757 / 767 Single Pack Operation
 

After a malfunction in the 757 / 767 that results in a single pack (engine failure, engine bleed malfunction, etc), checklists direct the crew to remain in single pack operation instead of opening the crossbleed valve to allow both packs to be sourced from a single bleed. Does anyone know the "why" behind this?

Thanks a ton.

Only a guess from a non type rated person...

If you had two packs operating from a single bleed, then had an engine failure of the engine with the bleed failure. You'd be trying a run an engine feeding both packs at MCT thrust and maybe that would mean that the core of the engine may be working a little too hard and single engine performance wouldn't be met... Especially if you drifted down to a level where wing/engine anti ice was needed too.

The remaining pack will go to 'high flow' - not as much output as both packs operating, but close.

The isolation valve/s are primarily there to 'separate the bleed air ducts into isolated segments' (B12) so that a leak on one side can be isolated and not fed from the other. The Pack Off QRH allows air to be taken from the other side in the event of a dual pack off condition using one of Mr Boeing's less intelligible statements : Consider an alternate bleed air source if dual PACK OFF condition is a result of engine bleed air loss not caused by a duct leak or engine start valve failure.

So, eg, a L pack failure followed by a R Eng failure would allow you to open the isolation valve/s to drive the R pack from the L Eng bleed (or the APU below 17000').

The protection given by keeping the ducts isolated is more important than having both packs working.

Hth