Hi.
Perf factor changes the fuel prediction of fmc. E.g. if trip is supposed to be with 3% degradation and fmc had none. Then lets say at a point fmc says you will burn 10 tonnes to your destination. When the correct factor was set in the fmc it will would predict 10.3 tones to your destination.
Idle factor changes the idling n1 of the engine. This affects the idle descent by making it shallower for +ve idle factor. Best recommendation to check Airbus fcom.
Regarding part 2. Generally default fmc factor is 0 to represent a new factory aircraft which should burn fuel by the book. For reasons unknown to me some Airbus aircrafts have +ve / -ve default factor (this is documented in fcom). So the new factory aircraft has a +ve / -ve default factor to match the book value. All fuel factor corrections are to be made against this default value. E.g if default factor is 2, fuel burn factor identified to be +1, then fmc requires 2+1=3 as the perf factor.
Fmc implies fmgc.
Last edited by vineet.mk; 3rd Jul 2013 at 02:17.