How is Mach number displayed on aircraft that display it?

• A. On a Mach indicator that uses ambient pressure alone.
• B. On a Mach display that ignores compressibility.
• C. On a separate altitude indicator display.
• D. On a Mach meter; it uses pitot-static data and compressibility corrections to determine Mach.

Answer: (D)

Mach number is a ratio of true airspeed to the local speed of sound, so you need a way to know both the air’s speed of sound (which depends on temperature and pressure) and the flow’s dynamic pressure. Aircraft accomplish this with the pitot-static system, which provides total (pitot) pressure and static pressure. The air data system uses those pressures, along with temperature data, and applies compressibility corrections to relate the measured pressures to Mach. The result is displayed on a dedicated Mach meter or Mach display. This is why Mach is shown on a Mach meter that uses pitot-static data with compressibility corrections—the reading comes from the air data measurements and the physics of compressible flow, not from ambient pressure alone or a separate altitude indicator.