The start of the aircraft engines is carried out using pneumatic motors, that is to say , turbines fed with high pressure air.
These turbines rotate at high speed and it is necessary to provide a very high starting torque, therefore one install a gear box between the turbine and the aircraft engines. This gear box reduces rotating speed and increases torque
When the engine is almost launched and rotates by itself, it is necessary to disconnect the starter from the engine, that is carried out with a free wheel system type.
One can see the starter of the CFM56-7 on this photograph
(the large clear cylinder)
The feeding air is provided either by a ground power unit set up by the airport, or by the APU, or by the other engines of the plane if those are already runing.
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