An accessory drive in an aircraft is a gearbox that forms part of a gas turbine engine. It drives the engine’s accessories including the fuel pump, generators, hydraulic pumps, engine starter, etc. They can handle between 400-500 horsepower (hp). The shaft that links the turbine and compressor also powers the accessory drive. This setup requires an internal gearbox that has a radial drive shaft that powers the external gearbox.
Internal gearboxes are difficult to design because there is a small and hot space in which the drive shaft can be connected. They’re often located between the compressor outlet and the combustor but may be placed ahead of the compressor for designs that include centrifugal compressors. The outer, shorter, shaft catches speed quicker when the engine is started, and the accessory drive is often taken from it. Drive and accessory gearboxes may be split into two in order to distribute the loads. Engine critical systems are arranged on the shorter, high-pressure shaft while the aircraft systems are located on the longer, low-pressure shaft. Allowing for thermal expansion, the drive from the main shaft may be taken from the direct drive, the stub shaft drive, or the idler shaft drive.
External gearboxes are located near the engine. The drive within the external gearbox casing is provided by a train of spur gears that run-in roller bearings. Idler gears are commonly used to increase the spacing between accessories. And helical gears are used for high-torque drives because they provide smoother running, but they generate end-thrust and require complicated thrust bearings for further support.
Bleed air and mechanical shaft drive may also be used to provide power for the accessories. If a source of compressed air is needed, this design may be useful; this may be for pressurized cabin air or to provide a cool air supply to various components. Another important use for bleed air is to cross start other engines within a multi-engine aircraft.
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