After much research I may have a solution as to why ATC and the engines won't start...Is there a limit to the APU run time? I start the APU (not ground power) after loading cold and dark. after say twenty minutes, I'm not finished programing, GSX etc.. the APU run light is still illuminated but ATC is gone so the engines don't start. This is a dumb question, but how (and how long) can I use external power before switching to the APU? On some add ons the APU run time seams to be unlimited or an actual external power unit can be seen (animated) externally.