Icelandair 757 Gear collapse at BIKF

[Zeffy]

METAR BIKF 071530Z 13042G56KT

​​​​​​https://www.ruv.is/frett/hvilir-a-odrum-hreyflinum-eftir-ad-hjolabunadur-brast

​​​​​​ (https://www.ruv.is/frett/hvilir-a-odrum-hreyflinum-eftir-ad-hjolabunadur-brast)JACDEC (https://twitter.com/JacdecNew)
@JacdecNew (https://twitter.com/JacdecNew)
Icelandair Boeing 757-200 (TF-FIA, built 2000) experienced right main gear collapse on landing runway 10 at Keflavik (BIKF) in very stormy weather. (up to 56 kts) None on board flight #FI529 (https://twitter.com/hashtag/FI529?src=hashtag_click) from Berlin-Tegel were hurt. METAR BIKF 071530Z 13042G56KT

[750XL]

Oops

https://scontent-lht6-1.xx.fbcdn.net/v/t1.15752-9/84881635_259216275067678_2316575294967775232_n.jpg?_nc_cat=1 00&_nc_ohc=FGK93KvFJDYAX81t6EX&_nc_ht=scontent-lht6-1.xx&oh=464317db464e3663a019bd7720042ed8&oe=5ECA9C5E

[Modular Halil]

Oops

https://scontent-lht6-1.xx.fbcdn.net/v/t1.15752-9/84881635_259216275067678_2316575294967775232_n.jpg?_nc_cat=1 00&_nc_ohc=FGK93KvFJDYAX81t6EX&_nc_ht=scontent-lht6-1.xx&oh=464317db464e3663a019bd7720042ed8&oe=5ECA9C5E

Pilot monitoring: "Set her down gentlyyy gentlyyyy gent--- bast@$d"

[Airbubba]

More pictures from social media.

https://cimg0.ibsrv.net/gimg/pprune.org-vbulletin/1920x934/ice3_768x373_large__884c38516d3fe95781fbd9b6b57aa41c08a3e3f5 .jpg

https://cimg7.ibsrv.net/gimg/pprune.org-vbulletin/1440x1080/84642862_2741074765968606_4522715654926630912_n_large__9f21e 8f8c5912ef850c6052ad2769e178a464593.jpg

[Ranger One]

Video of landing shot by pax:

https://twitter.com/beggi_north/status/1225849291891249152

[OntimeexceptACARS]

To be fair gear collapses on 757s are very rare, that must have been a pretty severe downdraught, I'd wager.

[ACMS]

Looked pretty smooth in that video........are we sure it suffered a downdraft hard landing?

[Rocade]

Second part of the video shows the landing from the collapsed side https://m.youtube.com/watch?v=ILdwSlxBkLI&feature=youtu.be
It looks like the gear collapses at touchdown (which appears to be relatively smooth) and the spoilers restow almost immediately, either by pilot action or failed air-ground sensor of the collapsed gear. It also appears the pilots pick the right wing up until the airplane slows down and then it drops again.
Rumors are the the collapsed gear was recently overhauled and fitted less than 3 weeks ago...

[ATC Watcher]

Great work by the PF .. looks like the wind is unrelated to this but that said when you are in 57 Gusty winds if there is an airline I want to be in is Icelandair .. .its breakfast for them ..

[jmmoric]

Great work by the PF .. looks like the wind is unrelated to this but that said when you are in 57 Gusty winds if there is an airline I want to be in is Icelandair .. .its breakfast for them ..

Agree, good work by the pilot.

The wind condition could favour a case like this, the groundspeed would be way less when the wing finally settles.

And Keflavik has crossing runways, and is located on flat land, so they often operate in quite strong winds. Problem comes with the air gates, and deboarding passengers when the wind picks up.

[procede]

What seems weird to me is that the gear collapsed to the outboard, so opposite from the direction of retraction.

[dixi188]

Side stay failure?

[DaveReidUK]

Side stay failure?

Hard to see what else might have caused it - the comment about it possibly being recently fitted may be significant.

I'd be surprised if there isn't some collateral damaged caused by the leg going in a direction it's not supposed to go.

[Bergerie1]

"Late on the round-out Hoskins."

[zerograv]

Side stay failure?

Probably !

From the first picture, looks like that the collapsed gear did not move rearward, it only moved outward, so as you rightly say, the Side Stay might have been the weakest link.

