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I was reading about the cabin pressurisation mechanism from Wikipedia and another article here. According to both of them, the aircraft's engine plays a vital role in the pressurisation process.

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My question is:

I have come across many air crash investigation articles mentioning that all the engines of an aircraft had failed, but the pilots were able to glide the aircraft and safely land it with no casualties. How is it possible for everyone onboard the aircraft to survive without cabin pressurisation (with all engines out) ?

UPDATE: What if a safe landing area is a bit too far? After losing altitude, the aircraft can't obviously gain altitude. Won't the plane crash due to low altitude (the descent is fast) gliding?So, isn't it better to maintain (sufficient) altitude to reach an emergency destination? I hope the altitude can be maintained provided cabin pressurisation is intact. But, how is this achieved in commercial aircrafts?

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    $\begingroup$ TLDR: the APU kicks in and everything stays alive long enough that the pilots can respond to the emergency. $\endgroup$
    – Nij
    Commented Aug 12, 2016 at 8:05
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    $\begingroup$ I guess pilots close the exhaust and the pressurization should hold long enough (time to glide to a flight level where pressurization is not needed) $\endgroup$
    – Manu H
    Commented Aug 12, 2016 at 8:30
  • $\begingroup$ What happened with Air Transat 236? That craft was out of fuel somewhere near FL345. Nothing I've read about it suggests that they did anything other than max glide before the approach. $\endgroup$
    – BowlOfRed
    Commented Aug 12, 2016 at 17:17
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    $\begingroup$ Your "update" is an entirely separate question. Please ask it independently, instead of having multiple questions in one. This is a Q&A, not a forum with threads. $\endgroup$
    – Nij
    Commented Aug 13, 2016 at 6:38
  • $\begingroup$ @Nij My update points to the same problem of loss of cabin pressurisation when all the engines are out. It just adds a scenario of when a safe landing area is far and will be impossible to reach when the aircraft drops down to lower altitudes in order to get breathable air (since dropping down to lower altitudes is the only way of saving the passengers). $\endgroup$
    – steam7137
    Commented Aug 13, 2016 at 6:48

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Closing the outflow valve would seal up the plane, so it wouldn't depressurize immediately. Once the pressure falls below a safe threshold the oxygen masks would drop. The masks are good for about 15 minutes.

Air Transat flight 236 which ran out of fuel over the Atlantic holds the record for the longest unpowered glide in an airliner. It lasted about 19 minutes.

The APU can provide pressurization, but if you're out of fuel that won't work either. Air Transat was running off the RAT and I doubt that can provide pressurization. BUT if you factor in the outflow valves being closed and the last part of the glide being below 10,000 feet the oxygen masks will provide ample time for the passengers to stay safe.

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If the engines of the aircraft fail, the APU can supply the required air for some time. During this period, the pilots will initiate descent to lower altitudes (10000' or lowest safe altitude permitted by terrain) where the passengers can breathe without cabin pressurization.

The cabin pressurization system is controlled by an outflow valve, which is shut in case the pressure differential goes above certain value or the pressure altitude rises above a certain value. This prevents the loss of air in cabin. The air coming to the cabin is prevented from going in the opposite direction by check valves.

Commercial aircraft usually carry supplemental oxygen, which is fed via masks in the cabin (which drop automatically once the pressure altitude reached 15000' or can be dropped by crew before that). As the supply is limited , the crew (who will don their masks first) will get the aircraft to low altitudes.

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    $\begingroup$ This creates an interesting dilemma, though: do you start an emergency descent without engines? Or would you rather not lose altitude you know you never can get back? Obviously, you need to keep the passengers alive, but do you need to keep them conscious? $\endgroup$
    – Jörg W Mittag
    Commented Aug 12, 2016 at 13:02
  • $\begingroup$ Some aircraft also have a maximum altitude for starting the APU, below typical cruising altitude. $\endgroup$
    – fooot
    Commented Aug 12, 2016 at 14:49
  • $\begingroup$ @JörgWMittag You need to keep then conscious so they can follow emergency procedures and an eventual evacuation $\endgroup$
    – jean
    Commented Aug 12, 2016 at 16:05
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    $\begingroup$ What if a safe landing area is a bit too far? After losing altitude, the aircraft can't obviously gain altitude. Won't the plane crash due to low altitude (the descent is fast) gliding? $\endgroup$
    – steam7137
    Commented Aug 12, 2016 at 16:08
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Oxygen for breathing is all that is required to survive the decent. That's what all the little masks that drop down are all about... don't forget to pull the little string. It may not be enough to keep you conscious in the event of a full loss of cabin pressure at cruising altitude but it will keep you alive.

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  • $\begingroup$ If it's not going to keep anyone conscious, what's the point? How will the pilots pilot the aircraft? And also remember that you will lose consciousness in a maximum of 15 seconds (due to hypoxia). $\endgroup$
    – steam7137
    Commented Aug 12, 2016 at 16:02
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    $\begingroup$ @steam7137 The pilots' oxygen system is separate from that of the passengers and delivers oxygen at a higher rate than that of the passengers. Also, while the passenger masks are flimsy affairs that allow considerable leakage, the cockpit masks are durable and have relatively little leakage. Also, individuals vary greatly in their tolerance of lack of oxygen. Additionally, recovery when breathable altitudes are reached is but a matter of seconds for a reasonably healthy person. $\endgroup$
    – Terry
    Commented Aug 12, 2016 at 18:45

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