What is an advantage of using Mach speed instead of indicated airspeed (IAS) at high altitudes?

Using Mach speed at high altitudes is advantageous because Mach numbers are directly related to the speed of sound, which decreases with altitude due to lower temperatures. At high altitudes, the air is less dense, and thus indicated airspeed (IAS) can give misleading information about an aircraft's true performance characteristics. Fly speeds expressed in Mach numbers remain relevant and consistent because they consider the changing atmospheric conditions.

In-flight operations often reference maximum Mach limits, which are typically imposed by aircraft manufacturers due to the compressibility effects of airflow at high speeds. Using Mach allows pilots to remain aware of their speed relative to the speed of sound, which is crucial for effective control of the aircraft, especially considering factors like aerodynamic drag and potential structural limits.

This relationship is particularly important for jet aircraft operating at cruising altitudes, where maintaining a specific Mach number can optimize fuel efficiency and performance, making Mach a more effective measurement at those altitudes compared to IAS.