Question on Engine Start
 

Hi,

For commercial aircrafts, APU bleed will help to start the engines. When we turn the ignition to ON and then select Engine 1, the startup is automatic. But I want to know a little more. When we select Engine 1 on the pedestal, does the APU bleed turn the low pressure compressor or turbine?

Care to share your knowledge?

Thanks

Neither one - the air starter (basically a small turbine) is connected to the high pressure rotor via the accessory gearbox and a 'one way' clutch. So what you have is a small air driven turbine that spins up the hp rotor to sufficient speed to achieve lightoff when fuel and ignition is applied.
The starter motor continues to apply torque to help the engine accelerate towards idle until the starter air valve closes (typically around 50% N2/N3)