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An aeroplane descends from FL410 to FL270 at its cruise Mach number and from FL270 to FL100 at the IAS achieved at FL270. Assuming idle thrust, a clean configuration and ignoring compressibility effects, how does the angle of descent change

(i) in the first and
(ii) in the second part of the descent?

  • A
    (i) Increases (ii) Decreases
  • B
    (i) Decreases (ii) Increases
  • C
    (i) Increases (ii) Remains constant
  • D
    (i) Remains constant (ii) Decreases

Descent at a Constant Mach Number in Standard Conditions

During a descent, LSS will be increasing, as temperature increases.
Therefore, if Mach number is being kept constant the TAS must be increasing (Mach number = TAS/LSS).
During the descent air density increases TAS is increasing and IAS also increases at a greater rate (Dynamic Pressure = 1⁄2 ρ V2).
Similarly, in a climb at constant Mach number the TAS and IAS both reduce.

During descend with a constant Mach no. the Lift increases as IAS is increasing.

Lift = 1⁄2 ρ V2 CL

To balance the lift CL is reduced by lowering the aircraft nose.

Lowering the nose, the aircraft will have a steeper angle of descent.

Flying with a constant IAS, the Lift will remain constant and there is no need to further lower the nose.

Angle of descent remains constant.

Your Notes (not visible to others)



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