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.
Par 300 Page 28
(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.
Par 400 Page 31 CHAPTER 4. INSTALLATION
400. PROPELLER INSTALLATION. Propellers must be installed by an appropriately rated mechanic or repair station. Certain propellers are only partially assembled for shipping prior to installation on the aircraft and must be assembled during installation. The propeller manufacturer will provide explicit instructions on assembly during installation that must be carefully followed.
Ensure that correct seals and O-rings are used during assembly. Only the propeller manufacturer’s bolt or nut torque requirements should be used for the installation. On some propellers, the correct installation torques may be shown on a propeller hub decal. Whether to use or not use lubrication during torquing is important, and the manufacturer’s instructions must be carried out. When the propeller is correctly torqued, the blade track should be checked to the tolerances specified by the manufacturer. New or reconditioned propellers will be statically balanced by the manufacturer or propeller repair facility. However, if after installation the propeller/engine combination begins to immediately run rough (vibration) on the ground or in flight, the propeller should be removed, rotated 180 degrees on the engine crankshaft (if not indexed), and reinstalled and the blade track should be checked again. Certain propeller installations may require additional dynamic propeller balancing on the engine. Chapter 3, paragraph 300 discusses the general guidelines for dynamic balancing. Proper pitch change action and revolutions per minute (RPM) parameters must be checked during run-up and the installation inspected for oil and grease leaks. Spinner assemblies should be inspected during installation in accordance with the propeller or aircraft manufacturers’ guidelines. Inspect spinners and backplates for warping, cracks, looseness, missing parts, fasteners, improper repairs, or unauthorized modifications (including addition of balance weights).
Appendix 1 APPENDIX 1.
Glossary of Common Propeller Terms Blade Angle. An angle between the chord line of a propeller blade section and a plane perpendicular to the axis of propeller rotation.
Blade Station. See “Stations.”
Blade Track. The path established by the tip of each blade as it passes a fixed point in rotation.
Blade. One arm of a propeller from the hub to tip.
Chord Line. A straight line drawn between the leading and trailing edges of the blade.
Chord. The distance from the blade leading edge to the trailing edge.
Corrosion, Pitting. Visible cavities extending inward from the metal surface due to chemical or electrochemical action.
Corrosion, Intergranular. Intergranular corrosion occurs in the grain boundaries of some alloys of aluminum. The presence of intergranular corrosion may be the result of fatigue, stress, or the continued presence of moisture such as under a decal or in a fastener hole. Sometimes called exfoliation corrosion when it occurs in forgings, extrusions, or rolled sheets.
Corrosion, Surface. Surface loss of metal due to chemical or electrochemical action. On polished surfaces, it begins as a dulling of surface, soon becoming frosted, followed by widespread pitting.
Corrosion. Deterioration of a metal because of reaction with its environment. For further references, refer to Advisory Circular (AC) 43-4, Corrosion Control for Aircraft.
Crack. A physical opening or fissure within the body of a material. May be either internal within the material or at the surface (surface breaking). On a propeller, cracks can be started by cuts, nicks, or corrosion.
Cut. A deep, long, narrow fissure usually caused by a sharp object.
Debond/Disbond. An adhesive or cohesive failure or separation along a bonded interface between two or more surfaces.
Delamination. Separation between adjacent layers of a laminate.
Dent. A depression in the blade as the result of impact from a blunt object.
Depression. Concave surface deformation.
Diameter, Propeller. The diameter of the circle circumscribed by the blade tips.
Appendix 1 Erosion. Surface removal of material by mechanical action of grit, sand, water, etc., usually present on the leading edge close to the tip.
Face Surface. The flat, or very slightly curved, side of the propeller blade against which the relatively higher pressure acts to produce thrust when the propeller is rotated.
Gouge. A deep groove on a blade formed by a heavy pressure contact with a solid object.
Horizontal Balance. The blade balance process that locates the center of blade mass along the radial direction to reduce unbalance of an assembled propeller.
Hub. Center section of the propeller that carries the blades and is attached to the engine shaft.
Impact Damage. Damage (visible or not) resulting from a blade striking or being struck, while in flight or on the ground, by an object, such as a snow bank, runway light, maintenance stand, birds, etc.
Inclusion. Material foreign to base metal but contained in it.
Leading Edge. The edge of the blade that leads the direction of rotation and first encounters the air.
Lightning Strikes. Damage to blades caused by lightning usually manifested by localized burning, discoloration, melted metal, and/or pits.
Nick. A sharp notchlike displacement of metal usually found on leading and trailing edges.
Overhaul. Complete teardown and reassembly associated with major repair or maintenance. The terms overhaul and reconditioning are synonymous for fixed-pitch propellers.
Pitch. The theoretical distance that the propeller blade reference station would move forward if it were moving along a helix with an angle equal to the blade section angle.
Pitting. Small irregularly shaped cavities from which material has been removed by erosion or corrosion. Corrosive pitting is usually accompanied by a deposit such as rust, a corrosion byproduct, formed by a corrosive agent on the base metal.
Propeller, Constant Speed. A propeller in which a governor is used to automatically provide constant revolutions per minute (RPM) as the pilot selects the propeller pitch setting.
Propeller, Controllable Pitch. A propeller with blades that may be altered continuously to any desired angle during flight.
Propeller, Fixed-Pitch. A one-piece propeller with fixed blade angles.
Propeller, Full-Feathering. A propeller with blades that can be rotated to a high positive blade angle to stop rotation (windmilling). This feature is common on multiengine aircraft because it allows an engine to be shut down and rotation stopped to reduce drag and asymmetric control forces.
Appendix 1 Propeller, Reversing. A propeller in which blades can be rotated to a negative blade angle where they will provide a rearward thrust to slow down, stop, or move the aircraft backward.
Propeller. A device for propelling an aircraft that has blades on an engine-driven shaft and that, when rotated, produces by its action on the air, a thrust approximately perpendicular to its plane of rotation. It includes associated control components normally supplied by its manufacturer.
Reconditioning. The repair of major or minor blade damage caused by erosion or striking small objects during normal operation. Reconditioning consists of penetrant inspection, refinishing, and rebalancing. It is accomplished on an irregular basis as necessary and required.
Repair. The correction, on an irregular basis as necessary, of minor or major damage caused by a physical mishap. The amount, degree, and extent of damage determine whether the propeller can be repaired as a minor repair in the field by a mechanic.
Score. Groove-like indentations from rubbing by a hard, rough surface. See “Gouge.”
Shank. The portion of the blade from the butt face to the first blade station.
Stations. Locations perpendicular to the blade center line at which dimensions are checked.
Tip. The portion of the blade outermost from the axis of propeller rotation.
Tracking. See “Blade Track.”
Trailing Edge. The rear edge of the blade where the air leaves the blade.
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