Alaska Airlines 737-900 MAX loses a door in-flight out of PDX

[Edoil17]

Hmmm. Looking at the setup for Wi-Fi installation, it does not look ideal. I'm sure they would need to cut / drill holes in upper fuselage (all this being done outside, albeit under cover of a tent).
Post installation, would, from experience require some form of cabin pressurisation checks, to confirm that the fuselage was sufficiently airtight to return to service post this work being carried out.

[vikingivesterled]

Originally Posted by EXDAC

I searched the inox.ie site for "set screw". I found no definition and no products. Where did you find the definition you quoted?

Under the tab Bolts, then select "Bolts & Set Screw - Hex Head"
It's in the 2 first lines of the Product Details:
"Stainless Steel Set Screws are fully threaded and have hexagon heads.
Stainless Steel Bolts are part threaded and have hexagon heads."

Question is also wether a Clewis Pin would have been the better application for this type of use.
It is what is seen as a stronger part than bolts for holding marine rigging together, also avoiding chance of deformation or fracturing the roller guide in this case, by overthigthening the bolt.
A partly threaded bolt would have had to be made to exact length specification of the non threaded part to avoid pinching. Again these bolts did not pull something together but just acted as a blocker for the rolling pin and the spring load mechanisme. A Clevis Pin would also avoid the possibility of forgetting the split pin since it is the only thing holding it in place. The added castellated nut of a bolt would probably be useless anyway without the split pin.

I alo see some comments on how the split pins before where bent in a certain way. This overbending has been abandoned in the marine industry unless snagging is a factor, because sharp bends is found to weaken the pin. A slight split of the 2 ends to an angle of about 30 degrees between the two is enough to keep it in place in even the storms of the southern polar regions, and simplifies removal.

[incompleteness]

Originally Posted by DIBO

I think, for now, better not call the NTSB...

That image is weird. The v-stab is perforated and its got odd blobbiness all over like you see in AI images.

[Solofast]

Originally Posted by DaveReidUK

Not forgetting the scenario where the bolts were initially fitted, but the nuts were never split-pinned ...

If the bottom bolts were installed, even without nuts and split pins they won't just fall out. The reason is that the springs would jam the lower fittings up against the bolts and that constant force would prevent them from coming out. Once those bolts are in place if force isn't supplied to push the door down off the bolts, then they are restraining the springs and the bolts aren't moving, let alone falling out. Doesn't matter if the bottom nuts were installed or not, those bolts won't fall out on their own. This is why it is clear the bolts were never installed.

[EXDAC]

Originally Posted by vikingivesterled

Under the tab Bolts, then select "Bolts & Set Screw - Hex Head"
It's in the 2 first lines of the Product Details:
"Stainless Steel Set Screws are fully threaded and have hexagon heads.
Stainless Steel Bolts are part threaded and have hexagon heads."

Thanks for the reference. It includes the statement - "Set Screws are manufactured to DIN 933 and Bolts to DIN 931."

The problem is that DIN 933 does not, at least as far as my search has gone, define set screws or even mention set screws. The primary characteristic of DIN 933 appears to be "full thread". They come with a variety of head types but none are set screws as set screws have no head.

https://www.fastenermart.com/files/D...ifications.pdf

As far as aviation fasteners are concerned I have more confidence in AC 43.13-1B, NAS specs, and MS specs, than an Irish bolt manufacturer's web site that appears to list no aircraft or aviation products.

[OldnGrounded]

Grabbed from Google Maps just now. https://www.google.com/maps/place/66...sy9_?entry=ttu

Originally Posted by incompleteness

That image is weird. The v-stab is perforated and its got odd blobbiness all over like you see in AI images.

I think it's most likely an AI-assisted enlargement of a 3D satellite image, either not the latest & greatest AI or one in which AI has not been permitted to get too creative. It seems to have filled in some spaces where there was limited information available, but not attempted to paint in every possible detail.

[Loose rivets]

Fractured roller guide can be clearly seen at the corner nearest the camera.

I think the vital thing to notice is just how slight the deformation of the out edge of the guide is. i.e., it really spells out just how short the roller/pin is compared to the available depth.

[BigJETS]

Originally Posted by DaveReidUK

Door plug on arrival at NTSB's Materials Lab:



Fractured roller guide can be clearly seen at the corner nearest the camera.

