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When climbing at a constant CAS in a standard atmosphere:

1 – TAS decreases
2 – TAS increases
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
    2, 4.
  • D
    1, 4.

If you maintain a constant CAS while climbing (air is getting less dense), you will need to increase your forward speed relative to the airflow (TAS) to pick-up the same dynamic pressure (=CAS).

M = TAS ÷ LSS

If the OAT decreases (because the aircraft is climbing), the LSS will decrease since LSS = 38.95 x √ (°K). Therefore, if TAS increases and the LSS decreases, the resultant Mach number (from the above equation) will increase.

If you struggle to visualize this relationship, try to replace the TAS and LSS with actual numbers and verify the resultant Mach number variation.

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