Text-only reference. Published from the recorded official FAA Chapter 7 PDF. Diagrams, photographs, and figure artwork are not reproduced here; use the official FAA PDF for those materials.
7-20
Autofeathering System
An autofeather system is used normally only during takeoff, approach, and landing. It is used to feather the propeller automatically if power is lost from either engine. The system uses a solenoid valve to dump oil pressure from the propeller cylinder (this allows the prop to feather) if two torque switches sense low torque from the engine. This system has a test-off-arm switch that is used to arm the system. The autofeather system automatically energizes the holding coil (pulling in the feather button) when engine power loss results in a propeller thrust drop to a preset value. This system is switch-armed for use during takeoff and can function only when the power lever is near or in the “takeoff” position.
The NTS device mechanically moves the NTS plunger, which actuates a linkage in the propeller control when a predetermined negative torque value is sensed (when the propeller drives the engine). This plunger, working through control linkage, shifts the feather valve plunger, sending the blades toward feather. As the blade angle increases, negative torque decreases until the NTS signal is removed, closing the feather valve. If the predetermined negative torque value is again exceeded, the NTS plunger again causes the feather valve plunger to shift. The normal effect of the NTS is a cycling of rpm slightly below the rpm at which the negative torque was sensed.
Unfeathering is initiated by pulling the feather button to the “unfeather” position. This action supplies voltage to the auxiliary motor to drive the auxiliary pump. Because the propeller governor is in an underspeed position with the propeller feathered, the blades will move in a decreased pitch direction under auxiliary pump pressure. The pitch lock operates in the event of a loss of propeller oil pressure or an overspeed. The ratchets of the assembly become engaged when the oil pressure, which keeps them apart, is dissipated through a flyweight-actuated valve, which operates at an rpm slightly higher than the 100% rpm The ratchets become disengaged when high pressure and rpm settings are restored.
At the “flight idle” power lever position, the control beta follow-up low-pitch stop on the beta set cam (on the alpha shaft) is set about 2' below the flight low-pitch stop setting, acting as a secondary low-pitch stop. At the “takeoff” power lever position, this secondary low-pitch stop sets a higher blade angle stop than the mechanical flight low-pitch stop. This provides for control of overspeed after rapid power lever advance, as well as a secondary low-pitch stop.
Propeller Inspection & Maintenance
Propellers must be inspected regularly. The exact time interval for particular propeller inspections is usually specified by the propeller manufacturer. The regular daily inspection of propellers varies little from one type to another. Typically, it is a visual inspection of propeller blades, hubs, controls, and accessories for security, safety, and general condition. Visual inspection of the blades does not mean a careless or casual observation. The inspection should be meticulous enough to detect any flaw or defect that may exist. Inspections performed at greater intervals of time (e.g., 25, 50, or 100 hours) usually include a visual check of: 1. Blades, spinners, and other external surfaces for excessive oil or grease deposits.
2. Weld and braze sections of blades and hubs for evidence of failure. 3. Blade, spinner, and hubs for nicks, scratches, or other flaws. Use a magnifying glass if necessary. 4. Spinner or dome shell attaching screws for tightness. 5. The lubricating requirements and oil levels, when applicable. If a propeller is involved in an accident, and a possibility exists that internal damage may have occurred, or if a propeller has had a ground strike or sudden stoppage, the recommendations of the engine and propeller manufacturer's maintenance manual need to be adhered to. The propeller should be disassembled and inspected. Whenever a propeller is removed from a shaft, the hub cone seats, cones, and other contact parts should be examined to detect undue wear, galling, or corrosion.
It is also vitally important to keep up-to-date airworthiness directives (ADs) or service bulletins (SBs) for a propeller. Compliance with ADs is required to make the aircraft legally airworthy, but it is also important to follow the SBs. All work performed on the propeller, including AD and SB compliance, should be noted in the propeller logbook. The propeller inspection requirements and maintenance procedures discussed in this section are representative of those in widespread use on most of the propellers described in this chapter. No attempt has been made to include detailed maintenance procedures for a particular propeller, and all pressures, figures, and sizes are solely for the purpose of illustration and do not have specific application. For maintenance information on a specific propeller, always refer to applicable manufacturer instructions.