Not only are the guides damaged, but both skins at top corner rib structure, appear separated; as if the bottom of the plug let go, pivoting on the upper guides and prying against top of opening until the top guides finally broke out. Perhaps due to both lower hinge guide fittings separated from plug the instant vertical motion clears 12 stops, but before top guides released. ?

Also Re: The wifi install photo, is pretty definitive. The plug occupies the 10th window placement from aft. Would appear to be sealed in this photo.

[Oro-o]

Originally Posted by BigJETS

Not only are the guides damaged, but both skins at top corner rib structure, appear separated; as if the bottom of the plug let go, pivoting on the upper guides and prying against top of opening until the top guides finally broke out. Perhaps due to both lower hinge guide fittings separated from plug the instant vertical motion clears 12 stops, but before top guides released. ?….

I was just about to ask about that. I had marked up the photo to highlight it. It would not have happened on impact in soft water-logged ground; it had to have been pre-existing or during separation.



Red indicators for highlighting.

[Solofast]

Originally Posted by vikingivesterled

But what if in the confusion of bolt definition they used an available fully threaded bolt (called set screw (with no head they are called grub screw)) instead of one only partially threaded (just called bolt). How much would that reduce the shear strength. And in this case it's bending and hitting/pushing on the middle by the roller pins strength we are talking about, not pull strength.
(Definitions on bolt type names are from the website of the supplier of stainless steel hardware inox.ie)
This could also have been done by a Spirit employee and especially if they thoght Boeing routinely would replace them later.
They could even have used non hardened bolts.

That Alaska plane needs to be taken apart to see if not at least one bolt have fallen into a gap and down into the belly of the plane. If they did fail its unlikely they all failed at the same time.
And I wouldn't worry to much about lose bolts on other planes. It's the precense of the bolt that is important. Not its thightness. And a castellated nut with a split pin would stop it coming out altoghether, unless it was broken in two.

The bolts used here would never be full threaded bolts. The bolts used would have unthreaded shanks and are clearly drilled for split (cotter) pins. To be drilled for split pins they aren't #6 and most likely aren't #10's (although they could be but I highly doubt it). It's far more likely that these are 1/4 inch AN bolts with an ultimate strength of 125ksi. They could even be A286 material as that is more corrosion resistant. A286 bolts have an ultimate strength of 160 ksi. A quarter inch bolt in double shear is good for about 5500 pounds of force each. That would apply to the lower bolts. You're looking at it requiring 11000 pounds of force to shear just the lower bolts, never mind that you have to get past the upper bolts too. The upper bolts are in bending since they are loaded by the roller, The person who designed this was very smart. In this design the upper bolts can bend some and thus share the load with the lower bolts. If the upper bolts were in pure shear then all of the bolts need to be in contact at the same time to make sure they share the load. With this design it's going to take significantly more load to get past the bolts if they are in place. The amount of vertical force to move the door off of the pads is likely well over 15,000 pounds. That is the lower bolts require bout 11000 pounds to fail in shear, and the upper botls are likely about half that and they fail in a combination of shear and bending since they're essentially a simply supported beam with a center loading. The bottom line is that if the bolts are in there the door isn't moving since there are no appreciable loads in that direction and it will require more than 7 TONS of load to move the door up with the bolts in palce.

[MechEngr]

Originally Posted by BigJETS

Not only are the guides damaged, but both skins at top corner rib structure, appear separated; as if the bottom of the plug let go, pivoting on the upper guides and prying against top of opening until the top guides finally broke out. Perhaps due to both lower hinge guide fittings separated from plug the instant vertical motion clears 12 stops, but before top guides released. ?

Also Re: The wifi install photo, is pretty definitive. The plug occupies the 10th window placement from aft. Would appear to be sealed in this photo.

Perhaps the last gasp from the restraining cables after the guide brackets let go.

[jimjim1]

Originally Posted by Oro-o

I was just about to ask about that. I had marked up the photo to highlight it. It would not have happened on impact in soft water-logged ground; it had to have been pre-existing or during separation.



Red indicators for highlighting.

If the door was freed from the 12 stop pegs the roller pin might still be retained within the top guide. If the bottom moved out under the influence of cabin pressure then damage to the top consistent with the images could occur. The very top lip would be forced inboard, pivoting round the guide, causing the separation of the skin from the frame shown until eventually the guide failed. Then both the top and bottom of the door would be free to open.

