Hi,. .A simple answer to your question is to look at the system on a large aircraft..... .. .The jet engine normally has a generator attached to it's own ancillary gearbox, as the engine is started, it rotates the gearbox and the gene' rotates also. The gene' gets "excited" and produces current, fed to the Voltage Regulator etc etc. If the battery fails the engines still rotate and the gene's are still producing power...all very well until the battery dies completely and the DC control circuits drop out and thats when you normally lose the AC and control!. .. .Hope this quick answer helps but you might be going a little too deep for PPL!. .Regards Rodders <img border="0" title="" alt="[Big Grin]" src="biggrin.gif" />