Originally Posted by
DaveReidUK
Apart from which, the same considerations would presumably apply to the other 180 or so door plug-equipped Max 9s, some of which have been in service for nearly 6 years.
Systematic faults tend to show up in random occasions, that's what safety is about. That's how the MCAS fault showed up, planes were flying with it for some time, people here pointed at the pilots with such argumentation in the first place.
If one plug pops out and 171 planes are inspected just for the plugs with >6 findings, how can this be?
a) Randomly distributed quality escapes: If you inspect just the pedals of 170 cheap chinese bikes and you find 6 to be loose, you would recall and trash them all, assuming they are all dangerous, having also loose brakes, wheels etc. So how many more parts than 20 nuts per plug are on an airplane? If 6 nuts of 20*2*171 are loose, what would this tell you about the whole fleet?
b) Systematic QM process fault regarding those plugs. Everything gets inspected, but somebody forgot to put those plugs on the quality checklist. This at least limits the rigorous reviews to the quality process and actual plugs, best possible outcome for Boeing.
c) Process deviation, the workfloor found a different way of handling things. Well, this would be a flaw in safety awareness, also affecting the whole plane and fleet, not limited to the plugs.
d) Systematic design flaw overstressing the plug system. This would explain why there is such a large number of findings on just such a small fraction of the plane despite (at least some) QM in place. So why would you exclude this possibility, just because those planes were still flying till the accident?
How do you explain such a high number of findings otherwise?
I did some estimate myself and yes, the fuselage movement is probably approx. 1mm/2m (just comparing max. tensile strength and young's modulus). But the doors and hinges are quite rigid, still a possibility. If the door just slipped from the pads, there needs to be some angle or a huge force. 12*0.5to on the pads with a friction coefficient of maybe 0,2 gives 1.2 to or 12,000 N of force needed to shift the plug compared to maybe 600N hinge spring force..
Even within the 'missing bolts' theory there are lots missing pieces, it's just too easy ot assume:
If all bolts are missing, why did the plug not instantly open on the ground due to spring force? Hinge spring force overcomes gravity on the photos shown.
Why would the bolts jump out if hinge spring force is holding them by friction?
Why should the door move just a bit instead of the full 3.8 cm as soon as the springs overcome friction of gaskets, paint etc.?
If the plug is pressurized being not aligned and slipped, why isn't there much more damage to the pads?
As long as the there is no clear answer to those questions and findings on other planes, I would not limit the search to the most trivial explanation. Things don't fit well enough to completely focus on the missing bolts in my opinion.