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The pressurization in the cabin requires the doors have their seals working properly. How do they prevent pressurized air from leaking from the doors?

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7  
Same way any other type of seal works, the sealing material creates enough of a barrier between one side and the other to prevent pressure to leak through the seal. – Ron Beyer Feb 23 at 14:28
    
Think cork plug in the inside of the neck of a bottle instead of on the top of it. – SMS von der Tann Feb 23 at 14:35

How do they prevent pressurized air to leak from the doors?

They don't. The seal does not have to be perfectly airtight.

The requirement is that it will create enough impediment to the air that wants to gush out, where "enough" is defined by the capability of the conditioning system to input new air into the cabin.

In other words, everything is fine as long as the air mass that gets out of the doors is less than the maximum air flow that the air conditioning system can input into the cabin.

Note also that, as stated on Wikipedia, aircraft are equipped with one (or more) "out-flow valve(s)" and "pressure relief valves": these valves intentionally let air out of the cabin to avoid over pressure in the cabin and thus they prevent possibly dangerous scenarios.

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darn, I'll never learn. Thanks @Simon. – Federico Feb 23 at 15:16
3  
Note that while small leaks don't cause pressure problems, they can still cause very unpleasant whistling sounds. – Jan Hudec Feb 23 at 16:44
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@Federico Think nothing of it, that's why I left no comment. Your English is many times better than most users on here are with Italian or Spanish, me included (sorry, don't know which one). – Simon Feb 23 at 16:52

Like any pressure seal they are generally rubber seals. You can find one of Boeings patents here which covers door seals. Some smaller planes (and unpressurized GA planes) use inflatable seals, the door is closed and then the rubber boot is inflated to form a tighter seal. You can find an interesting discussion on the seals here but in general they may have tiny leak issues. This was the cause of at least one A380 diversion recently, but it looks like airbus is working through it and has a solution. Since the cabin has an outflow valve to keep it properly pressurized a tiny leak in the door will be compensated by the outflow valve. This airbus briefing warns that they can be hazardous and hard to detect if one does occur the crew may not know until the O2 masks deploy.

Slow/Insidious decompression involves a very gradual decrease in cabin pressure. Slow decompression may be the result of a faulty door seal, a malfunction in the pressurization system, or a cracked window.

Slow decompression may not always be obvious. The cabin crew may not notice the changes in the cabin, until the oxygen masks drop down from the Passenger Service Units (PSUs).

It has been noted that leaky seals on both pressurized and unpressurized doors can lead to a very loud noise in the cabin.

The seals do at least to some level try to keep water out. This is mainly because excess water may build up and then freeze as the airplane climbs. This ice build up in the door could in theory cause an issue (although I cant find any cases where it has).

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Looks like that A380 diversion may have been more than just a leaky seal. (I trust AVHearld more than crikey.com.au...) – FreeMan Feb 23 at 16:55
    
Very Correct, If you dont mind ill use your link instead of the one I had. I know it at least prompted a door seal repair. – Dave Feb 23 at 18:36
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@FreeMan Thanks for the link, interesting stuff! "After the aircraft had levelled off at cruise altitude FL370 the supervisor informed the flight crew, who observed the cabin pressure at 6000 feet." <-- Is that an over-zealous application of the sterile cockpit rule? I would think that "the door may be loose" is something a pilot would want to at least be aware of during takeoff. – yshavit Feb 23 at 19:26
    
@yshavit, it's likely the flight took off below the noted cabin altitude of 6000', therefore, there would be nothing for the pilot to notice at takeoff. i.e. cabin altitude ≈ runway altitude. I'm not sure what the cabin altitude of an A380 would normally be, but I'm guessing it's lower than 6000', thus triggering concern with the cabin crew. – FreeMan Feb 24 at 16:57
    
@FreeMan I was actually more looking at the "after the aircraft had leveled off at cruise altitude" bit. Why wait that long? My only guess would be because of the sterile cockpit rule. – yshavit Feb 24 at 18:02

A lot of aircraft with pressurised cabins have plug type doors which press against the fuselage when in closed position. There is a (silicone) rubber seal where it touches against the fuselage, which minimizes air leak. The image below shows the seal in an aircraft door.

aircraft door seal

Image from aviationtroubleshooting.blogspot.in

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2  
The door in the picture clearly are not plug type, since the seal is on the inside. – Jan Hudec Feb 23 at 16:42

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