[squirtle24]

Very interesting video that shows a clear picture of how the plug is held in place. Some interesting spots in the video, 8:44 shows how the upper two locking bolts lock the roller pins into the upper door tracks, and 10:00 show the lower two locking bolts onto the sliding hinge posts. 13:10 shows how the door blew cleanly off, with very little damage to the roller pins and lower hinge posts. Some comments say the same plug have been used on the 737-900 which didn't have this issue.

My armchair speculative guess: there are only 4 bolts effectively holding the door onto the roller pins and lower hinges. Somehow I don't think it's a simple case of someone forgetting to tighten the bolts; since they're using castle nuts, forgetting to torque them down would leave behind extra cotter pins. Those bolts look positively tiny, probably no more than M12 diameter, and are subject to intense shearing forces. In the case of the upper roller pin with locking bolt, they are effectively two cylinders perpendicular and on top of each other, which causes extreme compressive forces to be concentrated on one tiny spot on the bolt. My guess is they cheaped out and switched to an inappropriate/softer bolt, which sheared and/or corroded. If one shears, the load quickly spreads to the remaining 3 bolts which all also shear off. This theory would still jive with OPs article about UA; "loose bolts" may not necessarily mean "untightened nuts and bad QA", but rather, signs that the locking bolts are all beginning to bend or shear.

[ooboe]

The 4 bolts are there to restrain vertical motion of the door which I presume would not involve large forces. They are meant to keep the door on the 4 cylindrical posts and 12 stop pads and it is those posts and pads that take the pressure differential forces on the door and keep it from blowing out. They are way beefier than the bolts and I believe they are still intact on the accident plane.

[albert_e]

do some of these bolts also carry the weight of the door?

what if the springs on the lower hinges (that are supposed to keep an upward pressure on the door, supporting its weight) are weaker than necessary or somehow failed and went unnoticed... would that put too much burden on the bolts and other fastening mechanisms and cause them to fail in turn over time?

[emchammer]

Shear forces on four bolts, isn't that the same thing that caused the disaster in that hotel with the suspended walkway in the 1980s? With more than a hundred deaths. Is this not a basic case in engineering school, like THERAC-25? Not a licensed PE.

[mkl]

https://en.wikipedia.org/wiki/Hyatt_Regency_walkway_collapse

Longitudinal forces on the bolts there, not shear. A change during construction doubled the force on the nuts and the bolts went through weak weld joints in beams. (Also not a PE.)

[yuliyp]

Most of the pressure of the door should be supported by the stop pads, rather than the bolts. The force on the bolts should generally be from vibrations, not from pressure differential pushing the door out.

[albert_e]

other than pressure differential...what about the weight of the plug door itself ... do the bolts support that weight?

or if some other mechanism like the springs on hinges below grew weaker over time .. would that in turn put too much weight on the bolts?