Machmeter

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Diagram illustrating the face of a Machmeter showing a Mach number of 0.83

A Machmeter is an aircraft pitot-static system flight instrument that shows the ratio of the speed of sound to the true airspeed, a dimensionless quantity called Mach number. This is shown on a Machmeter as a decimal fraction. An aircraft flying at the speed of sound is flying at a Mach number of one, expressed as "Mach 1.0".

As an aircraft in transonic flight approaches the speed of sound, it first reaches its critical mach number, where air flowing over low-pressure areas of its surface locally reaches the speed of sound, forming shock waves. The indicated airspeed for this condition changes with ambient temperature, which in turn changes with altitude. Therefore, airspeed is not entirely adequate to warn the pilot of the impending problems. Mach number is more useful, and most high-speed aircraft are limited to a maximum Mach number, also known as as a "Mach limit".

For example, if the Mach limit is Mach 0.83, at 30,000 feet where the speed of sound under standard conditions is 590 knots, the corresponding true airspeed is 489 knots. The speed of sound varies with air temperature, so at Mach 0.83 at 10,000 feet, where the air is much warmer, the corresponding true airspeed would be 530 knots.