Alaska Airlines 737-900 MAX loses a door in-flight out of PDX
Lift Assist Spring Force
Most loads are accelerating the airframe up, pushing the door down, harder against the springs. At 16,000 feet they had 5 psi delta; about 4000 lbf against the stops; far more than mild turbulence. If the friction is as low as 0.1, that is 400 pounds to move the door, over 10gs.
The damage to the stops is what happens if the door moves up far enough that it cannot come back down because of the step between the hardened contact and the rest of the bracket. That would leave a small gap at the bottom or top of the door that might let the pressurized air leak out until the pressure controller dumped enough to pop it into full contact. This is another feature I was hoping for in the report - the exact configuration of the door seals.
The damage to the stops is what happens if the door moves up far enough that it cannot come back down because of the step between the hardened contact and the rest of the bracket. That would leave a small gap at the bottom or top of the door that might let the pressurized air leak out until the pressure controller dumped enough to pop it into full contact. This is another feature I was hoping for in the report - the exact configuration of the door seals.
I still haven't got how the unsecured door managed to stay in place for some 150 pressurised flights before being blown out. Why didn't it go out on the first test flight.
There is still no doubt in my mind that a single temporary pin was keeping the plug closed. Doesn't anyone else wonder what was hidden under the insulation blanket covering the aft upper corner of the plug?
Last edited by EXDAC; 7th Feb 2024 at 11:50.
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…also interesting to see figure 15 which shows the door opened much further than 15 degrees, so not using the steel cable (strap assy), … not immediately clear if it was supported on the outside or just supported by the hinges with maybe something preventing it from going the full 90…?… is there a kind of support visible through the window behind the ