I have gone through the cabin pressurization article in Wikipedia. It says there will be some number of Air Cycle Machines (ACMs, or packs) to supply pressurized air into the cabin.

What happens if all the ACM machines stop working? Does the flight have enough oxygen on board supply all passengers until they can safely land the flight somewhere?

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    $\begingroup$ The point of the oxygen masks is not to provide oxygen to passengers for the remainder of the flight, it is only to supply oxygen to the passengers until the aircraft can descend to a breathable altitude, usually only about 15 minutes of O2 per passenger is provided. $\endgroup$
    – Ron Beyer
    Jun 23 '16 at 15:54
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    $\begingroup$ Even if all of the packs fail at once, the pressure will drop slowly. "Outflow" valves in the fuselage will close. No rapid descent is required. The crew will simply fly to an altitude which maintains cabin pressure. $\endgroup$
    – Simon
    Jun 23 '16 at 16:13
  • $\begingroup$ Hi @TylerDurden, I have seen that sherpas carrying some oxygen cylinders with them (atleast for some emerygency). Pls Correct me if I am wrong. $\endgroup$ Jun 24 '16 at 1:55
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    $\begingroup$ @NANDA Those are just the ninny sherpas. Real sherpas hike to the top carrying Swiss "mountaineers" on their backs. $\endgroup$ Jun 24 '16 at 11:32

If all ACM failed, an alarm would sound in the cockpit. And two messages would appear on the ECAM/EICAS¹. One indicating that the ACMs failed and the other indicating that cabin altitude² is rising above desired level.

Pilots would execute corresponding checklist, and the one for rising cabin altitude calls for rapid descent to 10,000 ft. That is the altitude where humans can breathe safely.

If the problem is a hole in the fuselage, the oxygen masks would drop to allow breathing until the aircraft descends, and emergency descent (with full spoilers) would be done. But if it is just failed pressurization, the outflow valve would close and the pressure would only decrease slowly, so the aircraft would most likely descend before the masks would be needed and they would not release. Even normal rate of descent is usually sufficient in such case.

If you're flying above mountains, there are additional rules where to fly to avoid any terrain rising above 10,000 ft, which will be part of the flight plan. For example flying along the mountains of Peru, you'd need to deviate toward the ocean during such descent.

Now at lower altitude, the aircraft flies slower while burning more fuel, so it probably won't be able to make it to the original destination any more. However when planning flight and calculating the required fuel this is taken into account, so the aircraft has enough fuel that it can make it to one of suitably selected diversion airfields if it needs to descend to 10,000 ft at any point along the route.

¹ Modern aircraft have a display on which information about any problems and abnormal conditions is printed, which is called ECAM by Airbus and EICAS by Boeing. Older aircraft had large panels of warning lights.

² Pressure in the cabin is usually given as altitude at which that pressure occurs in standard atmosphere. Normal cabin altitude is 7,000–8,000 ft, 10,000 ft is maximum safe altitude (with generous safety margin). The oxygen masks fall out if the cabin altitude exceeds 14,000 ft.

  • $\begingroup$ Since the ACM's in most aircraft run off bleed air I assume the pilots can shut off the bleed? If they couldn't for some reason, how hot is the bleed air? I'm imagining something blocking the fresh air inlets. Would the cabin get uncomfortably hot or dangerously hot? $\endgroup$
    – TomMcW
    Jun 23 '16 at 17:21
  • $\begingroup$ "Normal cabin altitude is 7,000–8,000 ft, 10,000 ft is maximum safe altitude. The oxygen masks fall out if the cabin altitude exceeds 14,000 ft." - could the pilot theoretically kill all the passengers by circling at 12,000 ft of cabin pressure? $\endgroup$ Jun 24 '16 at 5:35
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    $\begingroup$ @JanDvorak, unpressurized aircraft are required to have oxygen for the crew above 12,500 ft if over 30 minutes, above 14,000 ft always and above 15,000 ft for everybody (see aviation.stackexchange.com/q/25746) and immediate danger is only above 16,000 – 17,000 ft. The 10,000 target is there to provide generous safety margin and to avoid risk of sensitive persons getting altitude sickness. $\endgroup$
    – Jan Hudec
    Jun 24 '16 at 7:07
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    $\begingroup$ @TomMcW: If the ACMs have failed then the pilots open an emergency ram air inlet (usually located on the underside of the belly fairing), which uses the plane's speed through the air to force outside (fresh) air into the cabin, bypassing the ACMs. Depending on the outside temperature it might get a bit warm or cold in the cabin (controlling the cabin temperature usually requires the ACMs and bleed air from the engines), but the passengers will not suffocate, and this gives the pilots enough time to land at the nearest airport. $\endgroup$
    – Lightsider
    Jun 24 '16 at 11:29

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