If an aircraft climbs in the standard atmosphere below the tropopause at constant TAS, what happens to the Mach number?

When an aircraft climbs in the standard atmosphere below the tropopause while maintaining a constant true airspeed (TAS), the Mach number increases. This phenomenon occurs because the speed of sound decreases with decreasing temperature as altitude increases.

As the aircraft ascends, the atmospheric temperature generally drops until reaching the tropopause. Since the speed of sound is directly related to the ambient temperature (specifically, it decreases as the temperature drops), a constant TAS will appear to be a higher Mach number at a greater altitude compared to lower altitude conditions. This increase in Mach number indicates that the aircraft is traveling a greater proportion of the speed of sound as its altitude increases.

Understanding this relationship is crucial for pilots and engineers since Mach number plays a significant role in the aerodynamic characteristics of the aircraft, including stability and control at various flight levels.