Originally Posted by exAC
The problem is the airplane and the mission they are trying to do with it.
YYZ-YVR is too far for that aircraft. In order for it to get airborne out of YYZ they need all of the power that they can get, thus the air conditioning packs are turned off.
The packs run on engine airbleed and thus remove some of the takeoff power.
Could they not just shut the AC down prior to takeoff, but leave it on while the aircraft is parked at the gate?