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An aircraft is cruising at FL410, Mach 0.86. The ISA deviation is -12ºC. What is the TAS of the aircraft?
  • A
    466 kt
  • B
    479 kt
  • C
    556 kt
  • D
    647 kt

Mach number can be calculated using the following formula:
M = TAS / LSS
Where M stands Mach number and LSS is local speed of sound.

The Local Speed of Sound is only dependent on absolute temperature and can be calculated as follows:
LSS = 38.95 x √ K
T(K) = ºC + 273

(1) Determine the OAT at FL410:
ISA temperature at FL410 = -56.5ºC
ISA dev = -12ºC
Thus, OAT = - 56.5 - 12 = -68.5ºC

(1) Convert ºC into K:
K = -68.5ºC + 273 = 204.5 K

(2) Calculate LSS:
LSS = 38.95 x √204.5 = 557 kt

(3) Determine the Mach number:
TAS = ? kt
LSS = 557 kt
M = 0.86

  • 0.86 = TAS / 557 kt = 479 kt
Note: In an International Standard Atmosphere (ISA) atmosphere, the temperature declines by 2°C for every 1 000 ft of ascent until reaching the Tropopause, which occurs at an altitude of 36,000 feet. Beyond this point, the temperature is considered constant and remains at -56.5°C.

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