Hartzell HC-B3-2 Series Owner's Manual And Logbook page 249

(2) The accuracy of the tachometer is critical to the safe
operation of the aircraft.
(a) Some tachometers have been found to be in error by
as much as 200 RPM.
(b) Operating the aircraft with an inaccurate tachometer
could cause continued operation at unacceptably
high stresses, including repeatedly exceeding the
maximum engine RPM.
(c) Continuous operation in a restricted RPM range
subjects the propeller to stresses that are higher
than the design limits.
(d) Stresses that are higher than the design limits will
shorten the life of the propeller and could cause a
catastrophic failure.
B. Tachometer Calibration
(1) Hartzell Propeller Inc. recommends that propeller
owners/operators calibrate the engine tachometer in
accordance with the National Institute of Standards
and Technology (NIST) or similar national standard
(traceable).
(2) Contact Hartzell Propeller Inc. if the propeller was
operated in a restricted RPM range because of a
tachometer error.
MAINTENANCE PRACTICES
Propeller Owner's Manual
139
61-00-39
Page 6-29
Rev. 17 Apr/20