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

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

Old 7th Jan 2024, 17:15
  #261 (permalink)  
 
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Originally Posted by AirScotia
It does seem surprising to me that an airframe with known pressurisation issues wasn't taken out of service right away. It also seems odd to me that two seats next to the relevant window were apparently empty, two of only seven unoccupied seats in a full flight - including a window seat at that. That suggests that these seats were not bookable, which further suggests that something was known about that area of seating. Whistling noises? Rattling? Cold breeze?
Those seats were assigned but the passengers apparently missed their PDX connection. I can't remember where I read this...
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Old 7th Jan 2024, 17:17
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Originally Posted by Flipfrog
Are you thinking that there is a bolt that goes through the plug door and into a tapped hole on the fuselage pin? It also looks like there could be a bolt that runs coincident to the outer door the secures the pin from moving upward that seems to be installed?
I was thinking that there is a tapped hole in each upper guide fitting and the headless screw lies just under the fuselage side guide pin. Any upward movement of the door would be blocked by contact of the screw and the guide pin. (To be clear - this is speculation.)
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Old 7th Jan 2024, 17:18
  #263 (permalink)  
 
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Originally Posted by Thruster763
I was second guessing myself and was correct the first time. This image of a DOOR shows the locking pins below the upper two striker plates which have the guide pin between them.

But it does look like the PLUG has bolts in a different location, possibly going into the guide pin.
And yet, both the door and the plug have roughly the same shape of frame -- and the frame is missing on the accident aircraft with at least the top four (out of twelve) bolts sheared off where it connects to the fuselage from the inside. Are those bolts the primary mechanism to keep the frame of the door/plug attached to the aircraft, or would you expect them to shear off?
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Old 7th Jan 2024, 17:23
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Originally Posted by EXDAC
I was thinking that there is a tapped hole in each upper guide fitting and the headless screw lies just under the fuselage side guide pin. Any upward movement of the door would be blocked by contact of the screw and the guide pin. (To be clear - this is speculation.)
Yes I think we are on the same page on that bolt/screw under the pin. I had read elsewhere in the thread that someone had suggested that there was a tapped hole in the actual pin that could show evidence of a surface defect indicating a snapped bolt.
I agree all speculation. But on the face of it there doesn't seem to be much redundancy or a clear flag/indicator that the door.is flight ready and secure like the normal human operated doors.
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Old 7th Jan 2024, 17:26
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Originally Posted by MechEngr
I expect they are to lift the door to disengage the fingers. If the door was unable to move up then the fingers cannot release.
Yes, and that's an entirely necessary function when an actual emergency door is fitted.

The door plug, on the other hand, is only ever going to be removed in the hangar, during a check, and it's going to have to be lifted out anyway once it's cleared the stops, so the value of the springs, if fitted, would be minimal.
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Old 7th Jan 2024, 17:28
  #266 (permalink)  
 
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Originally Posted by ThreeIfByAir
And yet, both the door and the plug have roughly the same shape of frame -- and the frame is missing on the accident aircraft with at least the top four (out of twelve) bolts sheared off where it connects to the fuselage from the inside. Are those bolts the primary mechanism to keep the frame of the door/plug attached to the aircraft, or would you expect them to shear off?
What photo are you looking at that shows four sheared bolts?
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Old 7th Jan 2024, 17:35
  #267 (permalink)  
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Is there confusion between "sheared bolts" and permanently inserted steel surface pressure-bearing rods set into the castings?


Note the two rods that slide in the casting with the lock bolts (or missing lock bolts). It's been suggested (kghjfg) that they are too short for all fuselage stressed states. I note one can see a wear mark indicating how far into the sliding slot they have been sitting. There's a lot of wasted length.

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Old 7th Jan 2024, 17:40
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Originally Posted by MechEngr
I expect they are to lift the door to disengage the fingers. If the door was unable to move up then the fingers cannot release.
Take a look at Chris Brady's latest, which shows that the plug (like the door) moves down to disengage the guide pin. It also makes clear most of the rest of the plug installation we've been speculating about.

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Old 7th Jan 2024, 17:41
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Question

Have they found the door/plug and any related parts that blew out yet? I imagine that there will be some telling evidence if a loose bolt pulled out of a bracket. Or no damage if the bolt was never installed.
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Old 7th Jan 2024, 17:43
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Originally Posted by ThreeIfByAir
And yet, both the door and the plug have roughly the same shape of frame -- and the frame is missing on the accident aircraft with at least the top four (out of twelve) bolts sheared off where it connects to the fuselage from the inside. Are those bolts the primary mechanism to keep the frame of the door/plug attached to the aircraft, or would you expect them to shear off?
The primary contact between the door/plug and the fuselage is by 12 pads (stop fitting and stop pad) that are each adjustable to ensure uniform contact. There are no retaining bolts in the stop fittings, only contact adjustment. The door must move vertically so each stop fitting is clear of the stop pad for the door/plug to open.

