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

[EXDAC]

Originally Posted by fdr

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.

How do you explain the photo (posted early and included in the linked 737 door videos) of an open door plug supported only by the wire tethers and the bottom hinges. That photo shows the springs are fully extended and the sliders are on the stops. Are you saying the plug is being held in that position by some force other than that provided by the springs?

Jack did indeed leave the box.

[Europa01]

Bolts and Stops

Originally Posted by fdr

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.

Agreed - the bolts could not have been sheared (unless they were made of toffee) or now I think of it perhaps some unsuitable temporary sprag which someone forgot to remove could have been substituted. I agree the comment about the stops and by inference the stop pins BUT I think so far we have all made the assumption that they were correctly adjusted. What if they weren’t? Supposing the gaps were too big - this would lead to some or all of the load being taken by the guide pins and tracks.

Comment/criticise/shoot down in flames

[DaveReidUK]

Originally Posted by fdr

the bolts aren't there as they never were.

Well they certainly weren't in their intended location when the door plug departed ...

[SLF3]

Originally Posted by DaveReidUK

Well they certainly weren't in their intended location when the door plug departed ...

i think the most likely scenario is the bolts were installed but were subsequently removed and the plug opened for access. Possibly following a work method (official or unofficial) adopted on other Max models on the same line. The bolts were then either not replaced (most likely) or not replaced correctly.

This explains defects on other doors. The NTSB can likely see the bolts were there at some stage but not when.

given there is no real load on the bolts in service they are very unlikely to fail. They either weren’t there or ‘fell out’.

[DaveReidUK]

Originally Posted by SLF3

i think the most likely scenario is the bolts were installed but were subsequently removed and the plug opened for access. Possibly following a work method (official or unofficial) adopted on other Max models on the same line. The bolts were then either not replaced (most likely) or not replaced correctly.

Assuming that you're referring to work done at Boeing, rather than once the aircraft had been delivered, then your "most likely" scenario would mean that the aircraft operated 151 flights with a door plug that could have departed at any moment.

That would mean upwards of 20,000 passengers have been extremely lucky ...

[incompleteness]

Originally Posted by chrisl137

They may have it a little easier than that. They've verified that the starboard side plug was correctly installed with all the fasteners. It's the same age and has been through as close to an identical environment as you can get. They can pull that plug and first inspect the edges of the holes, then slice open the brackets and inspect the inside of the holes, giving them a control sample of what possible witness marks there could be in a plug of that age and history. The difficulty is that in normal operation the retaining bolts don't see more force than the weight of the plug, and during flight the door is locked hard against the stops, but it might chatter enough during taxi to leave marks on the paint, so they may not be prone to leaving a lot of marks when correctly installed.

There is one terrible possibility that I'm not sure how they are going to address, and that is: what if the bolts were installed but they were bogus parts? Made from some very low strength, low ductility material that just sheared under the force of normal operation (spring forces, fuse deformation) without leaving much in the way of marks? I guess they will try to find the lot # of the bolts and test bolts from that lot or at least review receiving testing. But in some ways it might have to remain a very unlikely but disturbing possibility.

They might be able to show that even the worst possible incorrect material/heat treat would have been enough to hold the door in position, hopefully so. I feel they will have to address that somehow because bogus parts are a legitimate concern.

[fdr]

Originally Posted by ozaub

Way back I insisted that thrust of assist springs must exceed weight of door, so that door engaged positively with rollers when closed. Any other arrangement is quite illogical.

I postulated spring force being 80% of door weight. That's when vertical.

When such a door is opened 15 deg its resolved force against the spring is reduced by 24% and springs extend. QED.

Can you revisit the first part of your statement, the latter part I do agree with.

[fdr]

Originally Posted by incompleteness

There is one terrible possibility that I'm not sure how they are going to address, and that is: what if the bolts were installed but they were bogus parts? Made from some very low strength, low ductility material that just sheared under the force of normal operation (spring forces, fuse deformation) without leaving much in the way of marks? I guess they will try to find the lot # of the bolts and test bolts from that lot or at least review receiving testing. But in some ways it might have to remain a very unlikely but disturbing possibility.

They might be able to show that even the worst possible incorrect material/heat treat would have been enough to hold the door in position, hopefully so. I feel they will have to address that somehow because bogus parts are a legitimate concern.

