The ratio of an airplane's true airspeed to the speed of sound in the same atmospheric conditions is called what?

• A. Mach number
• B. Indicated airspeed ratio
• C. Sound-speed ratio
• D. True airspeed ratio

Answer: (C)

Mach number is the ratio of an airplane’s true airspeed to the local speed of sound. This is a dimensionless quantity because it compares how fast the aircraft is moving through the air to how fast sound travels in that same air, which depends on temperature. Since the speed of sound increases with temperature, Mach numbers change with altitude and atmospheric conditions even if your TAS stays the same. When TAS equals the local sound speed, you’re at Mach 1 (the sonic speed); higher Mach numbers bring compressibility effects into play, defining subsonic, transonic, and supersonic flight. The term that expresses this TAS-to-sound-speed ratio is Mach number; the other options aren’t standard terms for this ratio.