What Is Mach Number? Definition & Meaning in Aviation
The Mach number is an aircraft’s speed as a fraction of the speed of sound in the surrounding air.
In plain language
At airliner cruise altitudes the air is cold and the speed of sound is about 1,060 km/h, so Mach 0.80 is roughly 850 km/h. Airliners usually cruise between Mach 0.78 and 0.85.
Example
A Boeing 777 or Airbus A320 crossing Pakistan typically cruises at about Mach 0.78 to 0.84.
Why it matters
Mach is the standard way to describe cruise speed at altitude, because the limiting factor is how close the aircraft gets to the speed of sound. It also explains why airliners cruise in a narrow speed band.