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When an airliner loses all engines while at cruising altitude, two things happen:

  1. The plane is a glider. It will descend no matter if you want to or not.
  2. The cabin pressure is no longer maintained. Although the valves are closed automatically, they are not air tight - air will slowly leak out until the air pressure is equal to the atmospheric pressure outside.

My question is, which one happens faster? Are the valves able to maintain cabin pressure until the plane has descended to 10,000 feet, or will the oxygen masks deploy before the pilots can complete their descent at best glide speed?


EDIT: For the purpose of the question, let's assume it's s fuel exhaustion and ignore the APU. I'm specifically interested to know if the crew would encounter a situation where they have to choose between oxygen and best glide.

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    $\begingroup$ I don't think the crew must choose between oxygen and best glide. At best glide speed, the sink rate will be great enough to allow oxygen masks to ensure enough breathing before the aircraft reach a flight level where you can breath normally. Even though, this is a quite good question. $\endgroup$ – Manu H Sep 27 '16 at 12:11
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    $\begingroup$ @ManuH I actually doubt that is a good assumption. The typical emergency descent due to a loss of pressurization requires a descent near max speed with the airbrakes and/or gear extended in order to get down before running out of oxygen. $\endgroup$ – Lnafziger Sep 28 '16 at 0:37
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    $\begingroup$ @Lnafziger The typical emergency descent is due to a rapid or explosive decompression, though, where the cabin pressure equalizes with the outside pressure immediately. The situation in the question is a gradual cabin leak due to pressurization packs shutting off (due to fuel exhaustion). So it would take much longer for the cabin altitude to climb sufficiently to trigger oxygen mask deployment. $\endgroup$ – Christian Lee Sep 28 '16 at 1:03
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    $\begingroup$ @ElliottTangoFoxtrot Agreed, in this context. Your statement was more general though and simply said that "At best glide speed, the sink rate will be great enough to allow oxygen masks to ensure enough breathing before the aircraft reach a flight level where you can breath normally." I don't want people to misconstrue that statement in a more general sense. It's only true here because the masks won't drop until much later in the descent and not at their cruising altitude. $\endgroup$ – Lnafziger Sep 28 '16 at 16:29
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    $\begingroup$ @Lnafziger Gotcha. That wasn't my statement though, it was another commenter's, not that it's a problem. $\endgroup$ – Christian Lee Sep 28 '16 at 16:54
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The valves will not be able to maintain cabin pressure until the plane descends to 10,000 feet at best glide speed. The oxygen masks will deploy before then.

At least that was the case for Air Transat Flight 236, an A332 that ran out of fuel over the Atlantic and glided to a deadstick landing. The occurrence report on page 96 (see the table) estimates that oxygen masks deployed at 06:37:50, when the cabin altitude reached 13,500 ft. This was about 11.5 minutes after fuel exhaustion, and only 7-8 minutes before landing at 06:45. So they would have landed within the 10 minutes of supplemental oxygen. Even a much longer glide would have allowed them to descend below 10,000 ft before the supplemental oxygen stopped. TSC 236 is AFAIK the longest glide in a transport category aircraft to date, so if they did not "encounter a situation where they have to choose between oxygen and best glide" (to use your phrasing), then probably no one else did either.

Note: The report used the 'worst case' leak rate of 700 ft/min for their calculation. The footnotes explain that the leak rate would decrease as the aircraft descended (because the pressure differential between the cabin and the outside would decrease as the aircraft descended). If anyone is interested, there is more info on pages 28-29 of the report about the cabin pressurization.

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Well if a dual engine loss occurred, the APU in the tail also drives a compressor to provide at least limited cabin pressurization until the engines can either be restarted or the aircraft can descend below 14,000 ft ASL.

The emergency RATs in airliners only drive a generator and hydraulic pump, not a compressor to provide cabin air.

Finally even if there was a failure in cabin pressurization because the air supply from engine bleed air or APU, the leak would be reasonably slow so as to allow for a controlled emergency descent. This combined with the chemical OBOGS to provide supplemental oxygen for the crew and passengers would allow for a safe descent without a high risk of hypoxia.

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Like the controls heaviness question, I find it very hard to find numbers. Especially when we ignore the APU. However looking at it from a different angle, let's say in a 737 both packs fail. Effectively rendering the operative bleed air useless.

This is what the QRH says:

enter image description here

... gradual loss of cabin pressure.

Since airliners are not graceful gliders, and the cabin climb rate will be gradual, I'd say they'll have enough time to descend past 10,000 feet. That is with no oxygen masks initially, and then with oxygen masks when the cabin altitude hits the critical mark.

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    $\begingroup$ Airliners are pretty good gliders in terms of L/D - around 20:1 is typical - though the sink rate at best glide speed is still rather high. $\endgroup$ – pericynthion Sep 27 '16 at 16:19
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    $\begingroup$ The 20:1 is only possible with the newest designs (A350 and B787); planes 20 years older reach about 18:1 and 40 year old designs reach about 16:1. $\endgroup$ – Peter Kämpf Sep 27 '16 at 20:24

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