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An aircraft is flying at FL440 with a TAS of 505 kt. If OAT is ISA -10°C, what is its Mach number?

  • A

    M .80

  • B

    M .87

  • C

    M .92

  • D

    M .90

Refer to figure.
There are two ways to convert TAS into a Mach No:

1. Method using the Mach formula: Mach No = TAS / LSS = TAS / 38.94 √ OAT(Kelvin)

  • Temperature decreases with altitude at a constant rate of -2°C/1000 ft approximately, up to the tropopause (36 089 ft).
  • So, ISA temperature at FL440 is -56.5°C.
  • The OAT at FL440 is: ISA temp. at FL410 + ISA dev = -56.5 °C - 10°C = -66.5 °C.
  • Convert Celsius into Kelvin: 273° - 66.5°C = 206.5°K.
  • Thus, Mach No = 505 kt / ( 38.94 √ 206.5°K) = 0.90.

2. Method using Flight/Nav computer:

  • Rotate the disc until the OAT -66.5°C is against the Mach Index.
  • Below TAS 505 kt on the outer scale, read Mach No 0.90 (90) on the inner scale.

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