<<If the aircraft is currently powered by the external Ground Power Unit (aircraft batteries not switched on), can I start the APU using only the GPU (without switching on the 2 aircraft batteries)?>>
No, in this case you would be able to switch on the master switch, but a short time after pressing start you will get a start fail on ecam.
<<will the APU take over the supply of electricity to the aircraft from the GPU? >>
No, external power button will be blue on with the GPU supplying. Once APU is available you will need to press the external power button to txfr over to the APU genny.
I would note I am a pilot not an engineer.
Last edited by tom775257; 7th Jun 2012 at 15:46.