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I know the usual scenario is where an airliner suffers from an explosive decompression and the oxygen masks drop and the plane descends to a safer altitude but.....

This is a purely hypothetical situation but if a passenger plane was cruising at 39000ft+ and it suffered a cabin leak that somehow went unnoticed by the crew and passengers and the oxygen masks didn't drop, how long roughly would it be before the passengers were incapacitated, and how long before the lack of oxygen became fatal.

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The depends entirely on how fast the cabin is leaking e.g. how big the hole is. –  casey Mar 26 at 16:12
Good question, but can we assume it takes 30 minutes to clear the cabin of something mid size like a 737, I am guessing a slow drop in pressure over that sort of time wouldn't be noticed. –  Ian Mar 26 at 16:20
Or if the cabin didn't pressurise correctly and was at outside pressure as it climbed to cruising altitude. –  Ian Mar 26 at 16:22
When an airliner suffers an explosive decompression, everybody dies of barotrauma (explosive decompression is defined as happening faster than the air can escape lungs and the pressure difference is enough to damage lungs) and the aircraft falls apart. The more normal event is a rapid decompression. –  Jan Hudec Mar 26 at 20:21

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up vote 8 down vote accepted

The question you're asking roughly translates to What is the time of useful consciousness at 39,000 feet?. The answer is "About 15-20 seconds, once the pressure bleeds off." The FAA has a handy table for this:

Time of Useful Consciousness
From Chapter 16 of the Pilot Handbook of Aeronautical Knowledge.
You can get a PDF of Chapter 16 here, or you can grab the whole thing from the FAA

The Time of Useful Consciousness will vary depending on personal physiological factors (e.g. if you're a smoker your blood doesn't oxygenate as well - you will probably have less time. If you're a mountain climber in excellent shape and used to breathing rarified air on your climbs you'll probably have a little more time).

In the case of a non-explosive decompression (say a pressurization system failure) you'll probably have more time than this from the start of the event to the point where everyone is unconscious: the cabin pressure will take time to bleed off. Whether that time is seconds or minutes depends on the nature of the leak.

Presumably while the pressure is bleeding off the aircraft's systems will complain about the loss of cabin pressure and give the pilots a chance to address the problem (fix the pressurization system or descend to a safe altitude). Unfortunately the effects of hypoxia can start setting in before the crew realizes what's going on (which is one of the contributing factors in the loss of Helios Flight 522, which suffered just this kind of gradual depressurization).

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Thank you, that was exactly what I was looking for. Thanks. –  Ian Mar 26 at 16:28
@Ian You may also find this article from AvWeb interesting. It discusses a lot of the other factors that I alluded to above. –  voretaq7 Mar 26 at 18:02

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