Editing Air-start system (section)

==Gas turbines==
[[File:Air Starter GALI in Engine Cummins.png|thumb|250x250px|Air-starter high pressure on a Cummins Engine. It is mounted at the flywheel housing in the engine. The starter when receive air or gas start to engage the pinion in the ring-gear. After engine gets the firing speed pinon disengage.]]
Compressed air has been used to start gas turbine engines using air impingement starting (in which air is directed at the engine turbine blades through nozzles in the turbine casing, used on US Navy [[General Electric J79]] engines). It is much more efficient to use an air turbine starter (ATS) which is usually mounted on an accessory gearbox.<ref>The Jet Engine 3rd Edition, July 1969 Publication Ref. T.S.D.1302, p.128/129</ref> An early axial compressor turbojet had an ATS located in the compressor nose cone (eg particular variants of the J79).

Air impingement starting was not used for US military aircraft after the F-4B, A-5A<ref>A5A Aircraft NATOPN Flight Manual, NAVWEPS 01-60ABA-1, p.14 Starting System</ref> and F-5 as the pneumatic energy requirement was several times greater than when using an air turbine starter. The gas turbine compressor required to start a J79 with impingement starting was sufficient to start two J79 engines simultaneously in a B-58 when using air turbine starters.<ref>Pneumatic Starting Systems, Robert J. Von Flue, The Garrett Corporation, ASME Publication 67-GT-15, p.3-5</ref>

An ATS has its own turbine and gears to change its low torque and high speed to low speed and high torque at the engine mounting pad. Further gears in the engine gearbox connect to the engine shaft (high pressure spool on multi-spool engines). Compressed air is sent to the ATS turbine from the aircraft [[auxiliary power unit]] ( [[bleed air]] from the gas generator or from a free-turbine load compressor, eg PW901 APU), from an already-running engine (bleed air) on a multi-engined aircraft or, for early jet aircraft, from an [[air compressor]] mounted on [[Ground Support Equipment|ground support equipment]].

Compared with electric starters, air-starters have a higher [[power-to-weight ratio]] so are used on large engines as an electric starter would be too big and, with its cables, too heavy and expensive. However, for smaller engines, which don't need as much starter power, an electric starter is more suitable. It has a dual function as a generator (is known as a starter/generator) at speeds above which the engine no longer requires starter assistance.<ref>Aircraft Electrical Systems, Third Edition, Pallett,{{ISBN|0 582 98819 5}}, p. 157</ref>