What is also peculiar from the first picture, is that because the right side of the aircraft is actually sitting on the engine, therefore something that is more forward than the gear itself, that in turn results in some weight being added to the rear, behind of the point of support. It is quite noticeable that the Nose Gear is very extended, indicating that it is not much weight is sitting on it. Still, that is a lot better than if it would be sitting on the tail.

Rwy 10, Wind form 130º, therefore half of the wind intensity is Crosswind.

As mentioned above, this is a very windy place. Went there a few times during a summer years ago. On one occasion after landing, taxing to the gate, parking brake On, even completing the after flight paperwork proved to be a bit difficult as the aircraft was constantly oscilating due to the wind.

[Airbubba]

Great work by the PF ..

"Late on the round-out Hoskins."

I'm sure if they had it to do over they would have gone around. :)

[oceancrosser]

I'm sure if they had it to do over they would have gone around. :)

Which would probably have given a much worse outcome, this happened at very low speed just before the runway turnoff.

[N707ZS]

As there are no scrape marks behind the engine did the undercarriage collapse as the aircraft was turning off at the end.

[capngrog]

That right main gear door hanging down indicates to me that a problem occurred during the L.G. extension sequence and was not related to touchdown forces at all.

Just my opinion.

Cheers,
Grog

[DaveReidUK]

As there are no scrape marks behind the engine did the undercarriage collapse as the aircraft was turning off at the end.

Looking at photos on the Net, the aircraft appears to be resting on the runway just before the 90° left turn onto Twy E-4 (runs parallel to Rwy 01/19), which could well have been its intended route to the gate.


That right main gear door hanging down indicates to me that a problem occurred during the L.G. extension sequence and was not related to touchdown forces at all.

I suspect that when the gear gave way, the engine would have impacted the runway with a pretty substantial thump, which could well have been sufficient to dislodge the gear door.

[Airbubba]

That right main gear door hanging down indicates to me that a problem occurred during the L.G. extension sequence and was not related to touchdown forces at all.

If they had a fault during landing gear extension would they still land in that wind? I would at least consult the QRH and do a Kumbaya session with the 'subject matter experts' at airline headquarters. Wouldn't you expect to get a DOORS light and a GEAR DOORS EICAS message if the door didn't close after gear extension?

[Nightstop]

Rwy 10, Wind form 130º, therefore half of the wind intensity is Crosswind
Don’t forget, METAR’s (and TAF) winds are in degrees TRUE. Variation at KEF is 14W and the runway is 104M. Therefore the wind was 40 degrees off runway heading, giving an average 32 knots crosswind.

[Airbubba]

Some guidance from Boeing on crosswind operations.

https://cimg0.ibsrv.net/gimg/pprune.org-vbulletin/1385x1040/crosswind_dc9e412753bdc820d38e010de0e339e6d83b3a03.jpg

[zerograv]

Don’t forget, METAR’s (and TAF) winds are in degrees TRUE. Variation at KEF is 14W and the runway is 104M. Therefore the wind was 40 degrees off runway heading, giving an average 32 knots crosswind.

Thanks for that !!! Forgot about that detail ... that in some parts of the world will make a pronounced difference.

It was even sportier than what I thought

[tubby linton]

A photo on the avherald quite clearly shows a bolt missing from the gear leg.

[Airbubba]

A photo on the avherald quite clearly shows a bolt missing from the gear leg.

From the AvHerald:

Passengers reported the aircraft bounced on landing, on second touchdown the right main gear collapsed and the aircraft skidded on the engine releasing sparks and smoke from the engine.

Here's the photo tubby linton mentioned:

https://cimg2.ibsrv.net/gimg/pprune.org-vbulletin/750x355/iceland_b752_tf_fia_keflavik_200207_5_9bb6744c433d8f046fed53 b44e22863aa576bfd1.jpg

To be fair gear collapses on 757s are very rare, that must have been a pretty severe downdraught, I'd wager.

Maybe a bad side load from landing hard in a skid? I've sure seen the wrinkled fuselage and the washboarding on the skin behind the gear before. Or, was it an improperly secured castellated nut on the missing bolt that was not found during multiple inspections and preflights?

These were not my landings ;):

https://cimg4.ibsrv.net/gimg/pprune.org-vbulletin/720x960/pri_80784106_b077fd42b4fc923dc90e07ac8e4fc15b25c333dc.jpg
https://cimg5.ibsrv.net/gimg/pprune.org-vbulletin/964x506/prc_80784410_f1f3487678cc6bb8cdff752430ab44808ae222f5.jpg
https://cimg6.ibsrv.net/gimg/pprune.org-vbulletin/914x577/united_757_ua627_5fdc310f4892c4798d97457752c9cb490e2ce4eb.jp g

[Onceapilot]

A photo on the avherald quite clearly shows a bolt missing from the gear leg.

