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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 21st Jan 2024, 05:07
  #1161 (permalink)  
 
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Something I have found odd about this plug design, given that it will hardly ever be opened, is why there is a need for springs in the hinges at all..
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Old 21st Jan 2024, 05:44
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Originally Posted by chrisl137
I don't mean non-destructive. I just don't think they'll be doing structural testing on the actual plug. More likely things like slicing open the brackets to look inside the bolt holes. That's not a structural test-to-failure but it is destructive. Removing the brackets changes the configuration (and possibly removes evidence) and slicing them open means they'll never be brackets again.

Destructive testing is fine if they've gotten all they think they can get from it as-is and have thoroughly documented the configuration.
You could be right in terms of not testing "the actual plug", as it makes more sense. The Reuters quote(with my bolding) was: "Homendy said the NTSB will move next week onto destructive testing of the door plug, or testing to the exact point of failure." May be a loose way of meaning "destructive testing of other identical door plugs".

Last edited by Mach2point7; 21st Jan 2024 at 06:24.
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Old 21st Jan 2024, 06:04
  #1163 (permalink)  
 
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Originally Posted by tdracer
I've was in this business long enough to know you 'never say never' - even a 10-9 event will happen one in a billion times.
That being said - that basic door plug design is on thousands of aircraft with millions of flight hours and cycles. If the design was truly deficient, the probability that it would show up - for the very first time - on a brand-new aircraft with just a few months of service is really, really low. Since it happened on a newly built aircraft, it almost certainly needs to be an assembly error of some sort - although not necessarily the bolts. It is possible some part of the interfacing hardware was no done properly.
I'm sure that's the focus of the NTSB if they can't verify that it was the bolts.
The 737 rudder PCU was reliable for years and countless flight hours until it wasn't. I'd avoid making assumptions like "it's been working fine" means "it's not the problem." When investigating complex systems you have to start by assuming nothing.
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Old 21st Jan 2024, 07:18
  #1164 (permalink)  
 
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Originally Posted by incompleteness
If my memory isn't failing me, Homendy said earlier that lab examination of the plug components would reveal whether or not the bolts were installed. I think that was a little optimistic. As far as i know there is no atlas of post-failure fastener holes that can be compared to the indident hardware to draw an indesputable conclusion. There are atlases of fractography for categorizing parts that have fractured, but this situation isn't like that, we are probably talking about deformed/damaged holes. So the testing they are talking about doing now is probably to generate such deformed holes to see what they look like.
It's the old problem with proving a negative. Visible witness marks would prove that the bolts were installed at some stage. Lack of witness marks wouldn't necessarily prove that they weren't.
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Old 21st Jan 2024, 09:36
  #1165 (permalink)  
 
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Originally Posted by tdracer
I've was in this business long enough to know you 'never say never' - even a 10-9 event will happen one in a billion times.
That being said - that basic door plug design is on thousands of aircraft with millions of flight hours and cycles. If the design was truly deficient, the probability that it would show up - for the very first time - on a brand-new aircraft with just a few months of service is really, really low. Since it happened on a newly built aircraft, it almost certainly needs to be an assembly error of some sort - although not necessarily the bolts. It is possible some part of the interfacing hardware was no done properly.
I'm sure that's the focus of the NTSB if they can't verify that it was the bolts.
Of course, you mean, the general principle of a plug door design is found in thousands of a/c. It is possible there is a significant distinction between this type of 'plug' and a real 'plug door'. I cannot help wondering if the weight difference, plus the springs, is maybe significant.
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Old 21st Jan 2024, 09:54
  #1166 (permalink)  
 
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Originally Posted by pax2908
Of course, you mean, the general principle of a plug door design is found in thousands of a/c. It is possible there is a significant distinction between this type of 'plug' and a real 'plug door'.
Well the difference is that a door plug isn't a door, but that's probably not what you meant.

