Alaska Airlines 737-900 MAX loses a door in-flight out of PDX
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SWAG Theories
1) The popular one. Rigging was incorrect, one or more bolts left out that would have prevented the door (not a PLUG) to move from its fully engaged position.
2) a) Photos of one guide pin on the accident plane appear to show a very short length of the guide pin/roller. Its length significantly less than its diameter. b) The dummy door (not a PLUG) differs from an Emergency Exit Door (not a PLUG) in a significant way. The cross bracing is removed to accommodate a larger window. Could the combination of the panel being weaker at that point have allowed the door (not a PLUG) to bow outward and dislodged the upper portion of the door from the guide pins/rollers? The four rigging bolts would do little to retain the door at this point.
1) The popular one. Rigging was incorrect, one or more bolts left out that would have prevented the door (not a PLUG) to move from its fully engaged position.
2) a) Photos of one guide pin on the accident plane appear to show a very short length of the guide pin/roller. Its length significantly less than its diameter. b) The dummy door (not a PLUG) differs from an Emergency Exit Door (not a PLUG) in a significant way. The cross bracing is removed to accommodate a larger window. Could the combination of the panel being weaker at that point have allowed the door (not a PLUG) to bow outward and dislodged the upper portion of the door from the guide pins/rollers? The four rigging bolts would do little to retain the door at this point.
Last edited by JamaicaJoe; 8th Jan 2024 at 02:33.
It exited the aircraft with a 100+ mph launch, and that's a 3 mile drop, so not too much. Now to find the t-shirt and the headrests. Oh, and the door.
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By the way, plenty of doors without handles in this world.
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I think the guide roller on at least the side shown in photos was not long enough to reliably engage the door. The four bolts really do nothing to secure the door , rather prevent movement of the mechanism.
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Flynerd You're looking straight at the locking bolt, end on. I'd imagine the centre hole is a machine turning indent, though I'm surprised it's there at all.
The length of that roller pin is still worrying me. Given that the door can move forward/aft a tad, the locking bolt is very near the end edge of that roller pin.
The length of that roller pin is still worrying me. Given that the door can move forward/aft a tad, the locking bolt is very near the end edge of that roller pin.
ASR-11 test requirement is detection of 1m2 RCS with a single pulse at a range of 55 nmi with a probability of 0.8. That is to say, even though it's smaller than an aircraft, an object the size of a door is precisely within the design of the radar to detect. What the RCS of a falling metal door plug is, who knows, but it's not stealthy.
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The locking pins on the upper assemblies appear to hold the roller pin only at the tangential point where the roller pin contacts the locking pin, which rather than being limited to the normal perpendicular shear force, is able to flex against the pressure from the roller pin. Is there a chance that, in the absence of the locking pins on the lower hinge slides, that this pressure point - in the absence of a radius block to spread that shear force at perpendicular points of contact on the locking pin, cause the locking pin to fail, moving just enough to gradually unseat the 12 stop brackets?
Just speculation at this point. Hopefully they find the door somewhere around Beaverton or Lake Oswego soon.
Just speculation at this point. Hopefully they find the door somewhere around Beaverton or Lake Oswego soon.
- It's a legacy from 1965 that never gave any problems, so it's kept the same to reduce certification headaches by 1. Just looking at it this looks like a real old design.
- The primary retention features are the locking pins on the hinges, the pins in the upper guide fittings are a reduncancy and should never touch the roller pin anyway
- Even the "crude" trapping pins are probably 5 times stronger in bending than the maximum load from the spring or general door flexing, maybe 10 or 20 times.
I'm really sure that any one of those locking pins should have been more than enough to keep the door from sliding off the stops. Either none of those pins were installed, or the door itself collapsed, a possibility with implications nobody wants to think about.
ASR-11 test requirement is detection of 1m2 RCS with a single pulse at a range of 55 nmi with a probability of 0.8. That is to say, even though it's smaller than an aircraft, an object the size of a door is precisely within the design of the radar to detect. What the RCS of a falling metal door plug is, who knows, but it's not stealthy.
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NTSB brief says this is a 'plug' or a 'door plug' but not a door. They tweeted a diagram of the complete structure of the 'door plug'. There were 12 'stop fittings' (circles) that hold the door in place. On left plug surround structure (frame), they identified several parts to be sent to lab. Will be inspecting the intact door plug on opposite side tomorrow. Three previous flights had pressure problems with cockpit indications on 12-7-23, 1-3-24 and 1-4-24. NTSB not yet sure if they correlate or not. Auto-pressurization system was tested and light reset by engineers. Alaska decided to restrict that plane from ETOPS routes, mainly Hawaii.
Survival group documented interior damage from decompression which was extensive. Rows 33, 32, 31, 27, 26, 25, 12, 11, 4, 3, 2 and 1 all had damage. 26A and 25A were both torqued with missing parts. Oxygen mask for 26C was functional and used by a passenger in that seatbut other three masks' plastic tubing were "sheared" off. No structural damage to the aircraft either inside or outside. Passengers were not moved out of the seats, the seats were dispatched vacant. Spokeswoman said they "just happened to be empty".
Two flight attendants (front of aircraft) interviewed today and two (back) tomorrow. Flight attendant says the cockpit door blew open in the decompression. It hit and damaged the lavatory door, jamming it closed. No one was inside. Flight crew was also interviewed. They heard a bang, immediately donned oxygen and verified cockpit door flew open. The quick reference checklist flew out the cockpit door so used the quick reference handbook. First officer (female) lost her headset while captain's was pulled almost off. Complimented both flight crew and cabin crew.
