The 3rd GPU works just fine and is no problem to disconnect after engine start.
If you don't have an APU then you get an external Ground conditioning supply like every other aircraft.
If Boeing wanted you to use The GND test enable switch for air conditioning it would be in the NOP or Boeing Supp procedure. Its not. They designed and fitted an external air connector for a reason.
We got to ask the Boeing design team about this GND test/pack issue a few months back. The Boeing chap pulled a face and said "don't do it".
Last edited by 8che; 8th October 2016 at 00:29.