Which statement correctly defines Mach number?

• A. The speed of sound divided by the speed of an aircraft.
• B. The speed of an aircraft divided by the speed of sound.
• C. The rate of climb at cruise.
• D. The stall speed divided by cruise speed.

Answer: (B)

Mach number is the ratio of how fast the aircraft is moving to the local speed of sound. It’s computed as Mach = V / a, where V is the true airspeed and a is the speed of sound in the surrounding air. The speed of sound depends on air temperature, so Mach changes with altitude and conditions, not just with the number on the airspeed indicator. That’s why this definition—speed of the aircraft divided by the speed of sound—is the right one. If you compare it to the other ideas, dividing the speed of sound by the aircraft’s speed would give the reciprocal and isn’t Mach, the rate of climb at cruise is about vertical performance, and stall speed divided by cruise speed isn’t Mach either. For example, at a given altitude if the true airspeed is 300 knots and the local speed of sound is 600 knots, Mach is 0.5. Mach is used to categorize flight regimes (subsonic, transonic, supersonic) and to compare performance under different atmospheric conditions.