The loads on the locking bolt are negligible. The force of pressurisation is normal to the panel, and that exerts no load to the bolts unless the panel distorts, and to do that, the stop plates would also need to distort. That is a vanilla flavour structure, my own jets have the same door stop structure, so not sure that there is that much load that would arise that would cause a soda straw to crush if used as a pin... The forces are just not there. The bolts used as pins have pretty severe shear load ratings, they would melt in the sun and leave a chocolate stain to be so open to failure. Draw a force diagram, and work out where the 63 lb door, with a 2300 lb total force is going to give a force that is normal to any of the pins (bolts). If the whole structure was distorting sufficiently under pressurisation to give any appreciable load on the pins, I would suggest to drive on your next vacation, as the stop plate design is common to almost all doors in some form or other, and should not result in a lateral load across their face that would give a shear load on the pins.

Even the upper guide that the upper rollers run inside does not have a high load on it when in the closed position, it is a guide. Once the loads on the face of the stops is eased by not being pressurised, there is only around 20-25 lbs of additional force needed to let the door slide across the stop faces, guided by the guide until the roller is free of the guide, at which point the plug can rotate outboard on it's lower hinges. The loads on this just aren't that high. If 25/2 lbs of load across an AN bolt will cause it to fail, then I'm definitely going to be walking rather than flying. Suggest the bolt isn't there as it never was, and it sat dutifully for a period with about a 20-25lb margin to its release, which is about -0.3g increment to normal g of 1.0, look for the last time the aircraft had a 0.7g load, and that is possibly when this started to get ready to depart the fix, exit stage right etc. As this is a station behind the CG, and the accelerometers of the B737 are forward of the center tank (EEC? I forget, Boeing uses different places and it does matter in investigation) then pitch rates and structural flex to a point load such as gear touchdown will give a change to the g at this longitudinal station.

Occam was an optimist.

[ozaub]

Way back I questioned ethics and legality of deliberately blocking escape routes. Imagine public outrage if any aircraft with plugged emergency exits is ever involved in an evacuation.

An article by Prof. Amy Fraher echoes my concerns - https://theconversation.com/why-did-...o-money-221263 .

Forty years ago when Boeing and BA dreamed up the idea of blocking EEs on the 747 it caused such consternation that FAA convened a Public Technical Conference on Emergency Evacuation and subsequently an Emergency Evacuation Task Force. Proceedings are in U.S. Department of Transportation (DOT) FAA Report Number DOT/FAA/VS-86/1, Task Force Report on Emergency Evacuation of Transport Airplanes, Volumes I and II.

Can’t find on FAA site but tracked it down at https://apps.dtic.mil/sti/pdfs/ADA171641.pdf

In short what BA did was legal but rules were tightened to specify max distance between exits as 60 ft, and that available exits are to be as evenly distributed as practical. Clearly plugging mid-cabin EEs on MAX9 does not comply with latter.

[fdr]

Originally Posted by Europa01

Agreed - the bolts could not have been sheared (unless they were made of toffee) or now I think of it perhaps some unsuitable temporary sprag which someone forgot to remove could have been substituted. I agree the comment about the stops and by inference the stop pins BUT I think so far we have all made the assumption that they were correctly adjusted. What if they weren’t? Supposing the gaps were too big - this would lead to some or all of the load being taken by the guide pins and tracks.

Comment/criticise/shoot down in flames

Good points. No criticism of flaming of that. If the stops are not adjusted, have excessive clearance, then that would result in ultimately a load being carried on the only item that remains, the guide forging that is attached to the door. That would have a point load from the roller on the forging face towards the plug proper, (facing outwards) and that would certainly have biggly witness marks on it, the load would be around 1100lbs exerted over a moment arm of 1/2 door height... say, 3', that is a good load. The load would not appear to give vectors towards release of the guide from its pin, although that would need a detail look at the geometry to verify. That force against a small surface on aluminium will leave a mark I would hazard to guess.

[remi]

Originally Posted by DaveReidUK

Well they certainly weren't in their intended location when the door plug departed ...

I wouldn't rule out scenarios where the door was loosened or translated, such as by forces on the frame, so that it came off of the pads enough for pressurization forces to shear the bolts.

That seems unlikely, but I wouldn't say "certainly" at this point.

