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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

#491

Post by acdha »

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.

This wasn’t related to autopilot and they removed mention of the MCAS system from the documentation to support the main selling point of the 737 MAX, which was that existing 737 pilots would be able to switch easily without recertification. They knew that they’d lose most sales to Airbus if the aircraft were compared on their merits so they were banking hard on their huge pool of certified pilots as the competitive edge.

If you listen to podcasts, these two episodes of Causality are excellent:

https://engineered.network/causality/episode-33-737-max/

https://engineered.network/causality/episode-50-737-max-ethi...

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

#492

Post by ssl-3 »

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…

Torque specs only apply to clean and clear threads that are free of corrosion and grease -- unless otherwise-specified.

Threads that are this way do not get stuck half-way.

If the threads are already boogered up for whatever reason, then the torque specification is already wrong: The threads aren't clean, clear, and free of corrosion and grease. This can result in under-tensioned fasteners when using a torque wrench as a guide, since boogered threads (rather obviously) can present an impingement that allows a torque wrench to click off before proper tension is reached.

However, grease is not a magical antidote to this condition.

If the threads (boogered or not) are greased, then the torque spec is also wrong: Greased threads are also not clean and clear, and free of corrosion and grease. This can result in over-tensioned fasteners when using a torque wrench as a guide, since grease is (rather obviously) a lubricant -- allowing things to slide more-freely in a way that doesn't allow a torque wrench to click off until somewhere beyond ideal tension.

---

So what do, then, as a home-gamer with a bicycle or maybe a car in the rust belt (but never an aircraft) full of dissimilar metals that are constantly rotting?

You could perhaps kit up to do the Junker test such as in DIN 65151, and make a study of how different greases affects things. You can even make a career out of publishing your studies.

Or: Just make sure the threads are clean, clear, and dry, and then assemble with an anti-seize paste which does not have lubricating qualities that affect final tension yield. (Permatex makes some, as do others.)

---

(And in aircraft, always do what the engineers say. If the engineers are wrong, then: Stop doing whatever it is that you're doing, and consult them.)

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

#493

Post by paranoidrobot »

deelowe wrote: post #465
paranoidrobot wrote: post #411

Earlier quoted context omitted.

It's built to be an actual emergency exit. It needs to be usable depending on how many passengers the interior is configured for. So it has all of the door bits there. Maybe some parts like the emergency escape slide are not installed. e: I should be clear that it's not usable as an emergency exit, as configured by Alaska. However the operator could choose to activate it later and install a usable exit.

Where did you see that? My understanding is that it's an optional plug door that's used to assist with interior installation. Once the interior is done, it's bolted shut and interior paneling is installed over top. From the inside, you can't tell it's there.

My source is [1], specifically at about 5:05.

Alaskan airlines chose a 178 passenger configuration for their 737-9, and so are not required to have a mid-cabin exit door.

Lion Air's chosen to go with a 221 passenger configuration, and so are required to have an operating door.

Obviously changing up the number of seats isn't done on-demand, you'd need to go for a refit/maintenance cycles.

But if Alaskan decided to change density, or sold the aircraft to someone else who decided to change density - then they could go and do this.

[1] https://youtu.be/nw4eQGAmXQ0?t=305 "The Boeing 737 Technical Channel"

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

#494

Post by paranoidrobot »

Pasorrijer wrote: post #442
paranoidrobot wrote: post #411

Earlier quoted context omitted.

It's built to be an actual emergency exit. It needs to be usable depending on how many passengers the interior is configured for. So it has all of the door bits there. Maybe some parts like the emergency escape slide are not installed. e: I should be clear that it's not usable as an emergency exit, as configured by Alaska. However the operator could choose to activate it later and install a usable exit.

This is not correct. To the passengers, this just looks like another seat next to a window with a plug installed. It's not a door. If there was a reconfiguration to a seating standard that required the extra exit, the plug would be removed and a proper door would be installed, with the associated interior pieces.

> To the passengers, this just looks like another seat next to a window with a plug installed. It's not a door.

This is true.

However there's still common hardware in there to allow the plug to be installed and maintained. This is why it's a complicated set of kit vs just bolting in a permanent fixture.

