FAA Aircraft Dispatcher (ADX)Flight Planning and PerformanceHard
An aircraft is cruising at Mach 0.80 in conditions where the local speed of sound is 590 knots. What is the aircraft's approximate true airspeed?
- A590 knots
- B472 knots
- C450 knots
- D500 knots
Show answer & explanationAnswer & explanation
Correct answer: B. 472 knots
True airspeed equals Mach number multiplied by the local speed of sound: 0.80 × 590 = 472 knots.
Why the other options are wrong
- A. 590 knots is the speed of sound itself (Mach 1.0), not the aircraft's TAS at Mach 0.80.
- C. This would require a lower Mach number or speed of sound than given.
- D. This value does not correspond to the given Mach number and speed of sound.
Mach Number and True Airspeed
True airspeed at a given Mach number equals the Mach number multiplied by the local speed of sound, which varies with outside air temperature.
- TAS = Mach × local speed of sound
- Speed of sound decreases with colder temperatures (higher altitude)
- Mach number is used for high-speed cruise performance and buffet margins
Memory trick: Mach × sound speed = true airspeed — like a percentage of the local 'sound limit'.