MAINTENANCE PRACTICES
61-00-36
Page 6-30
Rev. 2 Feb/17
Propeller Owner's Manual
136
9. Tachometer Calibration
WARNING:
OPERATION WITH AN INACCURATE
TACHOMETER CAN CAUSE RESTRICTED
RPM OPERATION AND DAMAGING HIGH
STRESSES. PROPELLER LIFE WILL
BE SHORTENED AND COULD CAUSE
CATASTROPHIC FAILURE.
A. All engine/propeller combinations have certain frequencies
(RPM) at which the propeller blade stresses begin to reach
design limits.
(1) In most cases, these frequencies are above the
maximum rated RPM of the engine.
(2) Some engine/propeller combinations have certain
ranges of RPM that are less than maximum engine
speed, where stresses are at a level considered too high
for continuous operation. This results in a “placarded
range” where continuous operation is not permitted.
(3) In other cases, the maximum permitted stresses occur at
an RPM only slightly above the maximum engine RPM.
(4) For these reasons, it is very important to accurately
monitor engine speed.
B. The accuracy of the tachometer is critical to the safe
operation of the aircraft.
(1) Some tachometers have been found to be in error by as
much as 200 RPM.
(2) Operating the aircraft with an inaccurate tachometer
could cause continued operation in a placarded range
of unacceptable high stresses, including repeatedly
exceeding the maximum engine RPM.
(3) Continuous operation in a placarded range of
unacceptable high stresses subjects the propeller to a
high amplitude frequency that causes stresses higher
than the design limits.
(4) Stresses that are higher than the design limits will
shorten the life of the propeller and could cause a
catastrophic failure.
