On the ground the NG is supposed to have 2 packs running for air con. Switching one off (the other remaining in AUTO) increases APU fuel burn from about 220lb/hr to 270lb/hr (approx figures) and increases APU EGT by about 100deg. due to the remaining pack reverting to high flow.
Since switching a pack off in flight will cause the remaining pack to go to high flow too, and following the logic demonstrated by the APU, I'd guess that the engines wil use more fuel too to power one pack rather than 2.
The only sensible reason I can imagine for operating a single pack is a failure in the other.
The FL250 restriction is in the MEL, but not the QRH or Part B limitations, so descending following an inflight pack failure is a matter of airmanship rather than procedure.
(Edited for bad typing!)
Last edited by Whippersnapper; 28th Jul 2004 at 09:15.