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An aircraft flies at a constant Mach number while climbing from sea level to the tropopause. Its true airspeed

  • A.Falls, because the speed of sound falls with temperature
  • B.Rises, because the air density falls
  • C.Stays the same, since the Mach number is constant
  • D.Rises, because the speed of sound rises with altitude

Answer: A

True airspeed is the Mach number times the local speed of sound, and that speed follows the square root of temperature. Climbing from 288 K to about 217 K takes it from roughly 340 to 295 m/s, so the true airspeed falls by about thirteen per cent even though the Mach number has not changed.

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