Not quite. I think you can see the sidestay/leg pin is still there...in the sidestay end fitting (with yellow colour). What does appear to be missing is the Nut etc that holds it in place. :ooh:

I am surprised that this thread has no more recent updates.

OAP

[DaveReidUK]

Not quite. I think you can see the sidestay/leg pin is still there...in the sidestay end fitting (with yellow colour). What does appear to be missing is the Nut etc that holds it in place. :ooh:

If I'm not mistaken, there's more than just a nut/bolt missing.

This entire bracket appear to be absent in the photo that's been posted:


https://cimg5.ibsrv.net/gimg/pprune.org-vbulletin/435x373/757_mlg_63d02d838b3a5a5f8238c571049504ca4891122b.jpg

It looks like it's the lower nut that has departed, allowing the entire bracket to pull out and subsequently part company with the strut.

https://www.airliners.net/photo/Xiamen-Airlines/Boeing-757-25C/772430/L

Alternative speculation welcome. :O

[Onceapilot]

No, I think that you are still not seeing that the leg Pin is still there in the end fitting of the sidestay. You can see the leg pin there, pointing downwards on the end of the disconnected sidestay, seen hanging down in Tubby's excellent picture.

OAP

[Diesel_10]

[QUOTE=Onceapilot;10684960]No, I think that you are still not seeing that the leg Pin is still there in the end fitting of the sidestay. You can see the leg pin there, pointing downwards on the end of the disconnected sidestay, seen hanging down in Tubby's excellent picture

Cardan painted yellow - for what it's worth standard procedure on Boeing U/C for modified items (probably 1st oversize bearings and pin) - so you don't try and use them on another unmodified gear with associated consequences! I'd be slightly more worried if painted Red. Anyways all survived - probably another Tin-Foil Bomber for scrap though.

[Zeffy]

AvHerald has updated the report with an account from a passenger:
Accident: Iceland B752 at Keflavik on Feb 7th 2020, main gear collapse on landing (http://www.avherald.com/h?article=4d2fd39b&opt=0)
On Feb 11th 2020 The Aviation Herald received permission to publish the narrative of a passenger describing the experience on board of this flight. While granting the permission the passenger re-iterated that the landing was really smooth and that the passenger noticed the missing pin from the landing gear, too. The narrative in its entirety:

For those curious about my trip, the most exciting part was on my way home landing in Reykjavik, Iceland on flight FI529. The flight tracker on the in-flight information system said the winds were 30-50 MPH at the airport. This is fairly typical for Reykjavik. The approach was smooth and not turbulent. Normal landing speed for a Boeing 757 is 155 MPH. According to another passenger we touched down at 120 MPH which means we had a strong headwind. Everything was smooth including the landing. We touched down and then rebounded on the shocks in the landing gear. It did not feel like we left the ground again. There were the normal sounds of landing. We settled back down and the nose wheel touched, there was a loud bang and thump from the right side of the plane. It felt like we were flying again as the right wing dipped and then came back to level. The bottom of the dip was soft like a normal correction to turbulence.

As the plane slowed the right wing dropped again and stayed down. I was on the left side one row behind the wing's emergency exits. Window seat. I looked across the aisle and out the window on the right. There were sparks coming back from below the right engine. We came to a straight and smooth stop.

All the passengers were calm and quiet the entire time. The flight attendant came on the PA and told everyone to stay in their seats and listen for instructions. Anticipating having to use the emergency slides I took everything important out of my computer bag including passport, car keys, wallet, cell phone charger, Twix bar, etc. and put them in my coat pockets. I put my coat on.

The pilot came on and in a slightly shaky voice told us that as we may have guessed there was a problem with the landing gear and that we were safe to stay right where we were. Emergency personnel were on their way and we would be told what to do. He kept the left engine running a little while. They he started the auxiliary power unit and shut off the main engines. We had lights and heat the whole time. Emergency vehicles began to show up. Quiet talking and a little laughter was heard in the cabin. The pilot announced that they were going to evacuated us by mobile stairs and buses. I was disappointed I couldn't go down the slide.

