Definition
A cockpit instrument that displays engine speed, expressed in revolutions per minute (RPM). On a fixed-pitch propeller airplane, it indicates both engine and propeller RPM, since they turn together. On a constant-speed propeller airplane, it indicates engine RPM, which the propeller governor maintains at the value the pilot has selected.
Plain English
A gauge that shows how fast the engine is spinning, measured in turns per minute.
Context Anchor
Seen on the engine instruments, especially when setting propeller speed during takeoff, climb, cruise, and descent.
Derivation
From the Greek 'tachos' meaning speed, plus 'meter' meaning measure. So literally: a speed-measuring device. Knowing this makes it easy to remember it measures rotational speed of the engine.
Why Pilots Care
Allows precise power management and prevents engine overspeed damage while confirming that the selected throttle and propeller settings are producing the intended RPM.
Intuition Check
A tachometer does not show airspeed, and it does not show engine power by itself. It shows how fast the engine and propeller are turning.
Example Sentence 1
During the run-up, the pilot advanced the throttle until the tachometer read 1,800 RPM and then checked each magneto.
Example Sentence 2
With the constant-speed propeller in high RPM, the tachometer remained at 2,450 while the pilot adjusted manifold pressure for climb power.