In this scenario the door's upward translation would be interrupted by being jammed between the outward force on the guide roller and the inward force on the top edge of the door on the frame.

[aox]

Originally Posted by EXDAC

Thanks for the reference. It includes the statement - "Set Screws are manufactured to DIN 933 and Bolts to DIN 931."

The problem is that DIN 933 does not, at least as far as my search has gone, define set screws or even mention set screws. The primary characteristic of DIN 933 appears to be "full thread". They come with a variety of head types but none are set screws as set screws have no head.

https://www.fastenermart.com/files/D...ifications.pdf

As far as aviation fasteners are concerned I have more confidence in AC 43.13-1B, NAS specs, and MS specs, than an Irish bolt manufacturer's web site that appears to list no aircraft or aviation products.

Reminder

Originally Posted by Pinkman

Set Screw (US) = Grub Screw (UK/IE)

And from other posts in the thread, set screws here have heads

There is something similar happening with split pins (UK) and cotter pins (USA). However here, and I suspect also in USA, a cotter pin is wedge tapered pin that secures a crank arm to a shaft or axle, such as old style bicycle cranks to the bottom bracket spindle before the development of tapered, square ended, cotterless crank systems.

[DaveReidUK]

Originally Posted by Solofast

The reason is that the springs would jam the lower fittings up against the bolts and that constant force would prevent them from coming out.

That's partly dependent on the weight of the door plug at any given point versus the spring force.

We know its mass, but I'd be very surprised if it's never subjected to more than 1g during flight.

This is why it is clear the bolts were never installed.

We'll find that out soon enough from the NTSB's metallurgists.

[NSEU]

Originally Posted by lateott

That is a different question from the spring force, but I agree with you.


I believe one of the Chris Brady videos states the maximum local wind specification for the plug (or door when installed). Definitely less than 271 KIAS.

Give Boeing engineers some credit, the self-ejecting plug design has breakaway features.
From Chris Brady video

What is the dynamic pressure at 271 KIAS at 16,000 feet?

Is this the right calculator for computing this?

https://www.spaceworks.aero/fcc/index.html

If so, I only get 1.05psi.

How tall/deep is the plug? i.e the side profile

[MarineEngineer]

I note that the lift assist brackets are completely missing.

[Jhieminga]

You need to convert the 271 KIAS to TAS (use https://indoavis.co.id/main/tas.html for example) and then convert that to ft/s. I have guessed at some of the values or used standard values, but that gets me 283.75 lbf/ft^2 or 1.97 psi.

[DTA]

Unfortunately there are still no photos that show what if anything remains of the forward green hinge bracket. The NTSB lady managed to get her arm in just the wrong place.

Originally Posted by Oro-o

I was just about to ask about that. I had marked up the photo to highlight it. It would not have happened on impact in soft water-logged ground; it had to have been pre-existing or during separation.



Red indicators for highlighting.

I think there was a photo showing that the plug ended up in a tree or bush. Not that it makes any difference.

Originally Posted by BigJETS

Not only are the guides damaged, but both skins at top corner rib structure, appear separated; as if the bottom of the plug let go, pivoting on the upper guides and prying against top of opening until the top guides finally broke out. Perhaps due to both lower hinge guide fittings separated from plug the instant vertical motion clears 12 stops, but before top guides released. ?

Also Re: The wifi install photo, is pretty definitive. The plug occupies the 10th window placement from aft. Would appear to be sealed in this photo.

Whilst having the bottom of the door plug move out would explain the separation of the skin at the top it is hard to see how the bottom could have moved out first. The two shafts remain attached and the rear bracket mounting bolts appear to have been broken by considerable force suggesting the rear bracket at least was correctly in place (though probably not with a lock bolt).

[Oro-o]

Originally Posted by DTA

…
I think there was a photo showing that the plug ended up in a tree or bush. Not that it makes any difference.

….

I agree, I don’t think that makes any difference. For reference, here is the reported position in which it was it was found. It passed through some canopy above, I believe. This was in the Seattle Times article yesterday.

[aox]

Sidetrack question: if a lot of the aircraft is made of carbon fibre, why isn't that door/plug?