Machmeter

A machmeter is an aircraft pitot-static system flight instrument that shows the ratio of the true airspeed to the speed of sound, 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.

Use

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 pressure, which in turn changes with altitude. Therefore, indicated 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 operating Mach number, also known as M_MO.

For example, if the M_MO is Mach 0.83, then at 30000 feet (9144 m) where the speed of sound under standard conditions is 590 knots (1093 km/h), the true airspeed at M_MO is 489 knots (906 km/h). The speed of sound varies with air temperature, so at Mach 0.83 at 10000 feet (3048 m) where the air is much warmer than at 30000 feet (9144 m), the true airspeed at M_MO would be 530 knots (982 km/h).

Operation

Some older mechanical Machmeters use an altitude aneroid inside the instrument that converts pitot-static pressure into Mach number. Modern electronic Machmeters use information from an air data computer system.

Translation

The word "Machmeter" occurs as such in the following languages: English, German.

Translation(s) in other languages: Arabic: مؤشر ماخ, Spanish: Machmetro, Polish: Machometr.