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How much oxygen do commercial airliners carry? Is is enough for every seat to have a 20-30 minute supply, is it a percentage of total flight time (ie 10% of total flight time), or is there a compressor on board?

I would assume the goal of the pilots during depressurization would be to repressurize whether that be by simply lowering altitude or other means. Thus, I would assume (hope) there would be sufficient reserves of O2 to do one or the other.

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The masks are connected to a chemical oxygen generator by a lanyard. Pulling the mask pulls the lanyard, which pulls a pin from the generator and starts the reaction. They typically last betweeen about 10 and 20 minutes, more than enough time to get down to 10,000 feet where additional oxygen is not required.

There is no need for a % of flight time since the crew will descend to a safe altitude then divert to the nearest suitable field. Therefore, the only requirement is to last during the (emergency) descent.

You cannot "repressurise" since the cabin altitude will be the same as the external altitude. The only thing to do is to increase the partial pressure of oxygen by descending.

It's worth mentioning that the flight crew each have their own, isolated supply from a bottle which lasts significantly longer. This is so that they can continue to fly if there are fumes in the cockpit. Cabin crew often have their own, smaller, portable bottles so that they can freely move about the cabin to assist the "self loading freight".

Many's a morning when I would arrive to work feeling, ahem, a little "under the weather" and nip into the cockpit for a couple of minutes on 100% oxygen. Works wonders - and don't worry, they are topped up to full as part of the pre-flight service.

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    $\begingroup$ Cool. Would there ever be case where it would be unsafe to fly lower (such as over mountains)? $\endgroup$
    – Matthew Peters
    Nov 13, 2014 at 20:28
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    $\begingroup$ Yep, pray for clear skies ;) 15-20 minutes should be enough to fly away from the 10,000' and above threats. But whatever, you got to get down before the oxygen supply runs out or your passengers start to croak. $\endgroup$
    – Simon
    Nov 13, 2014 at 20:29
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    $\begingroup$ Thanks, this kinda reminds me of my IP's response to why we dont fly out over the coast more during our cross country (TPA-BTR) he said, "I want to limit how far I need to swim" imply that we shouldnt go father than we could glide back in... $\endgroup$
    – Matthew Peters
    Nov 13, 2014 at 20:39
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    $\begingroup$ It's also worth noting that the flight crew's masks are pressurized (or can be, by flipping a switch) which allows them to clear smoke/fumes from the mask, as well as to continue breathing at altitudes where simply enriching the ambient air with oxygen would not be sufficient. Also cabin crewmembers get portable "walkabout" cylinders to enable them to attend to passengers who might have trouble with their masks. $\endgroup$
    – voretaq7
    Nov 13, 2014 at 20:59
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    $\begingroup$ @Antzi No, because you make sure you have enough fuel that that's not a problem. There are a few situations you have to have enough fuel for; one is the situation where you get out to the point where you're the absolute furthest from your diversion airports (around halfway across) and the plane depressurizes. You have to have enough fuel to proceed at standard post-depressurization cruise to an alternate. You don't just put the bare minimum fuel; you have enough to deal with contingencies. $\endgroup$
    – cpast
    Nov 14, 2014 at 6:04
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They don't carry oxygen tanks for each passenger but instead use chemical oxygen generators that activate can provide at least 15 minutes of oxygen. They activate when the person pulls down the mask.

These 15 minutes is enough for an emergency descent to a safe altitude ~10k ft where there's enough oxygen in the air for people to breathe without needing the mask any more.

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While some pressurized aircraft use oxygen generators as others have described, others use pressurized (up to 3000 psi) gaseous oxygen canisters. An engineer who designs such systems showed me how central oxygen systems work, but I believe there are smaller units (per seat group) on B787s and A350 XWBs.

They would carry at least 15 minutes of oxygen, but how long it lasts depends on how many people are using masks.

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    $\begingroup$ Welcome to aviation.se Gabe. Do you know how much oxygen is in those canisters, i.e. how long would they provide oxygen flow? $\endgroup$
    – DeltaLima ♦
    Nov 14, 2014 at 9:41
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    $\begingroup$ @DeltaLima: I can't find a reference for how much O2 they contain, but they would have 15-20 minutes worth. Unlike an oxygen generator that has a chemical reaction lasting for a fixed amount of time (15 min. usually), though, the duration of flow depends on how much is used. $\endgroup$
    – Gabe
    Nov 14, 2014 at 13:58

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