Are you reading “Tacky Tachs” !?
Tachometer inspections are important. 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. It is important to know if an airplane has a new propeller model installed (e.g. STC or Upgrade Service Bulletin) to be sure that the tachometer markings align with the acceptable rpm ranges for that new propeller.
CAUTION: Be aware there may be multiple false RPM solutions if the instrument sees only one blade passage.
You can find the subject of this maintenance tip and more enlightening maintenance information by reading Advisory Circular 20-37E. You can access and download a free copy of Advisory Circulars using FAASafety.gov or www.FAA.gov.