7-21 Lightning conductor earthed to metal root Carbon/glass fiber envelope All over polyurethane spray coat Carbon fiber spars Polyurethane foam core Nickel leading-edge protection
Wood Propeller Inspection
Wood propellers should be inspected frequently to ensure airworthiness. Inspect for defects, such as cracks, dents, warpage, glue failure, delamination defects in the finish, and charring of the wood between the propeller and the flange due to loose propeller mounting bolts. Examine the wood close to the metal sleeve of wood blades for cracks extending outward on the blade. These cracks sometimes occur at the threaded ends of the lag screws and may be an indication of internal cracking of the wood. Check the tightness of the lag screws, which attach the metal sleeve to the wood blade, in accordance with the manufacturer’s instructions. In-flight tip failures may be avoided by frequent inspections of the metal cap, leading edge strip, and surrounding areas. Inspect for such defects as looseness or slipping, separation of soldered joints, loose screws, loose rivets, breaks, cracks, eroded sections, and corrosion. Inspect for separation between the metal leading edge and the cap, which would indicate the cap is moving outward in the direction of centrifugal force. This condition is often accompanied by discoloration and loose rivets. Inspect the tip for cracks by grasping it with your hand and slightly twisting about the longitudinal blade centerline and by slightly bending the tip backward and forward. If the leading edge and the cap have separated, carefully inspect for cracks at this point. Cracks usually start at the leading edge of the blade. Inspect moisture holes to ensure that they are open. A fine line appearing in the fabric or plastic may indicate a crack in the wood. Check the trailing edge of the propeller blades for bonding, separation, or damage.
Metal Propeller Inspection
Metal propellers and blades are generally susceptible to fatigue failure resulting from the concentration of stresses at the bottoms of sharp nicks, cuts, and scratches. It is necessary, therefore, to frequently and carefully inspect them for such defects and make repairs promptly. The inspection of steel blades may be accomplished by either visual, fluorescent penetrant or magnetic particle inspection. The visual inspection is easier if the steel blades are covered with engine oil or rust-preventive compound. The full length of the leading edge (especially near the tip), the full length of the trailing edge, the grooves and shoulders on the shank, and all dents and scars should be examined with a magnifying glass to decide whether defects are scratches or cracks.
Tachometer inspection is a very important part of the overall propeller inspection. Operation with an inaccurate tachometer may result in restricted rpm operation and damaging high stresses. This could shorten blade life and could result in catastrophic failure. If the tachometer is inaccurate, then the propeller could be turning much faster than it is rated to turn, providing extra stress. Accuracy of the engine tachometer should be verified at 100-hour intervals or at annual inspection, whichever occurs first. Hartzell Propeller recommends using a tachometer that is accurate within ± 10 rpm and has an appropriate calibration schedule.
Aluminum Propeller Inspection Carefully inspect aluminum propellers and blades for cracks and other flaws. A transverse crack or flaw of any size is cause for rejection. No repairs are permitted to the shanks (roots or hub ends) of aluminum-alloy, adjustable-pitch blades. The shanks must be within manufacturer’s limits. Multiple deep nicks and gouges on the leading edge and face of the blade is cause for rejection. Use dye penetrant or fluorescent dye penetrant to confirm suspected cracks found in the propeller. Refer any unusual condition or appearance revealed by these inspections to the manufacturer.
Composite Propeller Inspection
Composite blades need to be visually inspected for nicks, gouges, loose material, erosion, cracks and debonds, and lightning strike. [Figure 7-32] Composite blades are inspected for delaminations and debonds by tapping the blade or cuff (if applicable) with a metal coin. If an audible change is apparent, sounding hollow or dead, a debond or delamination is likely. [Figure 7-33] Blades that incorporate a “cuff” have a different tone when coin tapped in the cuff area. To avoid confusing the sounds, coin tap the cuff area and the transition area between the cuff and the blade separately from the blade area. Additional nondestructive testing (NDT) techniques for composite materials, such as phased array inspections, and ultrasound inspections, are available for more detailed inspections.
Repairs to propellers are often limited to minor type repairs. Certificated mechanics are not allowed to perform major repairs on propellers. Major repairs need to be accomplished by a certificated propeller repair station. 7-22 LIBERTY 2008 LIBERTY 2008 Coin tap along the entire surface of the erosion shield to check for debonds. Coin tap the composite blade suface to check for delaminations. Wood block ± 1/16" Cowling fixture
Propeller Vibration
Although vibration can be caused by the propeller, there are numerous other possible sources of vibration that can make troubleshooting difficult. If a propeller vibrates, whether due to balance, angle, or track problems, it typically vibrates throughout the entire rpm range, although the intensity of the vibration may vary with the rpm. If a vibration occurs only at one particular rpm or within a limited rpm range (e.g., 2200– 2350 rpm), the vibration is not normally a propeller problem but a problem of a poor engine-propeller match. If a propeller vibration is suspected but cannot be positively determined, the ideal troubleshooting method is to temporarily replace the propeller with one known to be airworthy and then test fly the aircraft if possible. Blade shake is not the source of vibration problems. Once the engine is running, centrifugal force holds the blades firmly (approximately 30,000–40,000 pounds) against blade bearings. Cabin vibration can sometimes be improved by reindexing the propeller to the crankshaft. The propeller can be removed, rotated 180°, and reinstalled. The propeller spinner can be a contributing factor to an out-of- balance condition. An indication of this would be a noticeable spinner wobble while the engine is running. This condition is usually caused by inadequate shimming of the spinner front support or a cracked or deformed spinner.
