(1) Evaluating propeller blade tracking can indicate much information about propeller condition. Accurate propeller tracking requires securing the aircraft in a stationary position and ensuring that the engine propeller shaft is tight against the thrust bearing. A blade-tracking datum can be made simply by placing a block on the ground in front of the aircraft in the propeller arc. Raise the block as required to obtain a clearance between the blade tip (blade vertical) and the datum block not exceeding 1/4-inch. Another method is to raise a block in front of the propeller with a small gap. A cowling fixture can be used as well. In some aircraft, it may be necessary to relieve engine compression (loosen spark plugs) and seek a calm day to eliminate outside disturbances that would cause an apparent tracking error. Draw a line on the block next to the blade tip position. Move the blade in a fore-and-aft direction and mark the limits of such motion. Pull all the blades past the drawn datum, checking fore-and-aft free play as before. No blade should deviate more than 1/16-inch from the plane of rotation, as defined by the drawn marks, unless the manufacturer’s service manuals define greater limits. Record any abnormal condition in the propeller log.

(2) It should be noted that some propeller blades require centrifugal load to seat properly in the hub so propellers of this type will show a large amount of free play. Follow the manufacturer’s tracking inspection instructions for these propellers. Blade-to-blade tracking difference could indicate a deformed blade. Free play differences on blades may indicate internal blade bearing, preload system, or actuating pin problems. Safety practices, such as ensuring that switches are off, or grounding the magnetos, are necessary to ensure that rotating the propeller does not cause the engine to start during the tracking inspection (see Figure 2-2). A repair station should not return propeller blades that exhibit any looseness or out-of-track conditions exceeding 1/16-inch (or the manufacturer’s specification) to service without inspection/repair.