Today I learned how airliner doors are typically an outward facing plug and cabin pressure would make them nearly impossible to open mid-flight.

It's easy enough to see that for the shape of Boeing 737 doors, for example.

But looking at this video of the A320 door operation, it doesn't exactly look like it forms a plug. So if it were unlatched while the cabin is pressured, would it get pushed open?


1 Answer 1


It's a plug door. The door nestles behind an array of cleats and a set of L shaped guide tracks and has to move up some distance, as you can see in the video, to clear the cleats and exit the L track. When it's dropped into the locked position, it's not dependent on any moving parts to resist pressure because it settles behind the fixed cleats in the outer frame. The cleats in the door bear on the ones in the frame to resist outward pressure and the door is opened by lifting it to mis-align the cleats so the door cleats clear the frame cleats. That makes it a plug door.

It looks like there are other levers and cams to assist with the door pull-in, and probably some form of locking pin(s) to block vertical movement when latched. If the door was, say, 36 inches wide by 80 inches high, at an 8 psi cabin pressure differential, there is about 23000 lbs pushing on it, so you won't be moving it unless you are The Hulk.

The CRJ Regional Jet main pax doors are an example of non-plug doors that just pull in flush on a simple hinge at the bottom, and latching shear pins extend into holes in the door frame. This door is dependent on the shear strength of the movable pins, and the shear loads when pressurized pretty much weld the pins in place.

  • $\begingroup$ Are you sure those are plug-doors? I thought A320 family doors were type-C which are not plug doors. $\endgroup$
    – Manu H
    Commented Jul 10, 2020 at 16:18
  • $\begingroup$ It operates the same as the door in the first video, with the same L tracks and perimeter fittings, but a bit smaller with different contours. It has to move up to clear a set of fixed cleats in the frame. When closed, pressure is pushing the fittings on the door against the fittings of the frame, making it a plug door. The fact that that skin surface of the door is not larger than the fuse opening is irrelevant. The structural load paths are door statically pushing against fuselage.On the CRJs, the right side service door works the same as these doors, but the main pax door is a non-plug... $\endgroup$
    – John K
    Commented Jul 10, 2020 at 17:24
  • $\begingroup$ ...that depends mostly on 6 shear pins operated by a complex assembly of levers and bellcranks, and is a nightmare to rig. That door just moves straight out with no vertical or lateral movments because the shear pins, a pull-in cam at the top, and the piano hinge at the bottom hold the door against cabin pressure, and the shear pins completely retract when opening to allow the door to swing straight out. $\endgroup$
    – John K
    Commented Jul 10, 2020 at 17:30
  • $\begingroup$ Does that mean the answers in the link question was wrong 5 years ago? $\endgroup$
    – Manu H
    Commented Jul 10, 2020 at 19:24
  • $\begingroup$ I would say so. $\endgroup$
    – John K
    Commented Jul 10, 2020 at 20:11

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