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What is the effect on the Mach number and TAS in an aircraft that is climbing with constant CAS under ISA conditions and below the tropopause?
  • A
    Mach number increases; TAS increases
  • B
    Mach number remains constant; TAS increases
  • C
    Mach number decreases; TAS decreases
  • D
    Mach number increases; TAS remains constant

Refer to figure.
Climbing with a constant CAS in Standard Atmosphere

The rapid rise of Mach no. is the reason why high performance aircraft are flown are flown on CAS (or IAS) for the first part of the climb before transferring to a constant Mach no. for the rest of the climb. In a descent at constant CAS, Was and Mach no. will reduce.

In annexed figure, for a constant CAS (blue line), TAS (green line) increases and Mach no. (red line) increases at a higher rate.

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