A Mach meter is a pitot - static flight instrument that indicates the ratio of true airspeed (TAS True Airspeed engl. ) for speed of sound. This ratio is called a dimensionless Mach number. On a Machmeter the Mach number is represented as a decimal number with decimal places. A flying speed of sound plane flies with the Mach number one (Mach 1.00).

If the air speed approaches the speed of sound, the critical Mach number is first reached at which the ambient air flows first to a portion of the aircraft to supersonic speed - Mostly this is the upper wing surface. There, shock waves, can interfere with flight stability of aircraft that are not designed for this speed range form. The determined by the airspeed indicator in this situation airspeed depends on the ambient pressure which in turn is affected by atmospheric pressure, and is therefore not suitable to display reliably approaching the speed of sound. The Mach number of the Mach meter informs the pilot detail.

For many aircraft a maximum operating Mach number (MMO engl. maximum operating Mach number) is reported. For example, the maximum permissible speed for an airplane with a MMO of Mach 0.83 at an altitude of 30,000 ft ( 9,144 m), in which the speed of sound under standard conditions the value of 1093 km / h is 907 km / h

Older mechanical Machmeter use an aneroid, which converts the pitot - static pressure in the Mach number. Modern electronic Machmeter calculate the reaction to Machmeter value of air pressure values.

The Mach meter is named after the Austrian physicist Ernst Mach Czech-.