Why do subsonic airliners fly a mach number?
Mach is a variable speed – the associated airspeed to any Mach number changes with temperature. Jets have what’s called Mmo, or maximum operating Mach speed. Since a Mach number will be a different indicated airspeed at different temperatures and Mmo is the limiting speed, airplanes fly Mach numbers at high altitudes. Even flying lower speeds than Mmo, Mach is still used because it is a ratio rather than an absolute speed, so it’s easier to chart performance as a ratio of the maximum speed instead of constantly varying indicated airspeeds. Indicated airspeed becomes somewhat useless at high altitudes. Since the air is so thin, there are extreme errors between indicated airspeed and true airspeed. A very small difference in indicated airspeed may be a large difference in both Mach number and true airspeed. EDIT: Machmeters work by taking input from both the pitot-static system and the air temperature sensors. On modern airplanes, this is most often done by a function of the Air Data Com
