The concept of plug doors is that pressure holds them in place. This is achieved by a row of stops down both sides of the door frame, and a row of stops down both sides of the door. These stops push together when the door is closed. Old Boeing doors B727 B737 B747 B757 are all similar and operate as described above. B777 and A320 doors use the same stops, but to open they are first lifted about an inch, then pushed out past the stops, but when closed the principle is the same.
B737 type cargo doors are true plug doors. they are bigger than the opening, but still sit on stops when closed.
The L1011 small cargo doors, and B767 small cargo doors operate like B777 pax doors. They are lifted then opened past the stops.
All these doors are called plug doors.
However the B747 fwd cargo door, L1011 fwd large cargo door, B767 fwd large cargo door (where fitted) ,B777 fwd cargo door (and rear door on BA
aircraft) are NOT plug doors. The top hinge is a piano type hinge, and the bottom and sides of the door are held in place with hooks when closed. These doors are all similar design (and made by the same firm).
Some pax doors are not plug type. Usually when an airstair is built into the door like older F28 and F100 fwd pax doors. When doors are not plug type the locking and latching mechanism is quite complicated to try and make it foolproof, but it has been fooled. TK
DC10 at Paris and the NW
? B747 over the Pacific, both times the latching mech failed and the door opened under pressure.