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An aeroplane is cruising at FL 220. The auto-throttle maintains a constant CAS. If the OAT increases, the Mach number:
  • A
    remains constant.
  • B
    decreases.
  • C
    increases.
  • D
    decreases if OAT is lower than standard temperature, increases in the opposite case.

M = TAS ÷ LSS

If the Mach number is constant, it means that the above equation needs to be balanced. If one of the variables is changing (for example TAS), the other one (LSS) needs to change in the same way to equalize the relationship.
If the OAT increases, the LSS will increase since LSS = 38.95 x √ (°K).
CAS is basically the dynamic pressure, which is the number of air molecules that will hit the pitot probe. If the density is decreasing, but you maintain a higher airspeed relative to the airflow (TAS), you will pick up the same dynamic pressure (CAS). Therefore, the TAS and the LSS increase, maintaining a constant Mach number.

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