I'm sure they have already measured the door to see if it has been deformed to a significant extent, but if that was the case, there would be additional questions: Did that happen before and/or during the blowout, or did it happen when it landed on the ground, or all of the above?

They should have good metallurgical evidence fairly soon one way or another about the bolts and where they were at the time the plug blew out.

[remi]

New from FAA:

Sunday, January 21, 2024

This information is preliminary and subject to change.

As an added layer of safety, the Federal Aviation Administration (FAA) is recommending that operators of Boeing 737-900ER aircraft visually inspect mid-exit door plugs to ensure the door is properly secured. The Boeing 737-900ER is not part of the newer MAX fleet but has the same door plug design.

In accordance with their Safety Management Systems, operators conducted additional inspections on the Boeing 737-900ER following the loss of a mid-cabin door plug on a Boeing 737-9 MAX airplane on January 5th.

Read the Safety Alert for Operators (SAFO) regarding the Boeing 737-900ER here.

https://www.faa.gov/newsroom/updates...max-9-aircraft

SAFO (not populated at this moment): https://www.faa.gov/media/74751

[chrisl137]

Originally Posted by remi

I wouldn't rule out scenarios where the door was loosened or translated, such as by forces on the frame, so that it came off of the pads enough for pressurization forces to shear the bolts.

That seems unlikely, but I wouldn't say "certainly" at this point.

I'm sure they have already measured the door to see if it has been deformed to a significant extent, but if that was the case, there would be additional questions: Did that happen before and/or during the blowout, or did it happen when it landed on the ground, or all of the above?

They should have good metallurgical evidence fairly soon one way or another about the bolts and where they were at the time the plug blew out.

What the NTSB reported in their last briefing that's on YouTube was that it was clear that the plug translated up and cleared the stops before blowing out and fracturing the roller guides. The roller guides are taller than the stops, so once misaligned, the bottom of the guide is all that was holding the plug. This is consistent with all the photos we've seen of both the opening and the plug - there's no obvious structural damage to the stops that bear the pressure load on either side of the interface. The 4 retaining bolts don't see any significant load if they're installed in the positions where they actually retain the door aligned with the stops (post #1192 just a few posts up details this). And if they're aligned, the stops can't clear the pads to expose the bolts to the pressure load that would shear them. A soda straw being strong enough is a slight exaggeration, but only slight. Probably a cheap screwdriver through one hole is enough to hold it aligned while the first of the bolts are installed.

So it's hard to see any way that the retaining bolts could have had enough load to shear even one of them, let alone 4 of them, and do it without leaving substantial evidence on the plug parts that did the shearing. Improper installation (i.e. missing or improperly installed nuts and/or cotter pins) would keep the plug in place for a while until the nuts (if present) fell of and the bolts (if present) eventually fell out from vibration. It might be possible for the bolts to do this without leaving evidence they'd ever been there.

[remi]

Originally Posted by chrisl137

What the NTSB reported in their last briefing that's on YouTube was that it was clear that the plug translated up and cleared the stops before blowing out and fracturing the roller guides. The roller guides are taller than the stops, so once misaligned, the bottom of the guide is all that was holding the plug. This is consistent with all the photos we've seen of both the opening and the plug - there's no obvious structural damage to the stops that bear the pressure load on either side of the interface. The 4 retaining bolts don't see any significant load if they're installed in the positions where they actually retain the door aligned with the stops (post #1192 just a few posts up details this). And if they're aligned, the stops can't clear the pads to expose the bolts to the pressure load that would shear them. A soda straw being strong enough is a slight exaggeration, but only slight. Probably a cheap screwdriver through one hole is enough to hold it aligned while the first of the bolts are installed.

So it's hard to see any way that the retaining bolts could have had enough load to shear even one of them, let alone 4 of them, and do it without leaving substantial evidence on the plug parts that did the shearing. Improper installation (i.e. missing or improperly installed nuts and/or cotter pins) would keep the plug in place for a while until the nuts (if present) fell of and the bolts (if present) eventually fell out from vibration. It might be possible for the bolts to do this without leaving evidence they'd ever been there.

Right, but as I suggested (trying to find a source for this but a few minutes of looking hasn't turned it up) NTSB might be trying to eliminate the presumably very unlikely scenario that there is distortion of the plug opening allowing it to become dislodged enough to blow out. Having investigated problems in extremely complex systems myself (in the cloud services world), I've found that the surest way to be wrong is to report the obvious problem. Investigations have to proceed from indisputable facts to a verifiable cause. The relevant facts may be non-obvious and distant from the observed anomalies.

