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Various sources on the internet indicate that the passenger cabins on pressurised airplanes are not 100% airtight. I'm not talking about ventilation/pressure valves/cabin pressurisation system, but about unintentional leakages due to various imperfections in the construction.

Is that true? If yes, what would be the "usual" value for some commonly used airplane in easy to understand units?

I'm trying to imagine if the (possible) unintended leakage might cause any trouble in flight, if the pressurization system failed.

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  • $\begingroup$ Air is always leaking through the cabin pressurization system, it is designed to. If the cabin pressurization system fails, the vent valves close and the captain starts to descend. Other sources of leakage (unless you blow out a window) should not depressurize the aircraft before it gets to a safe altitude. $\endgroup$ – Ron Beyer Jul 26 '18 at 21:39
  • $\begingroup$ AT236 flew for 5 minutes with no engines before masks deployed. Not sure how to calculate the leak rate from that figure. $\endgroup$ – BowlOfRed Jul 26 '18 at 21:45
  • $\begingroup$ Possible duplicate of Is constant air pressure maintained inside the fuselage? $\endgroup$ – fooot Jul 26 '18 at 22:08
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    $\begingroup$ @BowlOfRed Masks typically deploy at a cabin altitude around 10,000 ft, don't they? If normal cabin altitude is maintained at 8,000 ft, that's a loss of 2,000 ft worth of pressure in five minutes, or a change of cabin altitude of +400 ft/min, which is in line with Therac's figures. $\endgroup$ – a CVn Jul 27 '18 at 12:25
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No, the are not even nearly airtight, and leak air quickly on their own. Air pressure is maintained with bleed air or a powered pump. FAA has a thorough, well-illustrated guide to the systems involved.

As for why they're not built completely airtight, aircraft are mostly riveted and glued, not welded, so their structure is inherently not solid. You'd need to use sealants throughout the structure, install and maintain seals on every opening, and compartmentalize a few systems that need outside air by design.

That's extra weight and maintenance, which doesn't have to be there, as cargo doesn't need pressure and humans need fresh airflow. The minimum rate is ~1.6 L/s/pax for survival and ~8 L/s/pax for comfort. More flow means more O2, giving more consciousness time if decompression happens (breathing low ppO2 greatly accelerates the onset of hypoxia), less CO2 (affecting cognitive ability and comfort), and less humidity, so that less odor is perceived and less water accumulates in the insulation.

With outflow valves closed, airliners normally lose about 400 feet (~5%) of their pressure difference per minute when new, this varies per airframe. The permitted limit is typically 750 FPM. High-flying aircraft like A380, A350, 787 are built with considerably lower leak rates, a safety margin that allows them to get certified above FL400 by providing more time to get to 10k ft in emergency.

As aircraft age, their leak rate can double and more, usually being close to the limit for an average airliner. A common offender is window frames rattling themselves a bit of clearance. When tested and found to exceed the limits, maintenance is required to bring the leak rate down. So even the current level of airtightness takes effort to maintain.

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  • $\begingroup$ Great analysis! "With outflow valves closed, airliners normally lose about 400 feet (~5%) of their pressure difference per minute when new." -> This number is what I've been looking for. That sounds like a lot to me. Is there any source (other than your answer) I can quote for this? $\endgroup$ – Michal Bryxí Jul 27 '18 at 7:48
  • $\begingroup$ @MichalBryxí It seems not. The 400 fpm number (±50%) just came through the grapevine. You could find the 750 fpm number in test procedures. Leakage is not generally given top attention, just a sign of age, the critical part is the packs. $\endgroup$ – Therac Jul 27 '18 at 15:35
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    $\begingroup$ In the old days, one could easily identify the outflow valve by the heavy brown streaking of smoking byproducts (tar and nicotine) running aft from the valve. $\endgroup$ – Walker Jul 27 '18 at 23:17

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