Plain English Meaning

A gauge that shows how fast the aircraft is going compared to the speed of sound. A reading of 0.80 means the aircraft is moving at 80 percent of the speed of sound around it.

Full Definition

A cockpit instrument that displays the aircraft's current speed as a Mach number — the ratio of the aircraft's true airspeed to the local speed of sound. The reading changes with altitude and temperature because the speed of sound varies with those conditions.

Context Anchor

Seen in high-altitude and high-speed aircraft operations, especially when monitoring speed limits based on Mach number instead of only knots.

Derivation

Named after Ernst , an Austrian physicist who studied the behaviour of objects moving at high speeds through . The suffix '-' means 'a device that measures.' So a Machmeter is literally a device that measures Mach number — speed expressed as a of the speed of sound.