Originally Posted by flensr

A320 has an issue with valves chattering if the APU is running the packs while there is external conditioned air plugged in, so don't expect 320s to have the APU running the packs while external air is plugged in. Getting ground to unplug it can take quite a bit of time.

737 may not have that issue but I've been told it's "not good" to have the packs running while external air is plugged in, and it seems like some crews won't turn on the APU bleed and packs until after external air is disconnected. Again, that can take quite a while at some stations.

Yea, excuses, whatever. The CAs I've been flying with have been perfectly happy to turn on the APU and packs at the first word or hint that it's getting warm in back and I've done it a couple times myself when the CA wasn't around to confirm first.

Finally, yea. If there is a crew swap, the APU gets shut down until the next crew gets on the plane. One can only hope the incoming crew checks cabin temps when they arrive to see if they need the APU back on immediately.