Do airlines need to change from external power to APU before pushing-back and starting the engines?

Question

I mean, normally, to start the engines, you need to have power. So, the APU would be great, since the engines (I think) are just turned on when the aircraft is pushed back, therefore an external power connection is not there anymore. Or is the ext. power even attached during push-back, so that therefore APU isn't needed?

Answer 1

On the flight line at airports the APU runs the whole time on the ground. You do see the cabin lights flash when they disconnect external power the switch is automatic. The co-pilot then begins the start up sequence by turning engine masters to on followed by the pilot switching the bleed air to a single selected engine at a time usually starting the engine closet to the APU.

Answer 2

Usually they do as airports now restrict the use of APU when being parked at the gate. (noise abatement or additional income for the host country airport). So prior push back the APU is started or if the tow tractor is equipped with ext. power they just reconnect it and as push back commences the APU is started to have the necessary pneumatic for engine start cause ext power supplies electrical only.

Answer 3

The aircraft needs pneumatic power to move the engine.

APU and LPU(Ex.p) given it at ground.

At pushing back , the power will be coming from the APU or engine only.

Answer 4

The bleed air of the APU is used to turn the engines to start them. The starters are not electric on any large turbofan aircraft.

Maybe the small ones do have them, but I have no recollection of that, Senator Kennedy.