We had been sitting for almost an hour, although I was not watching the clock, before the first passengers came off. They said they were stabilizing the plane. I could feel the wind shaking the plane. I saw a tug come out but I don't know where they put it. They started the evacuation by quickly unloading the rearmost few rows. They did this because the plane was unbalanced. The plane was resting on the right engine which is ahead of the point where the gear is. It was also tipped back a little. This moved the balance point forward and made the nose wheel light. I think they were afraid a strong gust could move the nose wheel sideways and cause problems getting onto the stairs.

As passengers came forward they had their luggage. They continued unloading from back to front. I put down the phone and prepared to leave. We went down the stairs and onto waiting buses. Once on the bus I took some more pics while waiting. It was only when on the bus and seeing the plane resting on the engine that I had a slight adrenaline rush.

When we got to the terminal there were a lot of staff waiting to help if needed. We were given bottles of water. They had emptied the entire wing of the terminal. As we got to the main terminal there was a person asking where I wanted to go. I said "Toronto." The couple behind me said "hotel." According to reports about a third of the passengers continued on to other destinations.

I grabbed some food and waited a long time in line but eventually made it to Toronto. Another person I knew was in line across from me. I said 'that could have gone better." She replied "it could have gone worse." Good point. A few of us were standing around discussing the event and more than a few people were eagerly listening. No announcement was made in the terminal except that all flights were delayed. No one knew there had been an accident. Not the kind of thing to tell people about to get on planes. While waiting in line to board my second flight two women behind me were complaining about the long wait. I told them that the airport could not use one of its two runways due to landing gear collapsing on a plane which had the runway blocked. I said I knew this because I had been on it. Their eyes went wide in a cartoon-like manner. No more complaining.

I went to go to my seat on the plane to Toronto but a flight attendant intercepted me and asked if I would like to move to a better seat up front. Was I on..."The flight from Berlin?" I interjected "Yes." I didn't make it to business class but I had a much nicer seat with my half of the row to myself. There was another girl from the same flight who was all the way on the right. The flight attendants checked on us more than usual.

Made it safely to Toronto while binge-watching "The Expanse." Good thing I had not downloaded the "Airport" movies from the 70's.

My opinion is that the landing was as smooth as any and that there was little cross wind. The landing in Toronto on an identical Boeing 757-200 was the same except for the gear collapse. I believed there was a mechanical failure due to maintenance but not due to the landing itself. The gear did not collapse until the plane settled down.

I found the below report (Editorial note: the passenger references this our AVH coverage about this event) which matches my recollection of the event. They had replaced the main gear on this very plane less than a month ago. Someone screwed up and didn't tighten a bolt. This pic (referencing our AVH picture showing the missing bolt) shows the brace which should have been attached to the gear.

In the end, no-one was hurt. The pilot did an excellent job of keeping the plane level and straight. It could have been much worse. I think the headwind kept the landing speed low which helped. When I look at the pic of the nose wheel I realize the pilot probably did not have great steering control due to the weight shift. I am even more impressed that he kept it straight down the middle of the runway.

Someone in the line listening to us relate the experience jokingly said he didn't want to fly with any of us. I disagreed and said that we had all used up a lifetime of bad luck and that there was no way it could happen to someone twice. I had no reservations about getting on the second plane.

Sometimes, people who travel frequently like to exchange travel stories and try to top each other with misadventures. I plan to keep this story in my back pocket for just such an occasion. If only I could have gone down the slide...

[ManaAdaSystem]

If they had a fault during landing gear extension would they still land in that wind? I would at least consult the QRH and do a Kumbaya session with the 'subject matter experts' at airline headquarters. Wouldn't you expect to get a DOORS light and a GEAR DOORS EICAS message if the door didn't close after gear extension?

Have you ever been to Island? Not like you have a lot of options in that area.
50 kts is not unusual flying into KEF, but the area is fairly flat, so not much turbulence.

[Lord Farringdon]

I have to give that passenger account a 10/10. It seemed to catch the atmosphere of the accident without emotion and appeared to be from someone who had flown as a passenger once or twice in his life time so that he knew a thing or two about whether the plane had actually landed or not. The description of the wing position during the event, and recall of the announcements, his assessment of the balance condition of the aircraft as unloading occurred post the event and of the pilots skills. A detailed account. I think the airline should track him down and let him take that ride down a cabin simulator slide!!

[MichaelKPIT]

Couldn't agree more - one of the best witness accounts I've seen. I especially liked the part where he put all the important stuff into his coat and put his coat on, obviously prepared to "leave everything" if it came to an evac.!

