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  Figure 7-32.  Composite blade construction  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. 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. 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. 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  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: Blade Tracking Propeller Vibration L $_{$_{I}$B}$E R T Y 2 0 8 L $_{$_{I}$B}$E R T Y 2 0 8 Coin tap along the entire  surface of the erosion shield  to check for debonds. Coin tap the  composite blade  suface to check  for delaminations. Figure 7-33.  Coin-tap test to check for debonds and delaminations.  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. 7-22