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True Mach No. Indicator

Technical Information

Catalogue No: C1382
Category: Air Data
Object Type: Indicator/Instrument
Object Name: True Mach No. Indicator
Part No: 81-25-01
Serial No: 132-69
Manufacturer: Elliott Bros (London) Ltd
Platform(s): Nimrod
Year of Manufacture: 1969
Dimensions: Width (mm): 83
Height (mm): 83
Depth (mm): 97
Weight (g): 568
Location: Triple Shelf Unit, RH (indicators) [Main Store]

True Mach No. Indicator
Type No. 82-25-01
Ser. No. 132-69
Ref. No. 6TP/1081721
Mod No. -
Safe Compass Dist. 12 ins.


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. 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, 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 Mmo (Mach Maximum Operating).
This True Mach Indicator use information from the air data computer system which makes calculations using inputs from a pitot-static system. Some older mechanical Machmeters use an altitude aneroid and an airspeed capsule which together convert pitot-static pressure into Mach number. The True Mach Indicator reading is compensated for non-standard attributes of the Pitot probe and is calculated within the Air Data Computer.

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