A consideration is time to descend to 10,000’ in the event of a pack trip when on one pack . Even if you can get there you don’t want to be at 39’0000 with a trip on the remaining pack. I have had a pack trip at top of descent as the thrust was brought back. Heat exchanger in the pack was blocked by fod it was later found. Mel dispatch requirements only ever apply after an MEL has been applied on the ground. QRH takes precedent airborne. QRH lets you stay at planned level because it assumes descending to 25,000 might cause endurance issues. But you wouldn’t plan to be in that position.
Last edited by Alt Flieger; 7th Dec 2021 at 04:37.