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You often on commercial aircraft (whilst on the ground to the runway) hear on the safety video:

in the event of a loss of cabin pressure, oxygen marks will be released from the panel ....

What could cause a loss of pressure? Is it at all even possible for it to just drop?

Season's greetings :-)

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    $\begingroup$ According to my company's training materials, an FAA study in the 1960s of depressurization events in business, airline, and military jet transport aircraft determined that the odds of experiencing cabin depressurization were one in 54300 flight hours. I have been unable to locate that study or substantiate the statement. Either way, though the data is now outdated, it is the only hard number I have ever heard. $\endgroup$
    – J Walters
    Dec 26 '16 at 17:53
  • $\begingroup$ @JonathanWalters: That looks a bit frequent for today, there are 100,000 commercial flights per day, at least 100,000 hours of flight (and actually much more). That would be 2 pressurization incidents per day, only for airlines. $\endgroup$
    – mins
    Dec 26 '16 at 18:14
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    $\begingroup$ Check avherald.com/… for an idea of how often cabin pressure is lost (and what a non-event this is thanks to the oxygen masks). Keep in mind that, even though Avherald does not report all incidents, there are over 100.000 flights a day, so it is still quite rare. $\endgroup$
    – Sanchises
    Dec 26 '16 at 18:20
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    $\begingroup$ @mins I agree; I'm sure the rate has improved by now. Our training materials still require that we give the rate as 1:54300, but that is another matter. $\endgroup$
    – J Walters
    Dec 26 '16 at 18:24
  • $\begingroup$ @Sanchises The AvHerald data would suggest an improvement of an order of magnitude since the 1960's study. $\endgroup$
    – J Walters
    Dec 26 '16 at 18:28
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Aircraft incidents are an extremely remote event (they say that you're more likely to be killed in a car accident on your way to the airport than a plane crash). Within all the incidents that engineers foresee, some are more likely than others. These incidents are included in pilot training (and passenger briefing if applicable) to increase the likelihood that it can be handled properly. Loss of cabin pressure is one of them.

Technically air pressure inside the cabin cannot be "lost", but the term is used to describe a significant drop in air pressure. The masks deploy when cabin altitude is around 14,000 feet.

Cabin pressure is provided by two methods:

  1. The engine actively pumping air into the cabin
  2. The fuselage is sufficiently sealed to contain air pressure inside (it is not 100% sealed, but the pump is powerful enough to overcome the small leakage)

Failure of any of these two components would lead to a loss of cabin pressure.

Historically, causes of cabin depressurization has included:

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  • $\begingroup$ You may well be more likely to be killed in a car crash on the way to the airport than be killed on your flight, but the question asks how likely you are to be involved in a depressurization incident, not how likely you are to be killed. $\endgroup$
    – David Richerby
    Dec 26 '16 at 22:36
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    $\begingroup$ @Emrakul The question says nothing at all about being killed and, as far as I'm aware, most depressurizations aren't fatal for most or all of the people on board. So actually there's a huge difference, and comparing the risk of dying on a plane flight with the risk of dying on a car journey tells us nothing at all about the risk of being involved in a depressurization. $\endgroup$
    – David Richerby
    Dec 27 '16 at 0:01
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    $\begingroup$ @David Oh, I see what you mean. That makes sense. $\endgroup$
    – user4273
    Dec 27 '16 at 0:16
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    $\begingroup$ While not statistically precise, a search of the NTSB accident/incident database for "depressurization" results in 31 items between 4/1988 and 8/2016 with 6 fatalities. That's a pretty low probability, but not much consolation to the six. $\endgroup$
    – Gerry
    Dec 27 '16 at 3:03
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    $\begingroup$ @Gerry actually, some of these 31 results reference entirely different events, like loss of hydraulic system pressure, and most of the fatalities are in small private aircraft. The single relevant victim is, curiously, a flight attendant who was thrown out of the plane by excessive pressure. $\endgroup$
    – IMil
    Dec 27 '16 at 8:56
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You can get a rough idea how common some kind of issue is by searching for it on The Aviation Herald.

In this case, you might want to search for loss of cabin pressure oxygen masks. Unfortunately, the search can't distinguish between “oxygen masks were released” and “oxygen masks were not released” and occasionally mixes in irrelevant result. Also not all incidents are necessarily reported here—there is no central registry of aviation incidents and AvHerald collects the reports from bulletins of the various accident investigation boards over the world, some of them being much more careful and detailed in reporting than others.

So from those results, I'd say there might be on order of ten or low tens of incidents where the oxygen masks release a year, worldwide.

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