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Assuming the flight level and Mach number remain constant, when the OAT decreases:
  • A
    CAS decreases and TAS increases.
  • B
    CAS remains constant and TAS increases.
  • C
    CAS and TAS both decrease.
  • D
    CAS remains constant and TAS decreases.

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 decreases, the LSS will decrease since LSS = 38.95 x √ (°K).
Therefore, in order to maintain a constant Mach number (from the above equation), we need to decrease the TAS as well.
CAS is the dynamic pressure, which is the number of air molecules that will hit the pitot probe. If the density is increasing, but you maintain a lower airspeed relative to the airflow (TAS), you will pick up the same dynamic pressure (CAS).

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