Performance and Speeds

Speeds

An airplane is cruising at FL 190. The auto-throttle maintains a constant CAS. If the total temperature decreases, the Mach number:

explain:

CAS = 1/2 rho V² where Rho is density and V is TAS.
Auto throttle is to maintain constant CAS.
If the OAT decreases the density will increase. The value of 1/2 rho V² would increase.
As the density increases, to maintain the value of 1/2 rho V² the value of "V" TAS must decrease.
Mach number = TAS/LSS. The local speed of sound changes with temperature. If temperature decrease local speed of sound decreases and visa versa.
Thus to maintain CAS (as temperature decreases) the TAS will decrease.
The decrease in temperature will also decrease the local speed of sound.
Mach Number = Decreased TAS / Decreased Local speed of sound.

The mach number remains constant.

remains constant

increases

decreases

decreases if OAT is lower than standard temperature, increases in the opposite case

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