Increasingly, modern types do not use plug doors to save complexity and weight.

As far as I know, the doors automatic/manual interlock only controls whether the slide inflates when the door is opened. The doors have electrically powered locks and EICAS alerts to show unlocked doors.

However, anything electrical can and does fail. What fail safe protections are in place to prevent non-plug doors from being opened in flight?

  • $\begingroup$ From what I can tell, the cabin door, e.g. in Airbus, don't have any electricity in them. They have big lever that pushes several massive latches to the frame on the sides. And the interlock can only be closed when the latches are in their end position. $\endgroup$ – Jan Hudec May 24 '16 at 9:16
  • $\begingroup$ @JanHudec So what stops someone pulling the big lever? $\endgroup$ – Simon May 24 '16 at 9:48
  • $\begingroup$ I believe when there is a pressure difference, the friction on the latches is more than human can overcome. I don't have any hard reference for this though. $\endgroup$ – Jan Hudec May 24 '16 at 9:51
  • $\begingroup$ Note that the cargo door do have electric mechanism and on 747 there was an accident where the locking mechanism got energized in flight (via short) and did have enough power to break the interlock, overcome the friction and opened the door (the plane was severely damaged, but managed to land). But humans don't have as much strength. $\endgroup$ – Jan Hudec May 24 '16 at 9:53
  • $\begingroup$ Which types are using non-plug doors for other than the fore and aft cargo doors? (Ignore main deck cargo doors on freighters for now) $\endgroup$ – UnrecognizedFallingObject May 24 '16 at 11:43

I am not sure about all aircraft types, but on the 777 cabin pressure is still used to positively lock the door into position in flight. When the door is open, the door is in a slightly raised position. As the door is closed, rollers on the door are lowered into tracks on the door frame. The tracks are curved slightly outwards, so to open the door the it most be moved slightly inward, which is impossible even with a slight amount of cabin pressure. The actual pressure load on the door is carried through the stops, which are inboard on the door and outboard on the frame. Once loaded with the pressure, these stops would also have a tremendous amount of friction preventing the door from sliding upward. Note the vent door near the top of the door in the picture below, this door is attached to the latching mechanism and it opens in the first movement of the handle, again preventing the door from unlatching if the cabin is pressurized.

Refer to this picture of the 777 door, to see the rollers, tracks and stops (Source). 777 Door Frame

I believe the 777 design is very similar to the Airbus design. There is a very good video showing the operation of Airbus doors on YouTube, here. It shows how the door must be lifted above the stops, before it can open outward. If you watch closely you can see the move slight inboard before lifting. This is a quote from the A340 maintenance manual: The lifting arms of the lifting shaft (which support the passenger door in roller fittings attached to the door frame) move the passenger door a small distance into the cabin and then upwards."

Note, some people (including Airbus and Boeing) still consider these doors plug designs since pressure still loads the door against the stops. For example, Boeing specifically states the 777 doors are plug type doors.

  • $\begingroup$ The main reason for my curiosity is that Airbus doors for example do not move inwards at all when opening and are not larger, in any dimension, than the door aperture so surely cannot be classified as plug doors? I think the answer to my question is that since the pressure applies an outward force to the door, the locking mechanism has a great deal of friction which it would be impossible to overcome regardless of interlocks. $\endgroup$ – Simon May 27 '16 at 10:54
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    $\begingroup$ @Simon they are very similar to the 777 door in that it slides down into its stops so pressure loads it against the stops. I've got some data at home I'll post later, but Airbus itself calls them plug doors. Also if you watch the video very closely you can see a slight inboard movement, just like a 777. $\endgroup$ – OSUZorba May 27 '16 at 20:25
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    $\begingroup$ @Simon Sorry for the long delay, this is from the A340 manual: "The lifting arms of the lifting shaft (which support the passenger door in roller fittings attached to the door frame) move the passenger door a small distance into the cabin and then upwards." $\endgroup$ – OSUZorba Jun 9 '16 at 3:21
  • $\begingroup$ Thank you! I'm now wondering if these are still correctly described as plug doors? $\endgroup$ – Simon Jun 9 '16 at 18:00
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    $\begingroup$ @Simon I think that is probably a little bit of a philosophical question. My take is that Boeing and Airbus call them plug doors, and they are definitely door experts. I think the term fits since the stops on the door are wider than the stops on the frame, so the door is technically bigger than the frame, just not everywhere. $\endgroup$ – OSUZorba Jun 10 '16 at 4:23

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