What is indicated by the Mach meter reading?

The Mach meter reading indicates the aircraft's speed in relation to the speed of sound. This measurement is critical in high-speed flight environments, particularly for aircraft operating at or near supersonic speeds.

The Mach number is a dimensionless ratio defined as the speed of the aircraft divided by the speed of sound at the same altitude and atmospheric conditions. For instance, a Mach number of 1 indicates that the aircraft is traveling at the speed of sound, while a Mach number greater than 1 indicates supersonic speed. This relationship is essential for pilots and engineers to assess performance, stability, and control characteristics of the aircraft as air density changes with altitude.

In contrast, the other options do not capture the primary purpose of the Mach meter. Altitude is indicated by other instruments such as the altimeter, while true airspeed is typically derived from airspeed indicators that measure dynamic pressure converted to a speed. Weight is not related to speed readings at all, as it pertains to the mass of the aircraft and its load, which is measured separately. Thus, the significance of the Mach meter lies in its role in providing a direct relationship between the aircraft's speed and the speed of sound.