In the real world, is there some reason that external power has to be connected before the APU can be started? Is it incorrect to view the aircraft like a car, battery cranks the starter (APU) and APU then recharges the battery? Thanks,Joe

Every airplane that I know can crank the APU from the battery. However, external power is easier on the aircraft systems.

In fact, in many aircraft the APU has as the main function to be a generator in case no external power is available.Bleedair is a secondary resource produced by them which helps start the engines.In fact, in prop aircraft the APU may well be a diesel generator and not produce any bleed air.

With battery power, you only have so much, so if you try and juice up the APU with the battery and for some reason it cannot ignite, then you are pretty much done. With the external power connected, you don't have to use up your battery in case APU won't crank.RH

its usually an economic decision.the external power is much cheaper than a 300lb/hr APU running.unfortunately the external power cannot heat/cool the cabin so usually the APU is turned on sometime during boarding or left on during quick turns.

But yes, once you have established external power, you crank up the APU.RH