Around 11,000 Airbus narrow-bodies have been built, each with a minimum of 4 doors that use basically the same semi-plug design as the 900ER/Max9 MED, not to mention a number of previous types.
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Old 21st Jan 2024, 10:26
  #1167 (permalink)  
 
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There is a good seating plan of Alaska Ailines 737 -9 Max here https://www.aerolopa.com/as-7m9 as it shows the windows. I think the seat with the obvious torque must be 25A



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Old 21st Jan 2024, 10:54
  #1168 (permalink)  
 
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Springs

I 42 quote.. Something I have found odd about this plug design, given that it will hardly ever be opened, is why there is a need for springs in the hinges at all..
It is most likely an HSW 'Health and Safety at Work' issue, or OSHA in the States. Which limits the amount one person is allowed to lift to 25kg. So if the door weighs 100kg, then 75kg+ needs to be supported by the Springs.
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Old 21st Jan 2024, 11:16
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Originally Posted by scifi
I 42 quote.. Something I have found odd about this plug design, given that it will hardly ever be opened, is why there is a need for springs in the hinges at all..
It is most likely an HSW 'Health and Safety at Work' issue, or OSHA in the States. Which limits the amount one person is allowed to lift to 25kg. So if the door weighs 100kg, then 75kg+ needs to be supported by the Springs.
That doesn't sound quite right. The springs will only give 75kg of support for their upward extent of movement, possibly just a few inches. Thereafter it will need to be fully manhandled, or more likely removed with relevant tools. Which could doubtless have been designed to give the initial lift as well.

And I agree, I don't see the need for the aircraft to cart around these springs for years when they are only needed, possibly once only in the aircraft's life. when it is converted to high density, and a proper door is installed. Given manufacturers' considerable efforts to minimise every aspect of airframe empty weight, it seems strange.
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Old 21st Jan 2024, 11:48
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Originally Posted by WHBM
That doesn't sound quite right. The springs will only give 75kg of support for their upward extent of movement, possibly just a few inches. Thereafter it will need to be fully manhandled, or more likely removed with relevant tools. Which could doubtless have been designed to give the initial lift as well.

And I agree, I don't see the need for the aircraft to cart around these springs for years when they are only needed, possibly once only in the aircraft's life. when it is converted to high density, and a proper door is installed. Given manufacturers' considerable efforts to minimise every aspect of airframe empty weight, it seems strange.
This plug being light, would it need to be pushed Down first (with significant force) before the pins/bolts/stops can be properly fitted ?
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Old 21st Jan 2024, 12:45
  #1171 (permalink)  
 
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For what it's worth (coming from an SLF) the springs as a permanently installed installation aid has always seemed odd to me as well. I guess one possibility is they are there to force the pins against the retaining bolts, stopping the door plug from rattling around, but still allowing it to be easily forced off the stops when the fuselage flexes.

Of course for that to work, the spring force would need to be at least twice the weight of the door plug (probably more) or the springs would promote rattling rather than preventing it.
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Old 21st Jan 2024, 12:55
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As a former mechanical and structural engineer, based on the evidences we have to date, I want to share what is my feeling about what could have happened in this case.

My first assumption is that none of the 4 bolts was assembled between the plug and the airframe structure.

Without the vertical motion upward arrestors that are the bolts, the plug is vertically retained downward on :

∑ the 2 springs on the hinges that restrained some part of the plug weight (~40 of the 63 lbs) for 2 Inches of displacement.

∑ The horizontal part of upper rollers tracks where the roller sits for the remaining plug weight or load (~20 Lbs)

When the plane was flying on phases with none or low delta pressure between the inside and the outside of the plane, there is no or low loads on the pads.

The plug with the springs is like a single degree of freedom dynamic system with no or low damping with a weight of ~63 Lbs and an estimated stiffness of 40lbs/2inches : 20 lb force/inch (3500N/m),

The damping is due to the friction between plug and frame pads and roller on roller track.

This dynamic system could be moved by forces due to vertical acceleration in flight phases likes taxing, take off, landings where delta pressures are low or absent.

This system can move upward by a process of slide and stick progressive motion or by a single motion due to a shock and rebound of the roller on the upper side on the roller track.

If the motion if long enough, it could disengage the lug pads from the frame pads. Once the pads are disengaged, it will be near impossible they could move back on their usual position because there will be some delta pressure which avoid them to move downward.

Once the pads are disengaged, all the delta pressure loads (aprox 2400 Lbs/ delta pressure Psi) are taken by bottom of the roller track on the roller and by the lower hinges and the hinge bolt on the frame.

The plug, roller tracks, hinges have not be designed to sustain those loads. The plug will bend a lot with this 4 points at the two ends of the plug instead or the 12 well distributed and designed for pads.

Once disengaged, when the plane climbs, the delta pressure increases until the capability of the roller track bottom is reached and then broke.