[Pilot DAR]

It seems that the wrong nut was used:

https://www.cbc.ca/news/canada/british-columbia/icelandair-landing-feb-7-2020-kelowna-bc-lawsuit-1.6344094

[A0283]

https://www.rnsa.is/media/4633/interim-report-tf-fia-right-main-landing-gear-collapse-during-landing-on-rwy-10-at-bikf-r1.pdf
English language interim report of 20 pages.

[First_Principal]

English language interim report of 20 pages.

Thanks A0283, my reading of the report is that the 'correct' nut was used, just that it had not been manufactured to the requisite tolerances by the overhaul supplier (that is to say, the next size up nut had possibly been supplied as 'correct')?

I've not worked on these, is the nut fitted during replacement of the assembly on the a/c, or is it kept intact as part of the assembly supplied by the manufacturer? If the former I'd be a little surprised that the engineer fitting the nut didn't detect there was something wrong given the nut minor dia was circa 1/16" larger than it should have been - or is that being unreasonable?

[Gargleblaster]

Icelandair has promptly sued the Canadian maintenance shop that allegedly fitted the new gear with an allegedly wrong nut (in Icelandic): https://www.mbl.is/frettir/innlent/2022/02/10/icelandair_stefnir_kanadisku_fyrirtaeki_vegna_flugs/

[DaveReidUK]

Thanks A0283, my reading of the report is that the 'correct' nut was used, just that it had not been manufactured to the requisite tolerances by the overhaul supplier (that is to say, the next size up nut had possibly been supplied as 'correct')?

I've not worked on these, is the nut fitted during replacement of the assembly on the a/c, or is it kept intact as part of the assembly supplied by the manufacturer? If the former I'd be a little surprised that the engineer fitting the nut didn't detect there was something wrong given the nut minor dia was circa 1/16" larger than it should have been - or is that being unreasonable?

Reading between the lines of the report, it would appear that the November 2019 overhaul (the one allegedly carried out by Kelowna) included an approved repair to the swivel attachment to the MLG strut. The repair involved recutting the threads on the swivel for a second time (having been done originally in 2008), which of course then required a correspondingly (further) undersized nut to be used during reassembly. It appears that instead of a 1/8" undersize nut being used, a 1/16" undersized one was fitted similar (or possibly even the same one) as that which had been removed.

In effect, most of the retaining force was then being provided not by the nut, but by the locking bolt, which sheared under landing loads, allowing the swivel to pull free from the nut and detach from the gear leg, with the inevitable result.

It's certainly not the first time that mismatched threads have resulted in failure of a component (a certain One-Eleven springs to mind) and it probably won't be the last.

[Jhieminga]

Icelandair has promptly sued the Canadian maintenance shop that allegedly fitted the new gear with an allegedly wrong nut (in Icelandic): https://www.mbl.is/frettir/innlent/2022/02/10/icelandair_stefnir_kanadisku_fyrirtaeki_vegna_flugs/
Can't blame Icelandair for doing that... the interim report is pretty clear! It looks like you would need to fabricate a new nut to the spec in the CMM. That's something that is hard to miss... so perhaps the nut was fabricated but the wrong one fitted? Or fabricated to the incorrect specs...? All speculation for now, we'll have to await the final report.

[oceancrosser]

Can't blame Icelandair for doing that... the interim report is pretty clear! It looks like you would need to fabricate a new nut to the spec in the CMM. That's something that is hard to miss... so perhaps the nut was fabricated but the wrong one fitted? Or fabricated to the incorrect specs...? All speculation for now, we'll have to await the final report.

Icelandair must sue Kelowna Flighcraft in order to get their insurance to pay. Insurance companies do not like to part with money and usually resist until a court orders them to pay in biz to biz dealings. These things take years, it´s already been two years. Covid will be an interesting twist in terms of trying to prove lost revenue.

[Winemaker]

The repair involved recutting the threads on the swivel for a second time (having been done originally in 2008), which of course then required a correspondingly (further) undersized nut to be used during reassembly. It appears that instead of a 1/8" undersize nut being used, a 1/16" undersized one was fitted similar (or possibly even the same one) as that which had been removed.

How could the wrong size nut be threaded on? It wouldn't even start......

[The Ancient Geek]

How could the wrong size nut be threaded on? It wouldn't even start......

It would fit if it had the same number of teeth per inch. Just loosely.

