What does the Mach meter display?

• A. The ratio of the speed of sound to the true airspeed an aircraft is flying.
• B. The rate of climb in feet per minute.
• C. The engine manifold pressure.
• D. The heading relative to magnetic north.

Answer: (A)

Mach number is what the Mach meter displays. It’s the ratio of the aircraft’s true airspeed to the local speed of sound, so Mach = TAS / a. This gives a dimensionless value (like 0.78 or 0.85) that shows how fast the aircraft is moving relative to the speed of sound. The local speed of sound depends on air temperature, so Mach reflects speed in terms of compressibility effects more than just raw airspeed. Pilots use Mach to manage performance and avoid issues that arise as speeds approach the speed of sound. This is different from the rate of climb (vertical speed), engine manifold pressure, or heading (direction relative to magnetic north), which are indicated by other instruments.