At what altitude will max operating speed shift from KIAS to Mach number?

The transition from using Knots Indicated Airspeed (KIAS) to Mach number as the measure of maximum operating speed occurs at higher altitudes due to the properties of air density and the effects of compressibility at those altitudes. The critical point for this transition is typically recognized around 18,769 feet MSL, which is just above the altitude where the transition begins.

Above this altitude, the relationship between airspeed and aerodynamic performance changes, making Mach number a more relevant measurement to ensure safe operation and maneuverability of the aircraft. At lower altitudes, KIAS is relied upon because the air is denser, and the impact of compressibility on airflow around the aircraft is minimized. Therefore, in the context of high-speed flight and safety parameters in aviation, above 18,769 feet MSL, it becomes essential to monitor speed in terms of Mach to account for these changing dynamics.