Definition
The flight Mach number at which airflow over some part of the airplane (typically the upper wing surface) first reaches the speed of sound, even though the airplane itself is still flying below the speed of sound. Beyond this speed, shock waves begin to form, causing increased drag, buffeting, and potential control difficulties.
Plain English
The speed at which air flowing over the wing first goes supersonic, even though the airplane is still flying slower than sound. Past this point, the airplane starts running into the problems associated with supersonic airflow.
Context Anchor
Seen in ground training for high-performance, turbine, and high-altitude airplanes, especially when discussing high-speed limits and control problems near the speed of sound.
Derivation
Named after Ernst Mach, the Austrian physicist who studied supersonic motion. 'Critical' here means 'the threshold point' -- the speed where something important changes, not 'dangerous' or 'urgent.'
Why Pilots Care
Exceeding this value introduces shock waves that can produce buffet, increased drag, and reduced control effectiveness, directly affecting safe speed management in high-performance aircraft.
Grounding Statement
At high altitude, the airplane may still be below the speed of sound, but air speeding up over a curved part of the wing can reach the speed of sound first.
Intuition Check
Critical does not mean the airplane has already failed or is automatically out of control. Here, it means the important threshold where high-speed airflow effects can begin.
Example Sentence 1
As the jet accelerated in the descent, the crew watched the Mach indicator carefully to avoid exceeding Mach Critical.
Example Sentence 2
Wing sweep on the training aircraft was designed to raise the critical Mach, permitting higher subsonic operating speeds.