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Given:

FL370
M = 0.74
OAT = -47° C

The TAS is:

  • A
    424 kt.
  • B
    415 kt.
  • C
    434 kt.
  • D
    428 kt.

Mach = TAS ÷ LSS (LSS = Local Speed of Sound). The LSS varies only with the temperature of the air => LSS = 38.95 × √Absolute temp in K. Absolute temperature in K = Temp in °C + 273°.

If the OAT is -47°C then this will be 226 K (-47°C + 273°). Using the LSS formula => 38.95 × √226 = approx. 586 kts. Now that we have the LSS (586 kts) and the Mach (0.74)

TAS => 0.74 × 586 = TAS of 434 kts.

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