During a descent at a constant calibrated airspeed (CAS) below the tropopause in ISA conditions, what can be said about TAS and Mach number?

When an aircraft descends at a constant calibrated airspeed (CAS) below the tropopause in ISA conditions, true airspeed (TAS) and Mach number exhibit specific behaviors due to the changing environmental conditions associated with the descent.

As the aircraft descends, the air density increases because the temperature typically decreases with altitude in the troposphere. Since CAS is a measure of airspeed corrected for instrument and position error but does not account for changes in air density, the TAS (which is the actual speed of the aircraft through the air) will increase. This is because, at lower altitudes, for the same CAS, the aircraft must be traveling faster in terms of TAS to maintain that same indicated speed.

Additionally, the Mach number, which is the ratio of TAS to the speed of sound, will also increase because the speed of sound is higher at lower altitudes due to the increased temperature. However, even though the TAS increases, the speed of sound also increases, but the increase in TAS often leads to an increase in Mach number. Thus, the primary point is that maintaining a constant CAS during descent means that both TAS and Mach number rise as the aircraft encounters denser air closer to sea level, where the temperature is typically higher than