Definition
A Mach number is the ratio of an aircraft's true airspeed to the local speed of sound in the surrounding air. Mach 1.0 means the aircraft is moving at exactly the speed of sound; Mach 0.80 means it is moving at 80 percent of the speed of sound. Because the speed of sound changes with air temperature, the same Mach number corresponds to different true airspeeds at different altitudes and conditions.
Plain English
A way of expressing how fast an aircraft is going compared to the speed of sound around it. Mach 0.80 means you are flying at 80 percent of the speed of sound at that moment.
Context Anchor
Seen in high-altitude flight, jet aircraft performance limits, instrument procedure discussions, and speed management during climb, cruise, and descent.
Derivation
Named after Ernst Mach, an Austrian physicist who studied the behaviour of objects moving at and beyond the speed of sound in the late 1800s. His name became the standard label for this speed ratio.
Why Pilots Care
Pilots use Mach numbers to identify the critical Mach speed beyond which shock waves form, causing buffet, control difficulties, and possible loss of lift.
Grounding Statement
As air temperature changes, sound travels at a different speed, so a Mach number can change even when the airplane's miles-per-hour speed stays the same.
Intuition Check
Do not treat a Mach number as a fixed speed like 500 miles per hour. It is a comparison to the speed of sound in the air the aircraft is flying through.
Example Sentence 1
At FL350, the crew set cruise speed to Mach 0.78 and monitored the Mach number on the airspeed indicator.
Example Sentence 2
At higher altitudes the same indicated airspeed produced a higher Mach number because the speed of sound had decreased.