Originally Posted by Dash8widget

If its hot out the two cooling units will NOT be fighting each other - even if they both have thermostats. If the temp settings on the thermostats are turned down there is a very good chance that the cabin temp will never get to the thermostat set temperature - which means that both systems will be working at full cold. Give it a try sometime - with ground air hooked up, turn on the APU air and turn down the pack temps - then watch the pack valve positions. I bet that they don't move off of full cold. On the 88, ground air + APU air is a good thing!

If you think about it there are really THREE cooling systems in play - ground air plus TWO packs. Each with its own thermostat. We don't turn off one pack to keep the packs from fighting each other do we?

Oh, and if the pack valves do move off of full cold then put them into manual.

The only thing that you would need to worry about is the ground air putting out warm air - for that to happen, the air around the jetway thermostat has get get colder then the set temperature - not very likely this time of year!