United Airlines Flight 811 - Cause

Cause

The accident was most likely caused by improper wiring and deficiencies in the door's design. Unlike a plug door which opens inwards and essentially jams against its frame as the pressure outside drops, the Boeing 747 was designed with an outward-hinging door which, while increasing capacity, required a locking mechanism to keep the door closed. Deficiencies in the design of wide-body aircraft cargo doors were already known since the early 1970s from flaws in the DC-10 cargo door. Despite the warnings and deaths from the DC-10 incidents, and early Boeing attempts to solve the problems in the 1970s, the problems were not seriously addressed by the aircraft industry and the National Transportation Safety Board until much later.

The 747's cargo door utilized a series of electrically-operated latch cams that the door-edge latch-pins close into, the cam then rotates into the closed position holding the door closed. A series of L-shaped arms, called locking sectors, actuated by the final manual moving of a lever to close the door, are designed to reinforce the now unpowered latch cams and prevent them from rotating into the unlocked position.

The locking sectors were made out of aluminum and of too thin a gauge to be able to keep the latch cams from moving into the unlocked position against the power of the door motors. If an electrical switch designed to cut electrical power to the cargo door when the outer handle was closed was faulty the motors could still draw power and rotate the latch cam to the open position. The same could happen if frayed wires were able to power the cam-motor even if the circuit power was cut by the safety switch.

It appeared in this case that a short circuit in the aging plane caused an uncommanded rotation of the latch cams, which forced the weak locking sectors to distort and allow the rotation, thus the pressure differential and aerodynamic forces to blow the door off the fuselage, ripping away the hinge fixing structure, the cabin floor and side fuselage skin, causing the massive decompression.