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An aircraft descends from FL410 to FL270 at its cruising Mach and then from FL270 to FL100 at the IAS achieved at FL270 with idle power and in a clean configuration. How does the angle of descent change during the first and second phase?

  • A

    Increases then constant

  • B

    Constant then decreases

  • C

    Increases then decreases

  • D

    Decreases then constant

Refer to figures.
When descending at a constant Mach number, the indicated airspeed (IAS) of the aircraft increases. In order for the aircraft to maintain the higher airspeed, the aircraft must pitch down further, which in turn increases the descent angle. As the IAS increases during the first phase, the descent angle will continuously become steeper and increase. The rise in IAS is primarily due to the increasing temperature and air density at lower altitudes, which affect the speed of sound as well as the aircraft's true airspeed (TAS) and IAS.

Once the aircraft transitions to a descent at a constant IAS, the pitch angle and descent angle stabilise and remain constant since the aerodynamic forces are balanced and do not change.

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