Mach Number

A dimensionless ratio expressing the aircraft's true airspeed as a fraction of the local speed of sound. Mach 1.0 equals the speed of sound (approximately 661 knots at sea level in standard conditions, decreasing with altitude as temperature drops). While Mach number is not directly relevant to typical training aircraft, private pilot students encounter it on the written test in the context of high-altitude operations and airspeed limitations. At high altitudes, jet aircraft reference a maximum operating Mach number (Mmo) rather than indicated airspeed to avoid encountering compressibility effects and shock waves on the wings.