Definition
The ratio of the true airspeed of an aircraft to the speed of sound in the surrounding air. Mach 1 equals the local speed of sound; Mach 0.80 means the aircraft is moving at 80% of that speed. Because the speed of sound varies with air temperature, the same Mach number can correspond to different true airspeeds at different altitudes.
Plain English
How fast the aircraft is moving compared to the speed of sound around it, expressed as a fraction or multiple of that speed.
Context Anchor
Seen in high-altitude flight, jet operations, aircraft limitations, and air traffic control speed assignments.
Derivation
Named after Ernst Mach, an Austrian physicist who studied how objects move through air at and beyond the speed of sound. Knowing the name comes from a person, not a Latin or Greek root, is itself useful — it explains why 'Mach' is always capitalised.
Why Pilots Care
Reveals when compressibility effects and shock waves begin to appear, directly affecting aircraft handling, drag, and structural loads.
Analogy
It is like saying a car is going half as fast as another moving object, except the comparison object is sound moving through the same air.
Grounding Statement
The same aircraft speed can produce a different Mach number when the air temperature changes, because the speed of sound changes with temperature.
Intuition Check
Mach number is not a fixed speed in knots or miles per hour. It is a comparison between the aircraft’s speed and the local speed of sound.
Example Sentence 1
ATC cleared the crew to maintain Mach 0.78 in cruise to sequence with traffic ahead.
Example Sentence 2
As the aircraft climbed, the pilot watched the Mach indicator closely to stay below the critical Mach number.