FAA Commercial Pilot Airplane (CAX)Aerodynamics and PerformanceMedium
An airplane is flying at an altitude where its true airspeed (TAS) is 150 knots and the outside air temperature (OAT) is -10°C. If the speed of sound at this altitude and temperature is approximately 630 knots, what is the airplane's Mach number?
- A0.40
- B0.15
- C0.63
- D0.24
Show answer & explanationAnswer & explanation
Correct answer: D. 0.24
Mach number is calculated by dividing the true airspeed (TAS) by the local speed of sound. In this case, Mach = 150 knots / 630 knots = 0.238, which rounds to 0.24.
Why the other options are wrong
- A. Incorrect, likely a miscalculation.
- B. Incorrect, likely a miscalculation.
- C. Incorrect, this is the speed of sound divided by 1000.
Mach Number Calculation
Mach number is a dimensionless quantity representing the ratio of an object's speed through a fluid to the local speed of sound in that fluid. It is calculated as True Airspeed (TAS) divided by the local Speed of Sound.
- Mach = TAS / Speed of Sound.
- Speed of sound varies with temperature (and thus altitude).
- Mach 1 is the speed of sound.
- Important for high-speed flight and compressibility effects.
Memory trick: Mach is how fast you are compared to the sound around you.