When powerplant vibration is encountered, it is sometimes difficult to determine whether it is the result of engine vibration or propeller vibration. In most cases, the cause of the vibration can be determined by observing the propeller hub, dome, or spinner while the engine is running within a 1,200- to 1,500-rpm range and determining whether or not the propeller hub rotates on an absolutely horizontal plane. If the propeller hub appears to swing in a slight orbit, the vibration is usually caused by the propeller. If the propeller hub does not appear to rotate in an orbit, the difficulty is probably caused by engine vibration.
When propeller vibration is the reason for excessive vibration, the difficulty is usually caused by propeller blade imbalance, propeller blades not tracking, or variation in propeller blade angle settings. Check the propeller blade tracking and then the low-pitch blade angle setting to determine if either is the cause of the vibration. If both propeller tracking and low blade angle setting are correct, the propeller is statically or dynamically unbalanced and should be replaced, or re- balanced if permitted by the manufacturer.
Blade Tracking
Blade tracking is the process of determining the positions of the tips of the propeller blades relative to each other (blades rotating in the same plane of rotation). Tracking shows only the relative position of the blades, not their actual path. The blades should all track one another as closely as possible. The difference in track at like points must not exceed the tolerance specified by the propeller manufacturer. The design and manufacture of propellers is such that the tips of the blades give a good indication of tracking. The following method for checking tracking is normally used: 1. Chock the aircraft so it cannot be moved.
2. Remove one spark plug from each cylinder. This makes the propeller easier and safer to turn. 3. Rotate one of the blades so it is pointing down. 4. Place a solid object (e.g., a heavy wooden block that is at least a couple of inches higher off the ground than the distance between the propeller tip and the ground) next to the propeller tip so that it just touches or attaches a pointer/indicator to the cowling itself. [Figure 7-34] 5. Rotate the propeller slowly to determine if the next blade tracks through the same point (touches the block/ pointer). Each blade track should be within 1⁄16 inch 7-23 Protractor TE97 Reference blade radius location Bench top protractor TE96 30 20 10 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 170 160 150 140 130 120 110 100 90 80 70 60 50 10 0 10 Degree scale on disc Vernier scale on ring Disc-to-ring lock Ring adjuster Center spirit level Disc adjuster Ring Ring-to-frame lockCorner spirit level Disc Edge used for checking against the propeller blade Edge used for checking against the propeller blade Nut—loosen to rotate face of level (plus or minus) from the opposite blade’s track.
6. An out-of-track propeller, may be due to one or more propeller blades being bent, a bent propeller flange, or propeller mounting bolts that are either over- or undertorqued. An out-of-track propeller causes vibration and stress to the airframe and engine and may cause premature propeller failure.
Checking & Adjusting Propeller Blade Angles
When you find an improper blade angle setting during installation or when indicated by engine performance, follow basic maintenance guidelines. From the applicable manufacturer’s instructions, obtain the blade angle setting and the station at which the blade angle is checked. Do not use metal scribes or other sharply pointed instruments to mark the location of blade stations or make reference lines on propeller blades, since such surface scratches can induce failure (stress concentrator), eventually resulting in blade failure. Use a bench-top protractor if the propeller is removed from the aircraft. [Figure 7-35] Use a handheld protractor (a digital protractor provides an easy measurement) to check blade angle if the propeller is installed on the aircraft or is placed on the knife-edge balancing stand. [Figure 7-36] Universal Propeller Protractor The universal propeller protractor can be used to check propeller blade angles when the propeller is on a balancing stand or installed on the aircraft engine. Figure 7-37 shows the parts and adjustments of a universal propeller protractor.
