Machmeter

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thumb|right|Illustration_showing_the_face_of_a_Machmeter_reading_a_Mach number of 0.83]

A Machmeter is an aircraft pitot-static system flight instrument thatshows 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 flyingat 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 flowingover low-pressure areas of its surface locally reaches thespeed of sound, forming shock waves. The indicated airspeedfor this condition changes with ambient pressure, which in turn changes with altitude.Therefore, indicated airspeed is not entirely adequate towarn the pilot of the impending problems. Mach number ismore useful, and most high-speed aircraft are limited to a maximum operating Mach number, also known as as "Mmo".

For example, if the Mmo is Mach 0.83, at 9144 m. /30,000 feet where the speed of sound under
standard conditions is 303 m/s / 590
knots, the corresponding true airspeed is 251 m/s / 489 knots. The speed of sound varies with air temperature, so atMach 0.83 at 3048 m. / 10,000 feet, where the air is much warmer, thecorresponding true airspeed would be 273 m/s / 530 knots.

Some older mechanical Machmeters not driven froman air data computer use an altitude aneroid inside theinstrument that converts pitot-static pressure into Machnumber. These systems assume that the temperature at anyaltitude is standard; therefore, the indicated Mach number isinaccurate whenever the temperature deviates from standard.These systems are called indicated Machmeters. Modernelectronic Machmeters use information from an air datacomputer system to correct for temperature errors. Thesesystems display true Mach number.

ee also

* Airspeed indicator
* Mach number
* Pitot-static system

References

*cite book
title = Instrument Flying Handbook
date = 2005-11-25
publisher = U.S. Federal Aviation Administration
location = U.S. Government Printing Office, Washington D.C.
id = FAA-H-8083-15
page = p. 3-8
*cite book
title = Instrument Flying Handbook
date = 2007
publisher = U.S. Federal Aviation Administration
location = U.S. Government Printing Office, Washington D.C.
id = FAA-H-8083-15A
page = p. 3-10
url = http://www.faa.gov/library/manuals/aviation/instrument_flying_handbook/