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An aeroplane is descending from FL390 to ground level at maximum speed. The limit(s) in speed is (are)…

  • A

    MMO only during all the descent.

  • B

    VMO only during all the descent.

  • C

    initially MMO, then VMO below a given flight level.

  • D

    initially VMO, then MMO below a given flight level.

In cruise, and generally at higher levels, for the same VMO, the TAS and Mach number increase due to the decrease in density. Therefore, at high levels, it is very unlikely that you reach the VMO. But let’s suppose you’re trying to reach the VMO (just to illustrate this relationship). By accelerating to reach the VMO, which is an IAS, your TAS and your Mach number will increase, and you will most likely reach your MMO before the VMO.

In the same way, at low levels close to the ground, the IAS and the TAS are very close because of the higher density, therefore the VMO becomes the limiting speed. To reach the MMO at a low level, you will need to drastically increase your TAS, which is impossible as you will reach the VMO and damage your structure way before.

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