aaa0808,
It still does not answer the primary question:
consider the case where the aircraft is dispatched with left bleed u/s.
as per maintenance procedure the valve is locked in closed position.
Now the aircraft departs with APU bleed supplying left pack and engine 2 bleed supplying right bleed after passing FL170 why cant we just turn of the left pack? :confused:

See the previous post on the matter:
The flight deck compartment is directly taken from the Left pack only

Because the flight deck compartment would not get ANY air if we were to take the LEFT PACK - OFF.

By ensuring the available (Engine) Bleed supply is feeding the LEFT PACK, we ensure pilots get some air to operate the aircraft. Feeding the Left Pack from the Right Bleed will ensure air is available in the Mix Manifold AND the Flight Deck Compartment.
If we only feed the RIGHT PACK as you are asking, the Mix Manifold will distribute air only into the passenger cabin but NOT into the Flight Deck Compartment (by design).

Thus to ensure the two people in control of the aircraft get some air as well, in (almost) every scenario we operate the LEFT PACK and turn the RIGH PACK - OFF.

Hi Skyjob,
that makes sense then the question that comes to mind is when you have a left bleed failure in flight why is it, that the QRH procedure does not require this cross supply? :confused: