Definition
An instrument that measures and displays the rotational speed of an engine's crankshaft (in a piston engine) or a turbine shaft (in a turbine engine). Piston-engine tachometers read in revolutions per minute (RPM); turbine tachometers typically read in percent of maximum rated RPM (% N1, % N2, etc.).
Plain English
A gauge that shows how fast the engine is spinning.
Context Anchor
Seen on engine instruments, maintenance checks, engine run-up checks, and engine operating limits.
Derivation
From the Greek 'tachos' meaning 'speed' and '-meter' meaning 'measuring device'. Literally a 'speed measurer'. The Greek root makes clear that it measures rate of motion, in this case rotational speed.
Why Pilots Care
Enables the pilot to confirm the engine is operating within its safe RPM range and to detect performance problems such as overspeed or failure to reach takeoff power.
Intuition Check
A tachometer does not show aircraft speed. It shows the turning speed of the engine or a rotating engine-driven part.
Example Sentence 1
During run-up, the pilot advanced the throttle to 1,800 RPM on the tachometer before checking the magnetos.
Example Sentence 2
After an oil change the mechanic verifies that idle RPM on the tachometer matches the value specified in the engine manual.