Definition
The lowest Mach number at which airflow over any part of an aircraft first reaches the speed of sound, even though the aircraft itself is still flying below the speed of sound. Also called critical Mach number (M Crit or MCRIT).
Plain English
The aircraft's speed, expressed as a fraction of the speed of sound, at which the air flowing over some part of the wing first goes supersonic — usually over the thickest part of the wing, where the air has to speed up to get around it.
Context Anchor
Seen in high-speed flight discussions, especially when learning why an aircraft can have speed-related problems before it actually reaches Mach 1.
Derivation
“Mach” comes from Ernst Mach, the Austrian physicist who studied the behavior of objects moving through air at high speeds. A Mach number is the ratio of an object's speed to the speed of sound. “Crit” is short for “critical” — meaning the threshold point where something important changes. So Mach Crit is the critical Mach number where airflow behavior begins to change in important ways.
Why Pilots Care
Exceeding Mach Crit causes a sharp increase in drag, possible buffeting, and loss of control effectiveness, requiring pilots to respect published limits on high-speed aircraft.
Grounding Statement
Picture the airplane still below the speed of sound, while the air speeding up over the curved top of the wing reaches the speed of sound first.
Intuition Check
Critical does not mean an emergency has already happened here. It means this is the threshold where high-speed airflow effects begin.
Example Sentence 1
The pilot reduced power slightly to keep cruise speed below Mach Crit and avoid the onset of shock wave formation on the wings.
Example Sentence 2
Designers shape the wing to raise Mach Crit and allow higher cruise speeds without compressibility problems.