An aircraft is climbing at a constant CAS in ISA conditions. What happens to TAS and Mach number?

When an aircraft is climbing at a constant Calibrated Airspeed (CAS) in International Standard Atmosphere (ISA) conditions, several changes occur in relation to True Airspeed (TAS) and Mach number.

As altitude increases during a climb, the air temperature generally decreases according to the ISA model until reaching the tropopause. Since TAS is directly related to the air density, and density decreases with altitude, an increase in altitude at a constant CAS leads to a higher TAS. This is due to the fact that TAS is the speed of the aircraft relative to the surrounding air and accounts for variations in air density.

Furthermore, as the aircraft climbs and TAS increases, the Mach number, which is a ratio of TAS to the local speed of sound, also increases. The speed of sound varies with temperature; and as the temperature decreases with altitude, the speed of sound decreases as well. However, since TAS is increasing, the Mach number consequently increases as well.

In summary, at a constant CAS during a climb in ISA conditions, the TAS will increase, and since the Mach number is derived from the ratio of TAS to the speed of sound, it will also increase, making the answer that reflects both of these increases the correct one.