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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

#481

Post by FredFS456 »

garyfirestorm wrote: post #479
nolok wrote: post #356

Earlier quoted context omitted.

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

Bolts are most likely tightened with a torque wrench or a gun that is set to a torque spec. Over tightening a bolt is as bad as a loose bolt. I speculate these passed QA from Boeing because they might have been correctly torqued to the spec. What happens in field is hard to understand. One possibility is vicinity to the engine can cause extreme vibrations, these can make them loose. Other possibility is the maintenan…

I would expect lock wire or some other method of ensuring the bolt does not un-torque itself. Especially for bolts that are not required to be removed past final assembly...

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

#482

Post by kevin_thibedeau »

garyfirestorm wrote: post #479
nolok wrote: post #356

Earlier quoted context omitted.

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

Bolts are most likely tightened with a torque wrench or a gun that is set to a torque spec. Over tightening a bolt is as bad as a loose bolt. I speculate these passed QA from Boeing because they might have been correctly torqued to the spec. What happens in field is hard to understand. One possibility is vicinity to the engine can cause extreme vibrations, these can make them loose. Other possibility is the maintenan…

When it's happening on a two month old plane it's a production problem.

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

#483

Post by userbinator »

fshbbdssbbgdd wrote: post #145

Check out this video showing how the plug is supposed to be installed: https://youtu.be/maLBGFYl9_o?t=540 Some of the bolts that would be loosened when the plug is opened during maintenance have a pin to prevent them from turning. That pin is present in this photo: https://x.com/byerussell/status/1744460136855294106?s=46&t=s... However, the same photo shows other critical bolts that hold the whole hinge on the door a…

I'm not an aerospace engineer, but it seems those hinge bolts might be better replaced with rivets --- if the hinge isn't frequently removed from the door.

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

#484

Post by arrakeenrevived »

free2023 wrote: post #477
arrakeenrevived wrote: post #463

Earlier quoted context omitted.

Aircraft are often sold between airlines, or reconfigured even with the same airline. If United ever wants to reconfigure their 737s to be higher density (which is pretty common), they might need to start using that door as a real emergency exit.

Yes I see, that makes good sense. Given the course of events, it's still a fair question from a design standpoint: Can this be engineered with a reversible option to switch it from functional to non-functional, depending on the seat configuration? It just seems like a lot of complexity and moving parts, for a feature that's not in use.

> Can this be engineered with a reversible option to switch it from functional to non-functional, depending on the seat configuration?

That's exactly how it's engineered right now, though.

This same design has been used for decades and apparently without problem, so it's probably just a case of someone designed it this way originally, and nobody thought to fix what wasn't (at the time) broken.

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

#485

Post by diabeetusman »

ethbr1 wrote: post #469
unyttigfjelltol wrote: post #453

Earlier quoted context omitted.

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 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.

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

#486

Post by johnwalkr »

heads wrote: post #307

Bolts and machine screws are very interesting. One mistake I, an amateur bike mechanic, made for years was not greasing bolts properly before assembly. It sounds counterintuitive but without grease a bolt (or machine screw) will bind early with a high torque well before it is correctly tensioned lengthways. The torque is just a proxy for the tension and it is this tension which is needed to fasten your components tog…

It's a complex topic. Depending on the design, lubrication may be required or forbidden. And if it's required, it should normally be on the threads only, under the head is usually forbidden.

And it's very important to understand that in most mechanical design with fasteners, the fasteners provide tension, and friction between 2 faces actually carries the load. Too little tension then fasteners can be sheared off. Too much, then the fasteners may not have remaining strength available for loads that act to add tension to the fasteners.

Using a torque wrench to reach target tension is normally only about +/-30% accurate. Usually design margin allows for this but in very critical applications where margin is not feasible, calibration is done on a sample of the same materials, etc, or a more direct measurement done. More direct measurement can mean hollow fasteners that allow you to measure the amount of stretch, use of ultrasonic measurement to measure stretch, washers with integrated strain gauges, or cleverly designed "tension indicating" washers or fasteners. There are many types so I just those keywords for anyone interested.

