Definition
In aerodynamics, the critical Mach number (M_cr or M_crit) is the lowest Mach number at which airflow over any part of an aircraft reaches the speed of sound.
Discussion
For all flying aircraft, the speed of the airflow around them is not the same as their airspeed. This happens because airflow speeds up and slows down as it moves around the aircraft structure. At the critical Mach number, local airflow near certain areas of the airframe reaches the speed of sound, even if the aircraft itself is flying below Mach 1.0. This creates a weak shock wave.
For aircraft not designed for transonic or supersonic flight, exceeding the critical Mach number can lead to a sudden increase in drag. This spike in drag can affect the airflow over flight control surfaces, making it harder to control the aircraft. Additionally, in planes that aren’t built for these speeds, shock waves over the wings and tailplane can stall the wing, render control surfaces ineffective, or even result in a loss of control.
