Purely hypothetically speaking now,
Thanks Firststep. So the APU is used sometimes in concert with pnem issolation valves to supply pack source/pressurization at 400 feet AGL to reduce total eng bleed requirement, thus, improving high altitude performance. That's what I wanted to know; (forget the four bleed off procedure everybody; that's negative transfer from another airplane.)
Earl had a post where he relayed (reliable source) buzz of multiple engine failures as well, however it is now gone for some reason.
Engine nacel fire warnings are often caused by bleed leaks at high power near the fire detection loops and sometimes troublesome bleeds are legally wired shut by mtc and defered is where I'm going with this.
It still doesn't make sense to me that the first responders, saw people wearing masks right after it happened. This bugs me. Perhaps the pack didn't get turned on, or couldn't get an air source at 400 ft in all the confusion and later the cabin altitude horn sounded? Mysterious.
At any rate, it's beginning to sound like the Columbian media is right: these guys were genuine heros in handling a nightmare scenario.
pac - out