Then the lug open and rotate very quickly (more than 100g) until the 2 opening arrestor cables broke also, the hinges after and then the plug is released away of the plane.
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Old 21st Jan 2024, 13:13
  #1173 (permalink)  
 
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Originally Posted by India Four Two
Something I have found odd about this plug design, given that it will hardly ever be opened, is why there is a need for springs in the hinges at all..
If the bolts aren't there the springs should prevent the door/plug from remaining closed, clearly demonstrating the bolts are missing. One would want a clear indicator that critical fasteners were missing.

Hence the mystery - what held the door/plug closed if the bolts weren't there?
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Old 21st Jan 2024, 13:41
  #1174 (permalink)  
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Thatís not how I understood it, though I may be wrong.
As far as I understood, the springs are called Lift Assist Springs and their intended function is exactly that, to help the Engineer lift the plug when required.
Iím not quite sure about the reason for their installation, except for ergonomyís sake, but have read they are part of what makes the entire ďsystemĒ certifiable in the same way as the actual emergency exit door. Not sure whether thatís actually correct or not, though.
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Old 21st Jan 2024, 15:29
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Originally Posted by pax2908
This plug being light, would it need to be pushed Down first (with significant force) before the pins/bolts/stops can be properly fitted ?
Many posts back the plug was stated to weigh 63 lbs. The lift assist springs clearly lift the plug (photos in previous posts) so they exert more than 63 lbs (otherwise they would just sit in the force balanced condition) - You could guess that they might exert something like 70 - 80 lbs as much more would make the plug difficult to re-close - especially if having pulled it down with one hand and one foot on the lower cross member the next thing is to insert a bolt somewhere with your free hand.

Last edited by Europa01; 21st Jan 2024 at 15:57.
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Old 21st Jan 2024, 18:01
  #1176 (permalink)  
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Originally Posted by DaveReidUK
What's your source for saying that they now know ?

The Reuters article quoted a couple of posts previously said "So far the investigation has not been able to establish whether the door plug was outfitted with the four bolts that prevent it from vertical movement".
Source? me.
If the bolts were present they will have leaved very persuasive evidence that they were, they wouldnt shear without first elongating holes or otherwise marking or deforming where they were affixed. this is something that can be determined very quickly. they must know one way or the other..shirley?
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Old 21st Jan 2024, 18:13
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Originally Posted by MechEngr
If the bolts aren't there the springs should prevent the door/plug from remaining closed, clearly demonstrating the bolts are missing. One would want a clear indicator that critical fasteners were missing.
Hence the mystery - what held the door/plug closed if the bolts weren't there?
An excellent point. I can't believe that four bolts were fitted without nuts or without cotter pins in the nuts and that they all fell out. I can only guess that one or more temporary hole fillers were fitted with the intention of fitting the correct bolts later. Evidently, to amplify your point, something restrained the springs.
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Old 21st Jan 2024, 18:40
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Spirit AeroSystems; the "Flawless Factory" video:

5 years old.

I wonder when the comments were turned off.
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Old 21st Jan 2024, 19:21
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Originally Posted by 639
Source? me.
If the bolts were present they will have leaved very persuasive evidence that they were, they wouldn't shear without first elongating holes or otherwise marking or deforming where they were affixed. this is something that can be determined very quickly. they must know one way or the other..shirley?
OK. I won't ask your source for the assertion that the bolts sheared.
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Old 21st Jan 2024, 19:39
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Originally Posted by JapanHanuma
A combination of the structural flexing of the aircraft and the lack of bolts might also be the answer. It's usually a multitude of factors leading to these incidents because numerous failsafe measures are usually implemented, but it would be very bad for the Boeing 737 brand.
Originally Posted by DaveReidUK
It's the old problem with proving a negative. Visible witness marks would prove that the bolts were installed at some stage. Lack of witness marks wouldn't necessarily prove that they weren't.
Generally true. However, th compelling evidence that exists already is a part of an aircraft sitting in a garden without bolts in bolt holes. Had the failure mode not involved the bolts at all, they would be proudly in place claiming their innocence. Instead of having the bolts occupying their rightful abode, they are confronted with the mute testimony of the bolt holes, exposed to public view. Sans bolts. The force provided by the springs against the plug mass needs to be less than the plug mass.g, otherwise it is a jack-in-a-box, and would not close without additional restraint being applied. the missing bolts are logically the proof that they were missing at 16,000' some short time earlier. These bolts have substantial shear strength, above 5450psi each, 4 bolts, differential pressure does not result in a vertical force, it reacts to the face plates of the stops, the only vertical loads coming from the spring compression, and the plug times applied vertical g force.

the bolts aren't there as they never were.
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