The tach(ometer) timer is an instrument used in
aviation
Aviation includes the activities surrounding mechanical flight and the aircraft industry. ''Aircraft'' includes fixed-wing and rotary-wing types, morphable wings, wing-less lifting bodies, as well as lighter-than-air craft such as hot air ...
to accumulate the total number of
revolutions
In political science, a revolution (Latin: ''revolutio'', "a turn around") is a fundamental and relatively sudden change in political power and political organization which occurs when the population revolts against the government, typically due ...
performed by the engine. The unit of measure is equivalent to the number of hours of running at a certain, specific reference speed of rotation. If the reference speed of rotation is 2400
RPM
Revolutions per minute (abbreviated rpm, RPM, rev/min, r/min, or with the notation min−1) is a unit of rotational speed or rotational frequency for rotating machines.
Standards
ISO 80000-3:2019 defines a unit of rotation as the dimensionl ...
then the timer runs in
real time when the engine is running at 2400 RPM, half speed while the engine is run at 1200 RPM (a fast idle for some aviation engines) or at 5/6ths real time at 2000 RPM (a slow cruise speed).
The tach timer integrates over time the instantaneous rotation speed displayed by the
tachometer
A tachometer (revolution-counter, tach, rev-counter, RPM gauge) is an instrument measuring the rotation speed of a shaft or disk, as in a motor or other machine. The device usually displays the revolutions per minute (RPM) on a calibrated analo ...
. The displayed number is incremented by one if the engine is run at its reference speed for one hour. The quantity recorded is referred to as tach(ometer) hours. If the reference rotation speed is 2400 RPM then the tach timer records
:
Uses
The tach timer is usually used to schedule engine maintenance, although it is just an approximation of "Time in service" which is used to time and schedule aircraft maintenance. Time in service is defined in 14 CFR 1.1
as the actual time in the air, whereas tach time measures engine revolutions, which would still count time on the ground while the engine is idling (at a lower rate).
It can also be used as a basis for charging for aircraft rental as opposed to charging for elapsed time. This encourages the renter to properly warm the engine before take off and not to run the engine continuously at maximum speed.
See also
*
Hobbs meter
Hobbs meter is a genericized trademark for devices used in aviation to measure the time that an aircraft is in use. The meters typically display hours and tenths of an hour, but there are several ways in which the meter may be activated:
# It c ...
References
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Aircraft instruments