Another interesting side note to all of this is the aviation news report I saw over the weekend. A crew of a twin engine jetliner (not sure what aircraft type) was doing a cross bleed start because they wanted to use the APU air to run one of the packs after starting the first engine. The problem was the Captain ran the power up too high to get the needed duct pressure and the aircraft started to move at the gate and the tug bar broke and the ground crew tried to tell the captain to shut down the engine. But that took a little time because the Captain did see his plane was moving but he thought it was the tug driver moving it. It wasn't as the tug driver jumped off the tug as soon as the tow bar broke. What a mess.

Amazing they tried to do a crossbleed start with the brakes off and only the tow bar holding them stationary. Of course if the Captain accelerated the engine beyond the HP bleed switchover point then they'd be trying to get 40 psi from LP air, so quite a lot of thrust. The type concerned must be narrowed down by the fact you can have the APU supply a pack and be isolated from the crossfeed manifold.

 

 

what Level D did with the 767 and effectively disabled a start with the packs running by setting duct pressure to zero if you try it. You can't even start with a pack running on the other engine with the isolation valve closed.

Didn't know that! It's been years since I flown the LDS763!

I think its about 30 PSI to the starter, not sure how much the pack running would take out of that pressure, but i would think that the engine would run hot on startup, if it managed to light!

I think its about 30 PSI to the starter, not sure how much the pack running would take out of that pressure, but i would think that the engine would run hot on startup, if it managed to light!

I think the APU can produce about 40 psi no load and 30 is the minimum to attempt a start so there's some margin over that could run a pack. I said at the outset the start might be hotter and slower.

 

The way it's simulated in the NGX a start with one pack running is almost no different to no packs. However with two packs on and the start valve open duct pressure goes to zero and prevents the engine cranking at all. So the effect of one pack on a start is perhaps underdone and the effect of two packs rather overdone. Not really important when most users will follow the checklist and have both packs off anyway.