What is the effect on Mach number and True Airspeed (TAS) when climbing at a constant Calibrated Airspeed (CAS)?

When an aircraft climbs at a constant Calibrated Airspeed (CAS), the effects on the Mach number and True Airspeed (TAS) can be understood by considering the definitions and relationships between these speeds.

As altitude increases during the climb, the ambient air density decreases. Since true airspeed (TAS) is the speed of the aircraft relative to the surrounding air, it increases with altitude for a constant CAS, as CAS is a measure of the true speed adjusted for instrument and position errors in airspeed measurement. This adjustment reflects the aircraft's performance against the thinner air at higher elevations.

Meanwhile, Mach number, which is the ratio of the aircraft's true airspeed to the speed of sound in the surrounding environment, is influenced by the decrease in the speed of sound as temperature typically decreases with altitude. Since TAS increases with altitude while the speed of sound decreases, the effect on Mach number can vary. However, in this scenario, with an increase in TAS generally outpacing any decrease in the speed of sound, the Mach number would usually increase as well.

Thus, while climbing at a constant CAS, TAS increases due to lower density and as a result, the Mach number can also increase. The correct choice reflects this positive correlation between