Definition
Cockpit instruments that display engine rotational speed. In a piston-engine aircraft, the tachometer shows crankshaft speed in revolutions per minute (RPM). In a turbine-engine aircraft, it typically shows the rotational speed of one or more compressor sections as a percentage of their rated maximum.
Plain English
Gauges that tell the pilot how fast the engine is turning.
Context Anchor
Seen on the instrument panel during power changes, engine checks, and instrument flying with analog cockpit instruments.
Derivation
From the Greek 'tachos' meaning 'speed' and '-meter' meaning 'measuring device.' A tachometer literally is a speed-measuring device, in this case measuring how fast the engine is rotating.
Why Pilots Care
Tachometer readings guide power adjustments, prevent engine overstress, and confirm proper operation across flight phases.
Intuition Check
A tachometer does not show how fast the airplane is moving through the air. It shows how fast the engine or propeller is turning.
Example Sentence 1
After starting the engine, the pilot checked the tachometer to confirm the RPM had stabilized within the normal idle range.
Example Sentence 2
During the climb checklist, both tachometers were checked to confirm they matched the target RPM for that altitude.