Where did you find any photo that shows the fuselage side stop pads are sheared off?

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Old 7th Jan 2024, 17:56
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Originally Posted by OldnGrounded
Take a look at Chris Brady's latest, which shows that the plug (like the door) moves down to disengage the guide pin. It also makes clear most of the rest of the plug installation we've been speculating about.

I think Chris misspoke; not a big deal for the pressure from the interest in the topic. The door is stopped from moving down by the top of the slot in the door guide. I think he missed that the closeup was of the other side of the door than he expected as the rest of the diagram labels are correct, showing the door moving up to disengage.
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Old 7th Jan 2024, 18:08
  #272 (permalink)  
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It would seem from his post that the bottom two bolts would also have to be missing to allow the roller pins to move down the guide tracks.

However, if they were, the lift springs would allow the door to be lifted - unless the top bolts were never fitted. So, never any top bolts and the loss of perhaps only one lower bolt. The most far-fetched scenario. But, those spring must be very strong - with all that implies.
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Old 7th Jan 2024, 18:17
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Originally Posted by MechEngr
I think Chris misspoke; not a big deal for the pressure from the interest in the topic. The door is stopped from moving down by the top of the slot in the door guide. I think he missed that the closeup was of the other side of the door than he expected as the rest of the diagram labels are correct, showing the door moving up to disengage.
I think you're right. Listening to him, I misunderstood the guide track diagram beginning at about 6:27. I didn't notice that the track is on the door and the pin on the frame. [Edit: Which seemed counterintuitive to me.]

So, without the bolts, the springs on the hinges are going to be doing their best to free the plug from the guide tracks, and are very likely to succeed before very long. I wonder how many cycles this aircraft had in a bit more than two months..
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Old 7th Jan 2024, 18:17
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According to the video posted above, the upper two retaining bolts are supposed to be in the guide fittings:


The lower two in the hinge brackets.
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Old 7th Jan 2024, 18:19
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Originally Posted by PuraVidaTransport
there were only SEVEN (7) empty seats
On landing. Possibly only six on takeoff.
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Old 7th Jan 2024, 18:36
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Originally Posted by Loose rivets
But, those spring must be very strong - with all that implies.
Except that nobody seems to have been able to verify yet that they are present at all on the plug door.
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Old 7th Jan 2024, 18:42
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Originally Posted by spornrad
According to the video posted above, the upper two retaining bolts are supposed to be in the guide fittings:


The lower two in the hinge brackets.
Only that they are remarkably short for my eyeball and gusts - compared to the depth of that fitting.


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Old 7th Jan 2024, 18:52
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In the latest video the springs are shown compressed. See the 10:12 time mark or so and the spring kinda snakes side to side as compression springs (which are labeled) tend to do if they don't have tight guides.

That double nut cap washer should have been able to retain the bottom part of the door or at least hang on well enough to twist the hinges, but no such damage is evident.
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Old 7th Jan 2024, 19:04
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On the DOOR the guide pins are just that, guides to stop damage to the pads. The top of the door is stopped from moving out by the guides until the stops have cleared the pad structure. Remember thare are no pressurisation loads on the DOOR at this point, the vent panel is open.
The DOOR is stopped from moving up by locking pins that are pelow the viewport between the second and third stops / pads.
It appears that on the PLUG there are no locking pins, just 4 bolts. Two apparently going through the guides and into the pins (near top of door) and two at the hinge / springs at the bottom of the door which I've seen no detail of.
Looking like the top bolts at least were not fitted. They would not be highly loaded, at most opposing the force of the lift springs.

There is however another possibility:
Some or all of the Stops / Pads were not adjusted correctly leaving the guide pins and locking bolts carrying presurisation loads leading to failure from low cycle fatigue.

EDIT: just found latest youtube info
This shows the bolts through the guide track and details of the lower bolts. Looks ike either all bolts were missing or the stops / pads were mis-adjuated causing pressurisation loads on the guides and guide pins.

Last edited by Thruster763; 7th Jan 2024 at 19:14. Reason: new info
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Old 7th Jan 2024, 19:59
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Originally Posted by MechEngr
In the latest video the springs are shown compressed. See the 10:12 time mark or so and the spring kinda snakes side to side as compression springs (which are labeled) tend to do if they don't have tight guides.
Thanks for that - I hadn't seen the latest video, which makes everything clearer. Mea culpa.

I note that all 4 bolts are retained by castle nuts and cotter pins (more usually known as split pins). Had those been correctly assembled, I can't see any way that the door plug could have let go. Incorrect assembly could be any of:

a) omitting the split pins
b) omitting the nuts and therefore the split pins
c) omitting the bolts and therefore the nuts and split pins

at any or all of the 4 bolt locations.

Any thoughts?
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