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When descending at a constant CAS in a standard atmosphere:

1 – TAS increases
2 – TAS decreases
3 – Mach number increases
4 – Mach number decreases.

The combination that regroups all of the correct statements is:

  • A
    2, 3.
  • B
    1, 3.
  • C
    1, 4.
  • D
    2, 4.

Refer to Figure.
CAS is the dynamic pressure, which is the number of air molecules that will hit the pitot probe. If the density is increasing (the aircraft is descending), you will need to maintain a lower airspeed relative to the airflow (TAS), to pick up the same dynamic pressure (CAS).

M = TAS ÷ LSS

Temperature increases with decreasing altitude. Since the LSS is directly proportional to the temperature, it means the LSS is increasing with decreasing altitude as well. The ratio between a decreasing TAS and an increasing LSS gives as a result a decreasing Mach number.

If you struggle to understand the equation as it is, try to replace the TAS and LSS with actual numbers and verify the resultant Mach number variation.

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