Originally Posted by Kbboy

That can be a dangerous assumption. Flight 811 was a prime example of a 747 in which there was an inherent design flaw in the locking mechanism. A system that "couldn't possibly have opened" actually did open. It worked ok for years and even after the accident, the NSTB concluded it was not a design flaw but improper locking or other human error. But the dad of one of the victims who died in the accident was a retired engineer and set to investigate himself. He went through a lot of trouble but ultimately proved that it was a design issue in the way the latches worked and even a small current can move the latch to a half-open position.

Ideally, a plug door should be fitted from the inside such that it cannot be opened as the air pressure would push it against the fuselage. The fact that this plug door actually has hinges and does open outwards necessitates special analysis. If we assume, as I have seen some reports (not sure if true or not) that the system to unlock is present and just the connection to the opening mechanism is missing. This means that some error (electrical or mechanical or human) can cause it to open itself and that is dangerous. A plug door is not something you would decide to use/unuse regularly so ideally, there should be no mechanism to open the door. It should be removed and replaced by a working door if needed. Not saying it is a design flaw but it is not the safest design either in my opinion.

I read Spirit Aerosystems sends the fuselage with the plug/door semi-installed. But it is removed in Seattle during final assembly and refitted later. So, if a bolt is missing, loose, bent, improperly installed, etc. it was probably done at Boeing's plant during final assembly, but if a bolt/hinge/clip broke or a system malfunctioned, then it could be from where it was manufactured. At this point, however, the focus should be on how it happened and not fixing responsibility yet.