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

#495

Post by ethbr1 »

9659 wrote: post #464
WalterBright wrote: post #439

Earlier quoted context omitted.

The emergency procedure for runaway trim was the same for both aircraft types, and was not followed. After the first crash, an Emergency Airworthiness Directive was issued to all MAX pilots reiterating the procedure, which was not followed in the second crash, as well as not reacting to an overspeed warning. Unreported by the media, there was another MAX incident before the first crash. The crew had no knowledge of M…

"Runaway stab trim". It is a memory item, every pilot should be able to perform it from memory. Turn off the motor, and the trim is manual. There is a crank right there in the cockpit. If it is too hard to turn, change aircraft configuration to reduce the forces required to. Pilot know how to do this. This pilot stuff, they understand the forces on the flight controls and what impacts them. Boeing made an engineering…

> And that is how aviation becomes safer every year; at a significant cost of customers lives.

"Significant" might be inaccurate.

It looks like FAA Part 121 accidents over the last 10 years with fatalities have been... 4. [0]

For a total of 6 fatalities.

[0] https://www.ntsb.gov/Pages/AviationQueryV2.aspx; 2018 (1 passenger fatality) https://www.ntsb.gov/investigations/Pages/DCA18MA142.aspx ; 2019 (3 crew fatalities, cargo flight) https://www.ntsb.gov/investigations/Pages/DCA19MA086.aspx and (1 passenger fatality) https://www.ntsb.gov/investigations/Pages/DCA20MA002.aspx ; 2022 (1 ramp fatality) https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/G...

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

#496

Post by bb88 »

dilyevsky wrote: post #448
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.

Would you pay at least 2x for your software to have couple more nines of reliability? I’m gonna guess that “no”. At places where it costs $$$ to have bugs shipped to the end customer (e.g phones) or where there’re regulatory requirements they still have dedicated qa.

It depends.

1% of 10000 is 100.

.01% is 1.

If someone came up to me and said, "Hey I can save you 99% of expected costs with 1% of your profit.", I might go for it.

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

#497

Post by brettnak »

jacquesm wrote: post #489
johnwalkr wrote: post #486

Earlier quoted context omitted.

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?

The desire of fastener tension is clamping force or fastener stretch in some cases, not fastener torque. I think what the parent is saying is that a perfectly torqued fastener is only about 30% accurate in terms of clamping force.

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

#498

Post by nkurz »

jacquesm wrote: post #489
johnwalkr wrote: post #486

Earlier quoted context omitted.

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?

I presume his point is that you are measuring torque (potentially very accurately) but what you actually care about is tension. It's the variable factors affecting the relationship between torque and tension that results in most of the +/- 30% accuracy in tension, not the measurement of the torque.

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

#499

Post by ethbr1 »

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.

Because the comment I was replying to

>> Because the pilots should have been trained to unplug the computer to stop it from crashing the plane?

Yes.

The fault lies with the airlines because I don't for a second believe they didn't put pressure on Boeing to get the MAX certified without mandating retraining.

And then once that was done, didn't dig into the details too hard about what changes were made.

I have a low tolerance for 'I set up all the conditions and incentives to encourage you to break the law... but you should take all the blame when it explodes.'

At some point, the customer has to take some responsibility for what they asked for.

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

#500

Post by SAI_Peregrinus »

9659 wrote: post #464
WalterBright wrote: post #439

Earlier quoted context omitted.

The emergency procedure for runaway trim was the same for both aircraft types, and was not followed. After the first crash, an Emergency Airworthiness Directive was issued to all MAX pilots reiterating the procedure, which was not followed in the second crash, as well as not reacting to an overspeed warning. Unreported by the media, there was another MAX incident before the first crash. The crew had no knowledge of M…

"Runaway stab trim". It is a memory item, every pilot should be able to perform it from memory. Turn off the motor, and the trim is manual. There is a crank right there in the cockpit. If it is too hard to turn, change aircraft configuration to reduce the forces required to. Pilot know how to do this. This pilot stuff, they understand the forces on the flight controls and what impacts them. Boeing made an engineering…

Boeing also reduced the size of the manual trim wheels, which let them become impossible to turn sooner than on previous 737s.
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