During a descent at a constant Mach number below the tropopause in ISA conditions, what happens to CAS and TAS?

During a descent at a constant Mach number below the tropopause in International Standard Atmosphere (ISA) conditions, the behavior of Calibrated Airspeed (CAS) and True Airspeed (TAS) can be understood through the relationship between airspeed and changes in altitude, temperature, and pressure.

As the aircraft descends, the atmospheric pressure and temperature increase. While maintaining a constant Mach number, the TAS will indeed increase due to the aircraft moving through a denser air mass at lower altitudes. This increase in TAS occurs because True Airspeed is the actual speed of the aircraft relative to the air around it, and under ISA conditions, a decrease in altitude means more air molecules are available, resulting in greater actual speed for the same Mach number.

At the same time, CAS, which is calibrated airspeed adjusted for instrument error and position error, is affected by the increase in air density. Because the air density increases during descent, the CAS will also increase as it reflects the dynamic pressure acting on the aircraft's surfaces.

Therefore, since both CAS and TAS increase during descent at a constant Mach number, this aligns with the correct response regarding how these airspeeds behave in the given scenario.