Cabin door
All main cabin doors of the 747-400 aircraft type were designed as outward opening‘plug doors’. A plug door is designed to be physically larger than the doorway opening, and mates with the frame around the full circumference when in position. It is designed to increase the security of the pressurised fuselage, with pressurisation loads serving to force the door more tightly against the frame.
Retractable gates at the top and bottom of the door serve to allow it to move inward and then sideways through the door frame during the opening and closing process when the aircraft is not pressurised. The plug door design provides for a level of protection against inadvertent or intentional attempts to open the door while the aircraft is in flight. A latch mechanism holds the door in the closed position whenthe aircraft is not pressurised.