Definition
Cockpit instruments that display the aircraft's current Mach number — the ratio of true airspeed to the local speed of sound — based on inputs of pitot (impact) pressure and static pressure. The reading shown is the indicated Mach number, which on most installations is uncorrected for instrument and position errors.
Plain English
The gauge in the cockpit that shows how fast the aircraft is flying compared to the speed of sound at its current altitude. The number you read directly off the dial is the 'indicated' value — the raw reading before any corrections are applied.
Context Anchor
Seen in instrument flying, high-altitude operations, and aircraft performance discussions where Mach number and Mach limits are used instead of, or in addition to, airspeed.
Derivation
Mach' comes from Austrian physicist Ernst Mach, who studied the behavior of objects moving at and beyond the speed of sound. 'Indicated' here means 'as shown on the instrument' — the direct reading, before corrections. So an indicated Machmeter is simply the cockpit instrument that shows you the Mach number it is sensing right now.
Why Pilots Care
Allows pilots to monitor proximity to critical Mach and avoid control difficulties or shock-wave effects at high speeds.
Grounding Statement
At altitude, a pilot may need to know not just how fast the airplane is moving, but how fast it is moving compared with the speed of sound around it.
Intuition Check
“Indicated” does not mean guaranteed exact here. It means shown by the instrument, before correction for known errors.
Example Sentence 1
As the aircraft climbed through FL310, the pilot transitioned from monitoring indicated airspeed to watching the indicated Machmeter to stay below MMO.
Example Sentence 2
In level cruise the indicated Machmeter showed 0.78, confirming the airplane was operating safely inside its Mach limits.