I notice that the cabin lights flicker after one of the engines begins to spool up during start-up which I suppose the reason for this being the engines powering the aircraft systems, taking over from the APU. What kind of systems govern such a transfer of power from different sources? I can imagine it must be pretty efficient to immediately transfer power to the APU once the pilots cut off all the engines at the gate.
Correct me if I am wrong, the power for these supplies from the engines will only be available when the gearboxes in that particular engine are driven at a sufficient speed. I suppose different engines drive gearboxes of different purposes and usage, can they be driven by any other means should a particular engine fail?
And what do these gearboxes drive? Alternators and such?
Many thanks guys!