Critical Mach Number: (M_CRIT) The critical Mach number is the Mach number of the aircraft when the speed of the airflow over some part of the aircraft first reaches the speed of sound.

At high altitude, a large high-speed jet transport aircraft will be cruising at a speed marginally above its critical Mach number, and it will have a small shock wave on the wing. If such an aircraft overspeeds, the shock wave will rapidly grow larger, causing the static pressure to increase sharply in the immediate vicinity of the shock wave. The locally increased adverse pressure gradient will cause the boundary layer to separate immediately behind the shock wave. This is called a ‘shock stall’.

Shock induced separation (shock stall) at high speed can also cause buffeting. Aerodynamic buffet is a valuable stall warning, but it can damage the aircraft structure.

As the shock wave moves aft over the surface of the wing, the CP also moves aft. This aft movement combined with the reduced downwash at the tail plane due to the presence of the shockwave will lead to and unstable nose-down pitching moment known as ‘Tuck Under’.