When you find an improper blade angle setting during  installation or is 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 to make reference lines on propeller blades, since  such surface scratches can eventually result 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]  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 disk adjuster before the disk  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 disk zeros  are at the top of the protractor. Figure 7-34.  Propeller blade tracking.  Figure 7-35.  Blade angle measurement.  7-23 Universal Propeller Protractor Checking and Adjusting Propeller Blade Angles 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  (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. Protractor TE97 Reference blade radius location Bench top protractor TE96 Figure 7-36.  Bench top protractor.  Wood block ±  $^{1}$/16" Cowling fixture