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If a jet aeroplane is climbing at a constant IAS, which operational speed limit is most likely to be reached?
  • A
    Mach limit for the Mach trim system.
  • B
    Minimum control speed.
  • C
    Stalling speed.
  • D
    Maximum operating Mach number.

Refer to figure.
Effect on Mach Number of Climbing at a Constant IAS

Temperature decreases with increasing altitude, local speed of sound will decrease as altitude is increases.
It is know that if altitude is increased at constant IAS the TAS will increase.

Mach no. = TAS / LSS

Climbing at constant IAS, TAS will increase and therefore the Mach no. as well.

But the Mach no. will increase at a bigger rate than TAS, because the LSS will decrease with altitude.
Therefore, the Mach number will increase if altitude is increased at a constant IAS and at a certain altitude MMO can be reached.

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