Engine Ignition System
A gas turbine engine is generally started by means of a starter input to the main accessory gearbox that will rotate the compressor. Rotation of the compressor eventually provides sufficient air to the engine that is needed for combustion and will aid the engine in reaching idle speed by self-acceleration.

Starter Systems
There are several starter units used on aircraft today but the focus will be on the “pneumatic starter” which is in operation on most commercial aircraft. This starter unit is a type of low-pressure air motor and has a high power to weight ratio compared to older electric variants. The starter is fed by the Auxiliary Power Unit (APU)(figure 1.1), ground power unit or from the “bleed air” source of another operating engine. Figure 1.2 shows an example of a typical engine pneumatic starter. Air drawn from one of the above-mentioned sources, enters the starter inlet and passes through a set of turbine vanes that will change pressure into velocity. The air then exits overboard through a cowl fairing. Eventually, the turbine drives a drive shaft that is connected to the accessory gearbox. Mechanical linkages will then transfer the force from the starter motor onto the compressor that in turn starts to rotate. After reaching a certain rpm, a sufficient air/fuel mixture is present inside the combustion chamber thus the ignition system is automatically activated that will take the engine up to idle speed. The starter system will terminate at five to ten percent RPM after self-accelerating speed is reached. There are different systems used to terminate starter input.

Figure 1.1 - Engine Starter Location

Some installations require pilots to manually shut of the air supply to the starter by a cockpit toggle switch. Modern installations incorporate an automatic shutoff system regulated either by a flyweight cut-out switch, Electronic Engine Control (EEC) or Full Authority Digital Engine Control (FADEC). All three systems operate a cut-out switch that will shut down the turbine starter. The clutch, directly attached to the drive shaft, automatically disengages from the output shaft causing the main engine to accelerate up to idling RPM under its own power.

Figure 1.2 - Engine Starter Unit

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