AND REVERSING MECHANISMS. Always ensure proper operation of governors and other propeller controls. Check governor control rods periodically to ensure that they are properly lubricated according to the manufacturer’s instructions and that there are no worn parts, corrosion, or leaks. For reversible propellers, the functionality of the beta lockout system somewhere on the aircraft or the cockpit should be verified before flight or in accordance with the manufacturer’s recommendation. A beta lockout system is intended to prevent the pilot from inadvertently using reverse thrust in flight. The functionality of the propeller feathering system should also be verified before flight or in accordance with the manufacturer’s recommendation.
303. PROPELLER DEICERS.
a. Propeller deicing systems ensure safe propeller operation during icing conditions.
Therefore, it is important to periodically inspect deicer or anti-ice boots to ensure they are in an airworthy condition. Check the condition of the bond of the boot to the blade, looking for cracks and disbands. Look also for bumps, loose spots, or wrinkles in the boot. Ensure the propeller can be moved through its entire operating pitch range without placing any tension on wire leads or permitting them to interfere with or rub on nearby parts. Check for propeller resistance values between the deicer leads and ground.
b. When conducting a detailed inspection, remove the spinner. Check the wiring leads and harness for looseness and wear. Ensure that wiring clamps are secure. Check slip rings and brushes for wear. Electrically isolate the deicer circuits from other aircraft wiring and check for intermittent open circuits by moving the deicer straps slightly. Repairs to propeller deicers
Par 303 Page 30 should be made in accordance with the manufacturer’s instructions. If the ice protection system uses liquid-based anti-ice boots, check the condition of the slinger ring and the feed tubes.