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.
Plain English
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.
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 Mach, an Austrian physicist who studied the behaviour of objects moving at high speeds through air. The suffix '-meter' means 'a device that measures.' So a Machmeter is literally a device that measures Mach number — speed expressed as a fraction of the speed of sound.
Why Pilots Care
Exceeding the critical Mach number can cause shock waves, buffet, and loss of control authority.
Analogy
A normal speed display tells you how fast you are moving across the ground or through the air. A Machmeter tells you how close you are to the speed of sound, which changes with air temperature.
Intuition Check
A Machmeter is not just another airspeed indicator. It shows speed as a comparison to the speed of sound, not simply as knots or miles per hour.
Example Sentence 1
Climbing through the mid-thirties, the captain transitioned from watching the airspeed indicator to watching the Machmeter to stay below the aircraft's maximum operating Mach number.
Example Sentence 2
As the aircraft climbed, the Machmeter reading rose even though indicated airspeed stayed constant.