Mach Number

Mach number is the ratio of the speed of an object to the speed of sound in the surrounding medium. At Mach 1, the object is travelling at the speed of sound. Subsonic flight is below Mach 1; transonic is near Mach 1; supersonic is above Mach 1. Commercial jet aircraft typically cruise between Mach 0.78 and 0.85.

Speed of Sound and Altitude

The speed of sound varies with temperature — it is approximately 661 knots TAS at sea level in standard conditions (15°C) and approximately 573 knots TAS at FL350 (−56.5°C). This is why the same IAS translates to a higher Mach number at altitude (where TAS is much higher than IAS) than at low altitude.

Compressibility Effects

As aircraft approach Mach 0.8–0.9 (depending on wing design), shock waves begin forming on the wing. These create wave drag, control surface buffet, and potential control reversal. This is why subsonic jet aircraft have swept wings — to delay the onset of compressibility effects to a higher Mach number.

Related Terms

See also: Mach Indicator, EAS, True Airspeed.