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April 16, 2014

What Happens When You Try to Open a Plane's Door at 30,000 Feet?

Via Instapundit, this Popular Mechanics article notes a recent attempt at opening a pressurized cabin's door.

Nothing happened. The door can't be opened due to the pressure differential. The inside of the plane is pressurized, and the outside is low-pressure; basically you have a thousand pounds of pressure holding the door shut.

Popular Mechanics, however, gets vague about how this works. Airplane doors open outwards. The pressure inside a plane also pushes outwards.

So why does the pressure keep the plane's door shut, rather than giving a would-be door opener a terrific advantage in opening it? Why does pressure fight against a door-opener, rather than fighting for him?

The reason concerns the way that the door is engineered. Some just have locking mechanisms. I suppose those could be opened in flight.

But many are sealed shut by the cabin pressure itself.

These kind of doors are called plug doors, because, like a plug in a sink, they're meant to fill a hole and stay there, sucked into the hole by negative pressure.

Here now some things I learned from Wikipedia and generally scouring the Internet to answer this question, which has long bothered me. (My information is spotty and wholly based on reading so there may be some mistakes here, and I'd appreciate any corrections.)

The inside edge of a plug door is fatter than the outside edge, like a plug that tapers towards its front. The low pressure outside the door hole and the high pressure inside it pushes the plug door into its frame, preventing it from being pulled inwards.


An emergency exit door might just physically come apart from the plane -- that is, it detaches completely. To open it, one has to pull into the body of the plane, then put aside or laid on the floor.

In the video below, note that fatter inside edge of the door, and the thinner outside edge.

In flight, at high altitude, when pressurized, the pressure would be pushing that door outwards into the hole.

Main doors have a tricky hinge, that requires opening it in two stages:

First, the door must be moved inward. Because the door has to be opened inward, it's at this stage that the high pressure of the cabin is helping keeping it sealed shut -- the pressure is pushing the door outward, and a person trying to open it at altitude would have to fight the air pressure to pull it back.

Once it's been moved inward, the hinge now permits it to move outward into the fully open position. Apparently the door can tilt or rotate on the hinge such that it can now move through the door-space, even though it's actually too big for the space. (Much as you can get a couch through a doorway by angling it.)


See? It moves inwards first. If you were trying to open that at 30,000 feet, you'd find it impossible to pull it towards you.

Another kind of door, on the Airbus, achieves this "Bigger on the inside edge" sealing factor by building a door with a top and bottom piece that slide out of the way when a locking mechanism is turned.

You can see that at around twenty seconds in here:

I don't know about that one, though. The true plug type doors just can't be pulled inward (at least not by human strength) at the great pressure differential at high altitude. But the Airbus door appears to open outwards only -- and is only blocked by doing so by the top and bottom pieces of the door, which can be moved aside by mechanical manipulation.

Thus that door seems to be relying only on mechanical safeguards, rather than air pressure, to keep the door shut.

Correction: Airbus actually says its doors open inwards, too. I guess I'm just not seeing the inward motion in that video.

When people say it's impossible to open a door at altitude, I think they're talking about the plug doors.

But not all airplanes have those. (This is unaffected by the correction.)

One more thing about all this: While people will say "You can't open a door in flight," that's not true. Even the plug-type doors can be opened in flight -- just not in flight at high altitudes while the cabin is pressurized.

When there's not such a great difference between the external and internal pressure -- and cabins are only pressurized to about the pressure you'd feel at 8,000 feet -- there won't be anything except the locking mechanisms keeping doors closed.

Of course, it's also not particularly dangerous to open an aircraft door at lower altitude -- no explosive depressurization, no Goldfinger exit -- but it's also not advisable. A sudden change in the airplane's aerodynamic profile can wreak havoc on the pilot's control, and of course no one wants to feel 500 mph hour winds gusting right outside the window.

So there's kind of three myths that need to be busted here: first, the myth that you can just open a door at high altitude and explosively depressurize the cabin.

You can't do that... with plug doors.

But it's actually not true that all airplane doors are plug doors. Some just rely on mechanical safeguards.

And further, the the contrary myth that you can never open a door in flight at all is also wrong.

Up to 8,000 feet, you'd have absolutely no considerations of pressure at all keeping doors shut -- only the mechanical locks would be holding the doors shut -- and I'm going to guess that up to around 15,000 feet, while there would be a pressure differential, it wouldn't be so vast that an adult man couldn't yank it open.*

So... don't be entirely calm when that weird looking guy goes for the door while you're descending for a landing.

And via the Washington Free Beacon, an aircrew does her best stand-up while giving the pre-flight safety instructions.

Listen closely; she packs a lot of jokes in there. Including her advisory that not only are you not permitted to disable the bathroom smoke detector, but that you're also not permitted to disable the bathroom webcam.

* Actually, I bet the handles and such are designed to break off if too much force is exerted on them. That is, if you have to exert such a significant amount of force on the door to pull it open (which would happen in a low pressure outside/high pressure inside situation), the handles are designed to just break off in your hand and leave the door in place.

They could manage that just by using weaker attaching hardware -- plastic screws designed to fail under too much force -- than they otherwise would.

But that's all conjecture.

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posted by Ace at 06:16 PM

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