Critical Mach number

Transonic flow patterns on an aircraft wing, showing the effects below and above the critical Mach number. Note that the top airfoil illustrates all speeds below the critical Mach number. It doesn't highlight reaching the critical Mach number by showing a point on the upper surface where the speed first becomes sonic (as is shown in published texts).

Airliners cruise at speeds just below the speed of sound. An important speed which the aircraft flies through just before reaching cruising speed occurs when the air flowing over the wing reaches the speed of sound. It is known as the critical Mach number (Mcr). Further increase in aircraft speed to the cruise value produces a region of supersonic flow which ends in a shock wave. Beyond the cruise speed, at the drag-divergence Mach number, the shock wave, and any attendant separated flow, produces too much drag for economical flight.

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