When climbing at a constant calibrated airspeed (CAS), what happens to the Mach number and true airspeed (TAS)?

When climbing at a constant calibrated airspeed (CAS), the true airspeed (TAS) actually increases. This is due to the fact that as altitude increases, the air density decreases. To maintain the same calibrated airspeed, which is based on the dynamic pressure from the airspeed indicator, the aircraft must move faster in terms of TAS to compensate for the lower air density.

Regarding the Mach number, which is a ratio of the aircraft's TAS to the speed of sound in the surrounding air, it generally increases during a climb at a constant CAS. As the TAS rises with altitude due to the decreasing air density, the speed of sound also changes, but typically does not increase as rapidly as the TAS. This combination of increased TAS and the rate of change of the speed of sound means that the Mach number will also rise.

Hence, the correct interpretation is that while climbing at a constant CAS, TAS increases, leading to an increase in the Mach number as well.