Definition
The speed at which an aircraft is moving through the surrounding air at exactly the local speed of sound. Mach 1.0 is not a fixed airspeed in knots — it varies with air temperature, so the true airspeed corresponding to Mach 1.0 changes with altitude and conditions.
Plain English
The aircraft is flying at the speed of sound for the air it's currently flying in. Slower than this is below the speed of sound; faster is above it.
Context Anchor
Seen in high-speed and high-altitude performance discussions, especially when comparing airplane speed to the speed of sound instead of to a fixed miles-per-hour value.
Derivation
Named after Ernst Mach, an Austrian physicist who studied how objects behave at and above the speed of sound. A Mach number is simply the aircraft's speed expressed as a fraction of the local speed of sound — so Mach 1.0 means 'exactly the speed of sound,' Mach 0.8 means '80 percent of it,' and so on.
Why Pilots Care
At Mach 1.0, shock waves form and aerodynamic forces change abruptly, affecting stability and control.
Grounding Statement
At sea level on a standard day, Mach 1.0 is roughly 661 knots; at typical jet cruise altitudes, it's closer to 573 knots. The number changes because colder air carries sound more slowly.
Intuition Check
Mach 1.0 is not one fixed miles-per-hour speed. It depends on the speed of sound in the air around the airplane, which changes mainly with air temperature.
Example Sentence 1
The flight crew monitored their Mach number carefully in cruise to ensure they stayed below MMO and well clear of Mach 1.0.
Example Sentence 2
Pilots monitor airspeed closely to avoid exceeding Mach 1.0 unintentionally in certain conditions.