At high altitudes, which speed measurement is most relevant?

At high altitudes, Mach number becomes the most relevant speed measurement because it represents the aircraft's speed relative to the speed of sound in the surrounding air. As an aircraft ascends, the air density and temperature decrease, which affects the performance and behavior of the aircraft. The speed of sound also changes with altitude, as it is influenced mainly by temperature.

Using Mach number as a reference is critical in high-altitude flight since it enables pilots to maintain the proper aerodynamic efficiency and avoid potentially dangerous situations such as shock waves, which can occur when an aircraft exceeds the speed of sound. Unlike indicated airspeed or true airspeed, which do not account for changing air conditions with altitude, Mach provides a standardized measure that factors in the effects of altitude on temperature and air density.

While indicated airspeed and true airspeed are valuable for various aspects of flight at lower altitudes, they may not accurately represent the aircraft's performance characteristics at higher altitudes. Ground speed is also less relevant in terms of aerodynamic considerations, as it pertains more to the rate of travel over the ground rather than the aircraft's performance in the air. Therefore, flying at high altitudes necessitates a focus on Mach number to ensure safety and efficiency.