Originally Posted by
Old Fella
Derfred says that my suggestion of telling the "Computer" that the Cruise Level is 1000' above actual will not work (in giving a buffer) in the 737NG. If that is so, why bother telling the "Computer" anything about Cruise Level?
Old Fella,
I can't provide a reference because, as I said earlier, the systems description in the FCOM these days is very limited. But I'm reasonably sure that if you wound up the cruise level to 1000' above actual, exactly nothing would happen. The cabin altitude would not change, and the diff would not change.
Obviously, prior to a step climb, we would do exactly that (wind up the cruise alt). But the cabin would not start climbing until the aircraft started climbing.
The reason we set the cruise altitude in the pressurisation is mainly two-fold: (1) so it knows which diff to apply of the three different diffs, and (2) so it knows when to transition from "climb" mode, to "cruise" mode, and to "descent" mode. (A simplification perhaps, it also gives it a target to apply it's cabin rate algorithm to achieve the most comfortable cabin rate).
The system is largely foolproof (when it's working!). For example, you could set FL200 in the pressurisation and climb to FL410, and I'm pretty sure the system would happily keep a safe diff, safe cabin altitude and safe cabin rate. Not recommended of course, but the system would actually cope.
BTW: I've done the F/E gig in a past life, so I know what your're referring to in terms of ye olde semi-auto pressurisation systems. You can hack the 737NG pressurisation system by setting landing alt above current cabin alt - that will achieve the aim you were looking for (immediate cabin climb and diff reduction) and indeed that technique is used in non-normals such as cracked windows and smoke removal, but I wouldn't be recommending it in this scenario. Based on the OP's subsequent feedback, that wouldn't have helped either. Only manual mode was capable of saving the day on this occasion.
At the end of the day, the Boeing checklists kept everyone alive, even though a system failure occured that had not been properly thought out by the designers. So I'm going to keep the faith, although I will keep my "Ole Skool" scans going as described by Rat.
