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United finds loose bolts on plug doors during 737 Max 9 inspections

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Re: United finds loose bolts on plug doors during 737 Max 9 inspections

#541

Post by AnthonyMouse »

selectodude wrote: post #476
AnthonyMouse wrote: post #434

Earlier quoted context omitted.

Type ratings work that way because there are so few types of planes, and because the incumbents like it that way. "Oh no, I guess it's impossible for anyone else to produce a new aircraft model because nobody's qualified to fly it, now we'll just have to rest on our laurels forever." If there were more competitors, they'd lobby to change how the regulations work so they didn't effectively destroy the market for new a…

The FAA doesn't work like that and I can assure you, the airlines find it to be an enormous pain in the ass.

The FAA is susceptible to Boeing, the huge conglomerate and defense contractor, not any random airline that could go out of business without anybody really caring. But the status quo is good for Boeing because they don't have to pay to design new stuff, or at least they're ambivalent to it. Change that and see what happens.

Re: United finds loose bolts on plug doors during 737 Max 9 inspections

#542

Post by johnwalkr »

adrian_b wrote: post #50

There were only 2 possible causes of the incident, either the bolts had been replaced with bolts made from an inappropriate material, so they broke, or the bolts had not been assembled correctly, and they became loose. It appears that the latter is what happened. Perhaps those who did the assembly of the doors at Boeing did not use the right kind of washers that are needed to prevent the unscrewing of the bolts, or t…

There's at least 1 million root causes of the incident and while I have my own predictions, this industry is a great example of checking all possibilities before jumping to "2 possible causes".

It's really unlikely that washers are a root cause (spring/split washers are useless and are definitely not used for this type of apllication). "Correct torque" is not trivial to determine and maybe it was calculated right or wrong based on right or wrong materials/conditions or maybe conditions were changed or maybe...

I am 99.9% sure a procedure was followed and a torque or other measurement applied and was documented. This is probably not a trivial error but a cascading failure in design and/or documention and/or integration.

Re: United finds loose bolts on plug doors during 737 Max 9 inspections

#543

Post by ooboe »

AdrianB1 wrote: post #388
nolok wrote: post #356

Earlier quoted context omitted.

Imagine the quality of the manufacturing and QA / final inspection to have that kind of issue.

Nope. It is most probably caused by operational stress - rudder assembly is moving, fuselage is also working (compression and decompression cycles on take off and landing, thermal expansion and compression). I bet they don't just put red Loctite on it to keep it from getting loose. My bet is design flaws, not manufacturing or QA. EDIT: I saw the pictures of bolts with pins and bolts without pins. The ones with pins c…

The 4 restraining nuts and bolts on the door have a cotter pin like mechanism to prevent them from loosening. If assembled correctly they cannot loosen unless the pin fails.

Re: United finds loose bolts on plug doors during 737 Max 9 inspections

#544

Post by mkl »

I_Am_Nous wrote: post #516
mkl wrote: post #419

Earlier quoted context omitted.

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.)

>A surgeon spent 20 minutes amputating one victim's pinned and unsalvageable leg with a chainsaw; that victim later died. Wow

Yeah, the page is a pretty harrowing read.

Re: United finds loose bolts on plug doors during 737 Max 9 inspections

#545

Post by dmix »

johnwalkr wrote: post #535
trebligdivad wrote: post #154

https://www.youtube.com/watch?v=maLBGFYl9_o is a good video explaining how the bolts/doors etc are supposed to go together.

Oh wow. I made another comment about how important tension in fasteners is, to provide a clamping load and therefore friction between two faces. I was thinking that there must have been a process failure. Fasteners often shear off if there is not enough tension and therefore friction. But in this case it looks like there is not any clamping load between parts for at least some of the fasteners, so those fasteners bas…

The recurring question is that it was only 2 months old so likely had those bolts underneath the roller thing was not fastened properly or the whole concept is flawed and this one simply had some bad turbulence/rough landings which created the perfect pressure on them.

Re: United finds loose bolts on plug doors during 737 Max 9 inspections

#546

Post by stickfigure »

irrational wrote: post #518
amluto wrote: post #478

Earlier quoted context omitted.

> We have software QA departments that operate very much like manufacturing or aerospace QA. I don’t work in this industry, but this seems fairly ridiculous on its face: software is not at all like manufacturing. In manufacturing, there’s a design and a manufacturing process, and a critical function of QA is ensuring that the manufactured produce is manufactured to spec. With software, the software is written, compil…

Ideally there should be a solid spec written and then qa can test against the spec. Maybe there is somewhere that does write solid specs, including accounting for corner cases, but in my 25 years working professionally in the industry, I’ve never seen it.

The only complete and precise specification of software is the code itself. If some other form of specification was complete, we would be able to auto-generate the code.

Re: United finds loose bolts on plug doors during 737 Max 9 inspections

#547

Post by ooboe »

squirtle24 wrote: post #343
trebligdivad wrote: post #154

https://www.youtube.com/watch?v=maLBGFYl9_o is a good video explaining how the bolts/doors etc are supposed to go together.

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 sam…

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.

Re: United finds loose bolts on plug doors during 737 Max 9 inspections

#548

Post by SoftTalker »

unyttigfjelltol wrote: post #453
actionfromafar wrote: post #412

Earlier quoted context omitted.

The problem was not really the software in isolation, but that pilots expected the 737 NG to behave exactly like the old version - because Boeing decided it was too expensive to retrain pilots.

The problem was software that prioritized input from a fauly external sensor, over pilot control, and literally crashed planes directly into the ground. At a certain step in the sequence it was not physically possible for a pilot to pull hard enough on the control element to counteract the software. Could they have disabled the system? Only if they could figure out the specific software trying to crash the plane. Is…

Pilots should (are supposed to) disable the auto-trim if it's doing something uncommanded/unexpected. Runaway trim can happen for reasons other than faulty software. MCAS was a new factor and they should have been told about it, I don't dispute that at all.

Re: United finds loose bolts on plug doors during 737 Max 9 inspections

#549

Post by numpad0 »

diabeetusman wrote: post #485
ethbr1 wrote: post #469

Earlier quoted context omitted.

Pilots are definitely trained how to disable the autopilot, if needed. Afaic, the fault apportionment was Boeing documentation > airlines >> pilots > Boeing technical design.

Not sure why you’re bringing up autopilot— the MCAS system runs even when the autopilot is disabled. Edit: Also, how does the fault lie with the airlines? Boeing didn’t document the existence of MCAS in the flight manual or training materials.

Wasn't MCAS designed to activate when A/P is disconnected, also?
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