Mcrit, or the ‘critical Mach Number’, is the lowest Mach number at which the airflow over some point on the aircraft reaches the speed of sound.
Generally, lift is generated when a wing accelerates the airflow over its upper surface. While the aircraft itself may not be flying at sonic speeds, the accelerated air can by itself. This, in turn, creates a weak shock wave. With aircraft not designed for supersonic flight, speeds greater than Mcrit will cause an increase in drag and a decrease in control of the aircraft.
With aircraft not designed to reach the critical Mach Number, shock waves may occur which can stall the aircraft and lead to ineffective or less effective control surfaces.
