Which measurements are used by a Mach meter to display Mach number?

• A. Pitot static data alone
• B. Pitot-static data with compressibility corrections
• C. Altitude and temperature
• D. Airspeed and vertical speed

Answer: (B)

Mach number is derived from how compressible air behaves as you move through the atmosphere, which is read from the airplane’s pitot-static system. The Mach meter uses the total pressure from the pitot tube and the static pressure from the static port, then applies compressibility corrections to translate that pressure ratio into a Mach value. At higher speeds, air becomes significantly compressible, and those corrections are essential to get an accurate Mach number. Temperature and altitude influence the speed of sound and air properties, but they’re not used by the Mach indicator by themselves to determine Mach; they’re accounted for through the compressibility model that sits on top of the pitot-static measurements. Using only altitude/temperature or only a simple pitot-static reading wouldn’t correctly yield Mach, because the relationship between pressure and Mach requires those corrections.