reply to post by 27jd
The passenger O2 mask generators last for about 10-15 minutes. Of course, below about 15,000 and most everyone will be fine. Just sitting there.
Panic-stricken hyperventilation aside......
It is SOP to go to at least 10,000....for the flight deck crew regulations. (Actually, you are allowed to be off O2 above 10,000, below 12,000 for up
to 30 minutes). There are cases, such as mountainous terrain, where the MEA/MSA (minimum enroute altitude/minimum sector altitude) is above
10,000...in which case, you descend to MEA/MSA...and flight crew keep masks on (enough for several hours)....while the people in back will be fine,
even if only a few thousand above 10,000.
The descent procedure is to be accomplished methodically, per procedure...not in a panic, and haphazardly. Also, you heard mention of "structural
damage suspected" or words to that effect? I always was amused because, you're well into the descent, by the time you read that bit in the QRH (quick
reference handbook) checklist procedure.
Of course, here's what we know, from practicing the drills: IF you see an obvious reason for the explosive/rapid decompression (a door warning light,
or known breach in the pressure vessel) then you descend at the SAME indicated airspeed that you are showing, at cruise....IF no structural damage
indicated (so, a mechanical/computer malfunction....there are several redundant systems, though...)....anyway, if that's the case then you just let
speed increase up to the "barber pole" which is maximum (Vmo). Along with speed brakes...this gives the maximum rate of descent.
About the latest Qantas -- I know the 737 (and 757/767) pretty well...will be interested to read the report on this....I have some ideas, though.
But, not enough info, now....whether they dispatched with some equipment inoperative, or not. Perfctly normal to do that, is why there's the MEL
(minimum equipment list). Used to authorize flights with some (redundant) equipment out of order, until repairs can be made later. Without the MEL
would be a lot more delays ... this gives airline some leeway to maintain a schedule, and defer maintenance...but only up to a point, and there are
multiple limitations imposed.
It's possible (just hypohesis) that the airplane dispatched with one engine bleed source inop....so, one engine supplied one pressurization pack, and
the APU was used for the other. A pack can fail, for many reasons...internal failure, internal overheat (and system will "trip" off. like a circuit
breaker)....APU can fail, flame out, etc. Just a few ideas, not saying these were the case.
Here, the panel for Boeing 737 NG pneumatic system, engine and APU bleed controls, and pack switches...and the pressurization control panel (you can't
see, cut-off on the left, the cabin altimeter and rate of climb nstruments. To the left of the altitude horn cutout button):
Condition, there is airplane on ground, engines not running, APU off, packs off. Engine bleed switches usually just left to ON position....except for
certain occasions. "Bleeds Off" takeoff, engine failure/fire procedures, duct overheat conditions, or when the Bleed is placarded inoperative.
edit on 31 January 2011 by weedwhacker because: (no reason given)