Machmeter

A Machmeter is an aircraft instrument that indicates the Mach number, which is the ratio of the aircraft's true airspeed to the local speed of sound. It is primarily used in high-speed aircraft, typically those designed to operate near or above the speed of sound (Mach 1).

A Machmeter typically consists of a sealed aneroid capsule that senses static air pressure and a mechanism that calculates the Mach number based on the difference between the total pressure (also known as pitot pressure) and the static pressure. This difference is then used to determine the airspeed, which is then divided by the calculated local speed of sound. The local speed of sound is primarily determined by the ambient air temperature. Some Machmeters incorporate automatic temperature compensation to improve accuracy.

The reading on a Machmeter is displayed on a dial or digital display, indicating the Mach number as a decimal value (e.g., Mach 0.85). This allows pilots to accurately monitor their speed relative to the speed of sound and avoid exceeding aircraft limitations, such as critical Mach number, which can lead to aerodynamic problems like shock wave formation and a loss of control.

Using a Machmeter is crucial in high-speed flight as indicated airspeed (IAS) becomes less reliable as the speed of sound is approached due to compressibility effects. Mach number provides a more accurate representation of the aerodynamic forces acting on the aircraft.