Definition
The Mach number reading shown on the aircraft's Machmeter, expressing the airplane's true airspeed as a ratio of the local speed of sound at the current altitude and temperature. It is uncorrected for instrument or position errors.
Plain English
The number on the Machmeter that tells the pilot how fast the airplane is flying compared to the speed of sound around it.
Context Anchor
Seen in high-speed and high-altitude instrument flying, especially when the aircraft is operated by Mach number instead of ordinary airspeed alone.
Derivation
Named after Austrian physicist Ernst Mach, who studied the behavior of objects moving at and beyond the speed of sound. 'Indicated' simply means 'as shown on the instrument' before any corrections are applied.
Why Pilots Care
Allows pilots of high-speed aircraft to monitor proximity to critical Mach and avoid compressibility effects.
Intuition Check
“Indicated” does not mean guessed or suggested here. It means the value shown directly on the cockpit instrument before correction for known errors.
Example Sentence 1
Climbing through FL310, the captain noted the indicated Mach number rising and reduced thrust to stay below the airplane's Mach limit.
Example Sentence 2
As speed increased the indicated Machmeter needle moved steadily toward 0.82.