I agree that it wasn't bolted at all, or the bolts were loose and unsecured, is the likely scenario. Boeing's corporate statement that it was a "quality escape" must be based on fact, as Boeing is not allowed to make statements of hypothesis during the NTSB investigation. But all that means that Boeing agrees that there was a quality escape. It doesn't say that the quality escape was the cause (Boeing can't say that) or detail the nature of the quality escape (Boeing could do that, but hasn't).

[remi]

SAFO is up, indicating findings on 737-900ER.

My emphasis added.

https://www.faa.gov/media/74751

Subject: Boeing 737-900ER Mid-Cabin Door Plug Inspection.

Purpose: This SAFO informs aircraft operators under Title 14 of the Code of Federal Regulations (14
CFR) Parts 91, 91 subpart K (Part 91K), 121, 125, 129, and 135 about information pertaining to the
Boeing 737-900ER fuselage plug assembly. This SAFO also recommends that operators perform key
portions of the Boeing 737-900ER Fuselage Plug Assembly Maintenance Planning Document (MPD)
Inspection Tasks, related to the 4 locations where a bolt/nut/pin installation is used to secure the door to
the airframe, as soon as possible. This information is applicable to all operators under 14 CFR that
operate the Boeing 737-900ER aircraft. The Federal Aviation Administration (FAA) continues to
evaluate data involving the mid-cabin door plug and will determine additional action as necessary.

Background: Following the in-flight loss of a mid-cabin door plug on a Boeing 737-9 MAX airplane, on
January 5, 2024, the FAA began collecting data on the incident airplane and related airplanes to scope the
applicability of any action to mitigate risk to the fleet. On January 6, 2024, the FAA determined an unsafe
condition existed with the Boeing 737-9 MAX and an Emergency Airworthiness Directive (EAD) was
warranted.

On January 6, 2024, the FAA issued an EAD (AD number 2024-02-51) to address the potential in-flight
loss of a mid-cabin door plug on Boeing 737-9 airplanes. The FAA also issued a Continued Airworthiness
Notification International Community (CANIC) providing notice of pending significant safety action.

Discussion: The Boeing 737-900ER mid-exit door plugs have an identical door plug design to the 737-9
MAX. As part of their Safety Management Systems, some operators have conducted additional inspections
on the 737-900ER mid-exit door plugs and have noted findings with bolts during the maintenance
inspections.

Recommended Action: Operators are encouraged to conduct a visual inspection to ensure the door
plug is restrained from any movements through the two (2) upper guide track bolts and two (2) lower
arrestor bolts. Please refer to the Aircraft Maintenance Manual and 737-900ER Fuselage Plug Assembly
Maintenance Planning Document (MPD) for more information, regardless of if this inspection has been
conducted under the existing maintenance program prior to the EAD. As Operators conduct the visual
inspections, they are also encouraged to report findings to their certificate management office.

[chrisl137]

Originally Posted by fdr

Good points. No criticism of flaming of that. If the stops are not adjusted, have excessive clearance, then that would result in ultimately a load being carried on the only item that remains, the guide forging that is attached to the door. That would have a point load from the roller on the forging face towards the plug proper, (facing outwards) and that would certainly have biggly witness marks on it, the load would be around 1100lbs exerted over a moment arm of 1/2 door height... say, 3', that is a good load. The load would not appear to give vectors towards release of the guide from its pin, although that would need a detail look at the geometry to verify. That force against a small surface on aluminium will leave a mark I would hazard to guess.

As long as the plug were still in vertical alignment, the load would transfer to the stops after deforming the guides and hinge rods (or the bolts), possibly resulting in some deformation of the door to make it fit the mis-adjusted locations of the stops. If they're way out, you might get enough deformation to let some wind in and peel the plug off. I suspect anything like that would leave measurable deformation on the door, and maybe bent the hinge rods back. The way the hinge rods seem to be pointing straight out really seems like the door was propelled. It was also found pretty far north of the flight path, which might also indicate shot out rather than peeled off.

