Mach Number is defined as which of the following?

Mach Number is defined as which of the following?

• A. The ratio of true airspeed to the speed of sound
• B. The ratio of indicated airspeed to true airspeed
• C. The speed of sound divided by air density
• D. The ratio of ground speed to airspeed

Answer: (A)

Mach number is the ratio of an aircraft’s true airspeed to the local speed of sound. True airspeed is the actual speed of the aircraft through the air, while the speed of sound in air depends mainly on temperature and thus varies with altitude. This ratio is dimensionless and shows how close the aircraft is to or beyond sonic speed: Mach 1 means traveling at the local speed of sound, Mach less than 1 is subsonic, and Mach greater than 1 is supersonic. Indicated airspeed isn’t used for Mach because it’s affected by air density and instrument errors, not by the actual sonic speed of the surrounding air. The speed of sound divided by air density isn’t a meaningful or correct way to define Mach, and comparing ground speed to airspeed doesn’t measure speed relative to the sonic in the air.

Mach number is the ratio of an aircraft’s true airspeed to the local speed of sound. True airspeed is the actual speed of the aircraft through the air, while the speed of sound in air depends mainly on temperature and thus varies with altitude. This ratio is dimensionless and shows how close the aircraft is to or beyond sonic speed: Mach 1 means traveling at the local speed of sound, Mach less than 1 is subsonic, and Mach greater than 1 is supersonic.

Indicated airspeed isn’t used for Mach because it’s affected by air density and instrument errors, not by the actual sonic speed of the surrounding air. The speed of sound divided by air density isn’t a meaningful or correct way to define Mach, and comparing ground speed to airspeed doesn’t measure speed relative to the sonic in the air.