Hi EI678.

The APU get its fuel from the left wing fuel tank. If the APU is running, you can't change the quantity of fuel in that tank. You need to turn the APU OFF to be able to change it. It is for this reason that I rarely use the APU (especially if you knew how I work out the correct amount of fuel I need for a flight!), I just select the external power and external bleed air. This allows me to take my time setting up my flight and starting the engines (as I stated in Reply#4).

Hope this helps.

Mark

EDIT. I replied to this post twice! I had installed Mozilla Firefox and it was placing my tabbed browsing differently to what I had originally had it set at. I thought I had closed the tab before posting the first reply, but I hadn't. I have deleted the second one now. Sorry.  :(