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During a descent with constant Mach number below the tropopause in ISA conditions, how do the TAS and CAS change? TAS...
  • A
    decreases, and CAS increases.
  • B
    increases, and CAS decreases.
  • C
    increases, and CAS increases.
  • D
    decreases, and CAS decreases.

Refer to figure.
Descent at a Constant Mach Number in Standard Conditions

During a descent, LSS will be increasing, as temperature increases. Therefore, if Mach number is being kept constant the TAS must be increasing (Mach number = TAS/LSS). During the descent air density increases TAS is increasing and CAS also increases at a greater rate (Dynamic Pressure = 1⁄2 ρ V2). Similarly, in a climb at constant Mach number the TAS or CAS both reduce.

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