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What are the most common reasons that cause an aircraft to depressurize while flying? For example, a broken window, a failure on the Outflow Valve, etc.

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closed as too broad by UnrecognizedFallingObject, reirab, curious_cat, aeroalias, Peter Kämpf Feb 12 '16 at 8:17

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

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    $\begingroup$ Without listing the probably hundreds of reasons, it boils down to the failure of a component that seals the inside pressure of the aircraft with the outside pressure. The failure can be caused by many factors from fatigue through sabotage. $\endgroup$ – Ron Beyer Feb 5 '16 at 15:43
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    $\begingroup$ @RonBeyer or failure of the pressurization pump though that is rarer (I haven't heard of any other than helios 522 and that was because of a miss-set switch). $\endgroup$ – ratchet freak Feb 5 '16 at 15:52
  • $\begingroup$ @ratchetfreak Even if the pump fails, the pressure has to leak back out somewhere, even through the pump. The pump itself acts as a "seal" in that it only allows a one-way movement of pressure, when it fails and the pressure backflows, its a seal failure in a sense :) $\endgroup$ – Ron Beyer Feb 5 '16 at 16:19
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    $\begingroup$ @RonBeyer the air would still go out the outflow valves. $\endgroup$ – ratchet freak Feb 5 '16 at 16:21
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    $\begingroup$ Speaking of closed... I tried to edit this to make it less broad. Pressure issues do happen and the typical causes should be answerable. $\endgroup$ – fooot Feb 5 '16 at 17:00
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The most common failure leading to depressurization I've personally seen is a pack/air cycle machine failure. The equipment that takes pressurized bleed air from the engine, cools and expands it. I've seen it happen two ways. 1) The aircraft is dispatched with one pack on MEL and the second pack fails in flight or 2) The aircraft is flying at high altitude, 1 pack fails, the other pack switches to high flow mode and fails soon after.

Pack failure is by far the most common I've seen. But have also seen a couple failed fuselage skins and a few burst PAX windows. Typically door leaks are enough to be noisy as heck, but not depressurize the aircraft.

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