Door plug still missing with one side being "yellowish green" on one side and white on the other. It is 26 X 48 inches and weights 63 pounds. General location is based on radar. Two cell phones were found and turned in, one in a yard and one on the side of the road. CVR and FDR have arrived at the lab. CVR was overwritten as circuit breaker was not pulled. NTSB is not happy and want 25 hour not 2 hour CVRs.
Survival group documented interior damage from decompression which was extensive. Rows 33, 32, 31, 27, 26, 25, 12, 11, 4, 3, 2 and 1 all had damage. 26A and 25A were both torqued with missing parts. Oxygen mask for 26C was functional and used by a passenger in that seatbut other three masks' plastic tubing were "sheared" off. No structural damage to the aircraft either inside or outside. Passengers were not moved out of the seats, the seats were dispatched vacant. Spokeswoman said they "just happened to be empty".
Two flight attendants (front of aircraft) interviewed today and two (back) tomorrow. Flight attendant says the cockpit door blew open in the decompression. It hit and damaged the lavatory door, jamming it closed. No one was inside. Flight crew was also interviewed. They heard a bang, immediately donned oxygen and verified cockpit door flew open. The quick reference checklist flew out the cockpit door so used the quick reference handbook. First officer (female) lost her headset while captain's was pulled almost off. Complimented both flight crew and cabin crew.
Door plug still missing with one side being "yellowish green" on one side and white on the other. It is 26 X 48 inches and weights 63 pounds. General location is based on radar. Two cell phones were found and turned in, one in a yard and one on the side of the road. CVR and FDR have arrived at the lab. CVR was overwritten as circuit breaker was not pulled. NTSB is not happy and want 25 hour not 2 hour CVRs.
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Last edited by Senior Pilot; 8th Jan 2024 at 04:52. Reason: Fix url
Door found!
From the BBC:
“A teacher called Bob found the missing section in his yard, the chair of the organisation leading the investigation said on Monday.”
No further details. But well done “Bob”
- GY
“A teacher called Bob found the missing section in his yard, the chair of the organisation leading the investigation said on Monday.”
No further details. But well done “Bob”
- GY
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I think we will find out that the lift assist springs were out of adjustment. I would guess they are not designed to be providing a constant upward force on the door, which would put a constant force on that locking bolt.
It's right there in the name, lift assist spring. So, for example, if the plug weighs 100 lb, then the spring may provide 70 lb of upward force, and a mechanic opening the plug would only have to provide 30 lb of force to lift the plug high enough for the door stop fittings to clear the fuselage stop pads. This setup would cause a constant downward force (from the weight of the plug) keeping the upper guide roller in the door upper guide track fitting. And no constant force on the locking bolts
If the lift assist spring was adjusted in such a way that there was a constant force applied to the locking bolts, then I could see how after 3 months of vibrations the two bolts wear down and eventually fracture, which would allow the door open on its own.
It's right there in the name, lift assist spring. So, for example, if the plug weighs 100 lb, then the spring may provide 70 lb of upward force, and a mechanic opening the plug would only have to provide 30 lb of force to lift the plug high enough for the door stop fittings to clear the fuselage stop pads. This setup would cause a constant downward force (from the weight of the plug) keeping the upper guide roller in the door upper guide track fitting. And no constant force on the locking bolts
If the lift assist spring was adjusted in such a way that there was a constant force applied to the locking bolts, then I could see how after 3 months of vibrations the two bolts wear down and eventually fracture, which would allow the door open on its own.
1) The spring hinges are present to keep the door from closing after it is opened. Therefore, it would seem like they need to have sufficient force to hold the weight of the door, or the plug, whichever is fitted to a given aircraft.
2) If the same springs are used with the actual emergency door for those aircraft equipped with actual mid-cabin exits, the spring force would be far greater than is necessary to lift the plug (doors are much heavier).
3) Is there an actual source for the term "lift assist spring" in this case?
Last edited by lateott; 8th Jan 2024 at 05:42. Reason: Additional description
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A couple comments:
1) Can we all agree that the missing part of the plane is a "door" that replaces an emergency exit "door, That it is NOT a "plug"?, it is not designed as a "plug", does not function as a "plug" under pressurization of the hull. In fact, it performs exactly the opposite. The outward forces work to stress the door against its retainers not the frame in any intrinsic or hermetic manner..
2) The missing door is fitted with a larger window than the emergency door it replaces. It seems from photos that there is less cross bracing in that design. However I am not jumping to conclusions that the door failed. It is more likely that the low altitude and missing bolts conspired to eject this door (not a plug).
1) Can we all agree that the missing part of the plane is a "door" that replaces an emergency exit "door, That it is NOT a "plug"?, it is not designed as a "plug", does not function as a "plug" under pressurization of the hull. In fact, it performs exactly the opposite. The outward forces work to stress the door against its retainers not the frame in any intrinsic or hermetic manner..
2) The missing door is fitted with a larger window than the emergency door it replaces. It seems from photos that there is less cross bracing in that design. However I am not jumping to conclusions that the door failed. It is more likely that the low altitude and missing bolts conspired to eject this door (not a plug).