- Machmeter
[
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 theratio of thetrue airspeed to thespeed of sound , adimensionless quantity calledMach number . This is shown on a Machmeter as adecimal 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 itscritical mach number, where air flowingover low-pressure areas of its surface locally reaches thespeed of sound, formingshock wave s. Theindicated airspeed for this condition changes with ambient pressure, which in turn changes withaltitude .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 correspondingtrue 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/
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