[DaveReidUK]

How could the wrong size nut be threaded on? It wouldn't even start......

It would start, but it probably wouldn't stop ...

The swivel pulled out of the nut with minimal damage to the thread on either, that's how loose it was.

[First_Principal]

The swivel pulled out of the nut with minimal damage to the thread on either, that's how loose it was.

I don't think there's any real argument over what occurred, to me the important thing - and by important I mean how to ensure this doesn't happen again - is how it came about.

Hopefully the two major components in this saga have a paper trail that will permit scrutiny and perhaps come up with some useful detail, but what interests me at this point is the final link or two in the chain.

To this end I had an in depth discussion with a colleague last night as to just at what point in our respective careers we'd have been able to determine simply by 'feel' that there was something awry with the fit of a nut+bolt, and was it something we were taught, or did the knowledge somehow just occur one day? Neither of us could say, we both started messing with mechanical things around 8-10 years of age and went through later training in our respective fields of expertise, but we didn't recall being specifically taught about what 'felt' right, yet we'd be pretty sure we could both identify an issue such as this, either by the nut simply being too loose on the shaft thread, and/or by the rotation of the torque wrench as the nut was tightened. Although we didn't reach a definitive conclusion it might be that we'd be able to do this by the age of say 18-20.

Thus this incident raised questions in our mind around what training is given on fundamentals such as tightening nuts (do apprentices even get the opportunity to overtighten, to fit 'wrong' nuts and to see/feel what that's like?), and just how one might ensure a system that is able to deal with possible issues such as occurred here. To some extent it would be useful to know if the nut was fitted by, say, a first year apprentice and then 'checked' by a senior person (I say 'checked' not to impugn that senior person, but simply because what can you really check once it's in place?), or whether it was fitted by a journey[wo]man.

Knowing the answer to this might allow a focus on certain aspects of training, environment, or even competency assessment, but in any event it behoves us all to think well around how we might better approach such a simple task as fitting a nut, or how we might properly train a new person coming on board. I hope it's clear that I'm looking here at the system, not the individual, although I don't believe we can totally divorce the vagaries of the individual from the mix (at the end of the day if someone, after a lot of training, just doesn't 'get' it then they just cannot adequately perform that task - and should not be doing so).

Quite possibly we'll find this issue has come about as another example of the holes in a cheese lining up; the supplier supplied mismatched components, the fitter fitted them and didn't notice something was awry, the inspector didn't see anything awry, the pilot (on their walkround) didn't see that the nut had moved or 'loosened' after a few cycles, but how do we go about making a cheese with less holes? Or is there another lateral way of better seeing the individual holes before they line up to result in a calamity?

There will, almost certainly, never be a perfect answer, but nothing should stop us from striving for it. It'd be interesting to hear what people think about how a seemingly simple task such as this might be better addressed, or if it's even possible?

FP.

[Winemaker]

Agreed, which is why I asked a question about this a couple of posts above How could the wrong size nut be threaded on? It wouldn't even start......
I used to race formula cars and had a business in Seattle maintaining and renting same. I can't imagine starting a nut and not feeling something was not right. Either it fit or it didn't and that would send a loud signal and ask for corrective action. I guess this is simple experience, but it sure seems basic..... especially when you apply the toque wrench.

[DaveReidUK]

Quite possibly we'll find this issue has come about as another example of the holes in a cheese lining up; the supplier supplied mismatched components, the fitter fitted them and didn't notice something was awry, the inspector didn't see anything awry, the pilot (on their walkround) didn't see that the nut had moved or 'loosened' after a few cycles

Fair point, though in this instance we can almost certainly discount the possibility of the incipient failure being detectable on a walkround, unless pilots have started carrying spanners, torque wrenches and spare split-pins.

[DType]

If it was torqued on re-assembly (and I guess it should have been), then I am v surprised it did not strip out immediately.

[Chu Chu]

The flats on the nut threads look pretty wide to me, and the minor diameter is around .020 above the spec for a 1/16" undersize nut. That makes me think that some material might have sheared off the thread tips uniformly, and the fit on assembly might have been a little tighter than the post-accident measurements indicate.

[Jhieminga]

If it was torqued on re-assembly (and I guess it should have been), then I am v surprised it did not strip out immediately.
I suspect that, because it keeps that swivel in place, which needs to be able to swivel, it doesn't need to be torqued to a very high value. It could just have been hand-tightened to a position where the locking bolt will fit. Does anyone have a CMM available to confirm? ;)