What is Mach number and how does Mach 1 behave at sea level versus other altitudes?

• A. Mach number is the ratio of true airspeed to the local speed of sound; Mach 1 at sea level is about 661 knots; Mach 1 varies with altitude due to temperature changes.
• B. Mach number is the ratio to a constant sea-level speed of sound; Mach 1 is always 700 knots.
• C. Mach number is the ratio of ground speed to speed of sound.
• D. Mach number is a measure of engine RPM.

Answer: (A)

Mach number is the ratio of the aircraft’s true airspeed to the local speed of sound. Since the speed of sound in air depends on temperature, Mach numbers change with altitude because the air temperature changes.

At sea level in standard conditions, the speed of sound is about 661 knots, so Mach 1 corresponds to roughly 661 knots true airspeed. As you climb and the air cools, the local speed of sound drops, so Mach 1 equates to a lower TAS in knots—roughly around 570 knots at typical cruise altitudes, depending on the exact temperature profile. This is why Mach 1 isn’t a fixed knot value across altitudes.

Ground speed uses the movement over the Earth and is not the Mach calculation, and engine RPM isn’t what Mach number measures.