Originally Posted by remi

Right, but as I suggested (trying to find a source for this but a few minutes of looking hasn't turned it up) NTSB might be trying to eliminate the presumably very unlikely scenario that there is distortion of the plug opening allowing it to become dislodged enough to blow out. Having investigated problems in extremely complex systems myself (in the cloud services world), I've found that the surest way to be wrong is to report the obvious problem. Investigations have to proceed from indisputable facts to a verifiable cause. The relevant facts may be non-obvious and distant from the observed anomalies.

I agree that it wasn't bolted at all, or the bolts were loose and unsecured, is the likely scenario. Boeing's corporate statement that it was a "quality escape" must be based on fact, as Boeing is not allowed to make statements of hypothesis during the NTSB investigation. But all that means that Boeing agrees that there was a quality escape. It doesn't say that the quality escape was the cause (Boeing can't say that) or detail the nature of the quality escape (Boeing could do that, but hasn't).

That's a tough story to create without leaving a lot of other evidence
- If the hull is flexing enough to open up the hole sideways, a) it's going to leave some creased sheet metal (think oilcan), and b) the retaining bolts would still be in the bottom of the guide slots in the plug, castle nut, cotter pin, and all.
- For the hull to expand in the circumference to let the stop slip off, you need the hole to get bigger by enough that at least one set of stops moves by at least 1.5" out of about 60" (or less!). that's a huge change in size, and again, you're going to see effects of that in the metal around the hole. And if you want something to flex locally that much, rather than distribute it over the whole, it means that much more bent metal in the hole.
It's structurally a hard story to reconcile with the state of all the pieces.

[A0283]

Originally Posted by fdr

Good points. No criticism of flaming of that. If the stops are not adjusted, have excessive clearance, then that would result in ultimately a load being carried on the only item that remains, the guide forging that is attached to the door. That would have a point load from the roller on the forging face towards the plug proper, (facing outwards) and that would certainly have biggly witness marks on it, the load would be around 1100lbs exerted over a moment arm of 1/2 door height... say, 3', that is a good load. The load would not appear to give vectors towards release of the guide from its pin, although that would need a detail look at the geometry to verify. That force against a small surface on aluminium will leave a mark I would hazard to guess.

Could we say this design might be too sensitive to proper adjustment?

Is there only a ‘pure blow out’ scenario, or is there also a ‘tear out’ scenario … where improper adjustment leads to deformation or partial opening, and then aerodynamic forces tearing the door out, or assisting in deformation, which can then lead to a ‘deformed blow out’? Each scenario leaving its specific witness marks.

[D Bru]

Originally Posted by remi

SAFO is up, indicating findings on 737-900ER.

My emphasis added.

https://www.faa.gov/media/74751

Finally, the -900ER's produced between 2007-2019 with the same door plugs also get the attention they deserve. And apparently some airlines that had started inspections on their own initiative have found issues. This is contrary to "sources" of Reuters, reporting on 10 Jan that "Boeing has checked the service records of earlier Boeing 737-900ER aircraft that had a similar door plug, but all have undergone extensive maintenance since being delivered and none has shown a sign of similar problems, the sources said".

The significance is in the number of -900ER's: out of the about 520 -900ER's in service worldwide, some 420 have the same door plugs as the 171 grounded MAX-9's. Of those 420, just under 390 are US-operated (Alaska (79), Delta (172) and United (136)), the other 30-something are registered to Turkish (15), El-Al (8), Korean (6), Ukraine Intl, Windrose and Global Jet Luxembourg (BBJ).

[639]

Originally Posted by DaveReidUK

OK. I won't ask your source for the assertion that the bolts sheared.

I didnt assert that at all, so what other mechanism do you suggest is possible that there were actually bolts present, that they didnt stop the door coming off yet the NTSB are still not sure if they were present
tore out of the holes?

I think you know what I meant, whatever mechanism, the question of whether or not the bolts were present or not should by now be known (source me)

[soarbum]

Originally Posted by ozaub

<br />When such a door is opened 15 deg its resolved force against the spring is reduced by 24% and springs extend. QED.

I'm struggling to figure out how you justify that statement considering that Cos(15deg) = 0.966

ps: more realistically, if the CoG of the door plug in the closed position is of the order of 15 degrees angle from the hinge point (guess) and opening it by another 15 degrees brought it out to 30, the effective weight reduction would be (1 - 0.866/0.966) or around 10%.