B767 APU bleed on for take off
I'm looking after a couple of B767's in Quito, Ecuador at the moment. Although it seems that these PW4000 powered jets aren't suited to this operation due to it's performance limitations.
I'm the engineer on these jets but I'm also a PPL, and I have a question that an experienced tech pilot might know:
Quito (UIO) is 9,300 msl. There are runways 17 and 35. On our performance charts there is almost a 20 tonne difference on MTOW for either runway. Primarily due the the terrain and single engine performance off 17.
The way that the operator loads these flights, we are almost always at MTOW and sometimes we have to wait for the wind to change or at least drop below a 10kt tailwind so that we can take RW35.
Sometimes a tonne extra payload can make a huge difference and we occasionally have to offload bags.
With a packs off T/O we could gain an extra 1000KGs, but it's not recommended at Quito as you would be above 10,000ft in only a few seconds, and the crew should really be wearing O2 masks.
Why can't you take off with the eng bleeds off, but with the APU bleed on, powering the packs? I've asked the same question to our pilots and they just reply, "Because it's not in the book!"
Does anyone have an explanation?