A319\20\21 PACK'S OPERATION
 

WITH THE APU SUPPLING AIR ,DO THE PACK FLOW CONTROL VALVES OPEN STRAIGHT AWAY OR IS THERE A DELAY BEFORE OPERATION.

My understanding is that it depends on wether you have just started the engine/s. There will be a delay on the pack opening post engine start but not if one has just turned off the packs for say a walkaround.

Theres no built in delay, but the apu pressure is normally a bit lower than engine air pressure, so the valves operate a bit slower. If you try to select crossbleed ´closed` and just switch on the left pack you will find that it opens almost as fast as with engine air.. .Brgds. .Doc