I am not saying that this design required such complex methods and sizing these fasteners should not be difficult. There is probably a mistake or lack of control in the assembly process.

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

#487

Post by acdha »

mc32 wrote: post #461
Groxx wrote: post #428

Earlier quoted context omitted.

In lots of ways, the "learning" there would just be "capitalism". It's inherently short-sighted unless forced to do otherwise by legislation. Cutting small corners pays off A LOT until the hammer falls, so there's a massive advantage to doing it / you need to do it if competition is doing it, or you eventually shut down as they take all your business. It's inherently a race to the bottom. Sometimes that's a net gain,…

All major economic systems of all major national economies over the last century have perverse incentives. It’s not a capitalist thing. Other systems had incentives such as, get it running by such and such date or have yourself and relatives sent to inhospitable place. So people rushed flawed designs into production. That said, upper management at Boeing needs a shake-up. People need to get fired. They need to do wha…

> All major economic systems of all major national economies over the last century have perverse incentives. It’s not a capitalist thing.

It should be a lesson against dogmatic pursuit of absolutes: capitalism comes in a wide range of flavors, and the worst is if it’s completely unrestrained. Communism produced worse and worse results the further it got from any sort of public accountability, etc.

The two problems that I see is that the concept of nuance is somewhat at odds with having a simple concept to teach kids at school, and there’s always a group which is more motivated to game the system than the average person who really just wants to hang out with their friends, raise a family, etc. rather than play political games. Boeing didn’t start it by any means but they’ve benefited enormously from decades of reduced oversight and elevated pay driven by a sort of cartoon libertarianism where letting people get enormously rich will motivate them to build great things unfettered by “red tape”.

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

#488

Post by bb88 »

vegetablepotpie wrote: post #405
ethbr1 wrote: post #371

Earlier quoted context omitted.

I expect even at its worst, software development could learn a lot from aircraft QA. Especially since most shops have pretty much tossed professional career QA out the window.

I work in the defense industry, it’s very much like the aerospace industry in that we deal with human life as a consequence of our work. We have software QA departments that operate very much like manufacturing or aerospace QA. Software QA provides nothing of value to software development; having it as a dedicated function works against the overtly stated goals of the function and counterintuitively acts to degrade q…

> Good software teams build quality into projects by introducing continuous integration, unit testing, creating feedback, and tightening these feedback loops.

No. Good software teams are led by competent, technical management. Managers who aren't afraid to get down into the dirty details. Managers who aren't afraid to roll up their sleeves and write code if they need to.

The process doesn't matter. The management of what is or is not important does. Agile is just one process out of many.

Imagine an accounting team led by someone who never did accounting in their life: "Just make the numbers work out! I don't care how you do it! My bonus is at stake!"

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

#489

Post by jacquesm »

johnwalkr wrote: post #486
heads wrote: post #307

Bolts and machine screws are very interesting. One mistake I, an amateur bike mechanic, made for years was not greasing bolts properly before assembly. It sounds counterintuitive but without grease a bolt (or machine screw) will bind early with a high torque well before it is correctly tensioned lengthways. The torque is just a proxy for the tension and it is this tension which is needed to fasten your components tog…

It's a complex topic. Depending on the design, lubrication may be required or forbidden. And if it's required, it should normally be on the threads only, under the head is usually forbidden. And it's very important to understand that in most mechanical design with fasteners, the fasteners provide tension, and friction between 2 faces actually carries the load. Too little tension then fasteners can be sheared off. Too…

Lots of good stuff in this comment but I'm confused about the +/- 30% accuracy quote, in my experience it is relatively easy to reach sub 10% accuracy based on comparing different (good quality) torque wrenches against each other and against a calibrated one.

Is there something I am missing here?

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

#490

Post by cf1241290841 »

ethbr1 wrote: post #371
nolok wrote: post #356

Earlier quoted context omitted.

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

I expect even at its worst, software development could learn a lot from aircraft QA. Especially since most shops have pretty much tossed professional career QA out the window.

Cockpit resource management is also something a lot of industries can learn from. As well as human error analysis. How an error came to be is often much more interesting then the personal shortcomings of the person who caused it.
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