Single Pack Cruise
Some interesting comments on this subject from a pilot's point of view. Dubya says the single pack would not be working as hard at FL400 as it would at FL250 due to the "warmer" ambient temperature at the lower level.
ISA temp of -35C at FL250 does not seem warm at all to me. I have had nought to do with B737 aircraft, however I have had considerable B707/B747 time as a F/E and I don't believe "Working less hard and less prone to falling over" would even come into the thinking as far as the pack is concerned. I would guess that the only part of the pack doing any "work", even at FL250, would be the heat exchangers. The ACM would likely be being by-passed at those sorts of levels once the initial cooling of the cabin was accomplished.
Purely from a pressurisation loss consideration I would not like to be in an aircraft operating on one pack at FL400. Precisely the reason the MEL does not allow for dispatch with a single operative pack for flight at levels above FL250 is that in the event of a further failure the time to descend, initially to below FL140, is minimised and later to 10000', thus probably avoiding the "rubber jungle". If fuel is a consideration most routes flown by the B737 would likely have suitable diversion airfields available for additional fuel uplift if required.
Last edited by Old Fella; 19th Mar 2008 at 02:58.