Originally Posted by fumje

Technically speaking, this seems like non-functioning APU, which provides AC on the ground. Typically it is safe to fly without the APU. In flight, there's no need for AC given the air temperature at altitude. I don't know of any rules proscribing operation without APU in hot weather, although perhaps it would make sense to have some for extreme conditions. I believe there is capability to pipe in cool air while at the gate, although I also don't know of any rules requiring that they do this.

Probably good questions for the pilot thread.
United Pilot Q & A thread