Definition
The critical Mach number (MCRIT) is the speed, expressed as a fraction of the speed of sound, at which airflow over some part of an airplane (typically the upper wing surface) first reaches the speed of sound, even though the airplane itself is still flying slower than the speed of sound. Flight at or above this speed produces localized supersonic flow and the shock waves associated with it, leading to increased drag, buffet, and changes in handling.
Plain English
It is the airplane's speed at which the air flowing over part of the wing first goes supersonic, while the airplane is still flying below the speed of sound. Past this point, shock waves start forming on the wing and the airplane begins to behave differently.
Context Anchor
Encountered in high-speed and high-altitude airplane discussions, especially when learning about speed margins, Mach limits, buffet, and swept-wing airplane behavior.
Derivation
Critical' here means 'the threshold point where behavior changes,' not 'dangerous' or 'urgent.' 'Mach' comes from Austrian physicist Ernst Mach, whose name is used for the ratio of an object's speed to the speed of sound. So the term names the threshold Mach value where something important first happens to the airflow.
Why Pilots Care
Crossing this speed introduces shock waves, a sharp rise in drag, and possible Mach tuck that can pitch the nose down and reduce control effectiveness.
Grounding Statement
Even when the airplane is flying below the speed of sound, the air accelerating over the curved top of the wing flows faster than the airplane itself -- and at the critical Mach number, that local airflow first reaches the speed of sound.
Intuition Check
Critical does not mean the airplane is already out of control. Here it means the first threshold where part of the airflow reaches the speed of sound.
Example Sentence 1
The pilot kept cruise speed well below the airplane's critical Mach number to avoid shock-induced drag and buffet.
Example Sentence 2
As the jet climbed, the critical Mach number decreased because the true airspeed for a given Mach number increased with altitude.