The following instructions for using the protractor apply to a propeller installed on the engine. Turn the propeller until the first blade to be checked is horizontal with the leading edge up. Place the corner spirit level at right angles to the face of the protractor. Align degree and vernier scales by turning the disc adjuster before the disc is locked to the ring. The locking device is a pin that is held in the engaged position by a spring. The pin can be released by pulling it outward and turning it 90°. Release the ring-to-frame lock (a right-hand screw with thumb nut) and turn the ring until both ring and disc zeros are at the top of the protractor.
Check the blade angle by determining how much the flat side of the block slants from the plane of rotation. First, locate a point to represent the plane of rotation by placing the protractor vertically against the end of the hub nut or any convenient surface known to lie in the plane of propeller 7-24 rotation. Keep the protractor vertical by the corner spirit level and turn the ring adjuster until the center spirit level is horizontal. This sets the zero of the vernier scale at a point representing the plane of propeller rotation. Then, lock the ring to the frame. While holding the protractor by the handle with the curved edge up, release the disc-to-ring lock. Place the forward vertical edge (the edge opposite the one first used) against the blade at the station specified in the manufacturer’s instructions. Keep the protractor vertical by the corner spirit level and turn the disc adjuster until the center spirit level is horizontal. The number of degrees and tenths of a degree between the two zeros indicates the blade angle.
In determining the blade angle, remember that ten points on the vernier scale are equal to nine points on the degree scale. The graduations on the vernier scale represent tenths of a degree, but those of the degree scale represent whole degrees. The number of tenths of a degree in the blade angle is given by the number of vernier scale spaces between the zero of the vernier scale and the vernier scale graduation line nearest to perfect alignment with a degree scale graduation line. This reading should always be made on the vernier scale. The vernier scale increases in the same direction that the protractor scale increases. This is opposite to the direction of rotation of the moving element of the protractor. After making any necessary adjustment of the blade, lock it in position and repeat the same operations for the remaining blades of the propeller.
Propeller Balancing
Propeller unbalance, which is a source of vibration in an aircraft, may be either static or dynamic. Propeller static imbalance occurs when the center of gravity (CG) of the propeller does not coincide with the axis of rotation. Dynamic unbalance results when the CG of similar propeller elements, such as blades or flyweights, does not follow in the same plane of rotation. Since the length of the propeller assembly along the engine crankshaft is short in comparison to its diameter, and since the blades are secured to the hub so they lie in the same plane perpendicular to the running axis, the dynamic unbalance resulting from improper mass distribution is negligible, provided the track tolerance requirements are met. Another type of propeller unbalance, aerodynamic unbalance, results when the thrust (or pull) of the blades is unequal. This type of unbalance can be largely eliminated by checking blade contour and blade angle setting.
Static Balancing The knife-edge test stand has two hardened steel edges mounted to allow the free rotation of an assembled propeller between them. [Figure 7-38] The knife-edge test stand must be located in a room or area that is free from any air motion, and preferably removed from any source of heavy vibration. The standard method of checking propeller assembly balance involves the following sequence of operations: 1. Insert a bushing in the engine shaft hole of the propeller. 2. Insert a mandrel or arbor through the bushing. 3. Place the propeller assembly so that the ends of the arbor are supported upon the balance stand knife- edges. The propeller must be free to rotate.
If the propeller is properly balanced statically, it remains at any position in which it is placed. Check two-bladed propeller assemblies for balance: first with the blades in a vertical position and then with the blades in a horizontal position. Repeat the vertical position check with the blade positions reversed; that is, with the blade that was checked in the downward position placed in the upward position. Check a three-bladed propeller assembly with each blade placed in a downward vertical position. [Figure 7-39] During a propeller static balance check, all blades must be at the same blade angle. Before conducting the balance check, inspect to see that each blade has been set at the same blade angle.
Unless otherwise specified by the manufacturer, an acceptable balance check requires that the propeller assembly have no tendency to rotate in any of the positions previously described. If the propeller balances perfectly in all described positions, it should also balance perfectly in all intermediate positions. When necessary, check for balance in intermediate positions to verify the check in the originally described positions. [Figure 7-40] When a propeller assembly is checked for static balance and there is a definite tendency of the assembly to rotate, certain corrections to remove the unbalance are allowed.
1. The addition of permanent fixed weights at acceptable locations when the total weight of the propeller assembly or parts is under the allowable limit. 2. The removal of weight at acceptable locations when the total weight of the propeller assembly or parts is equal to the allowable limit. The location for removal or addition of weight for propeller unbalance correction has been determined by the propeller manufacturer. The method and point of application of unbalance corrections must be checked to see that they are according to applicable drawings.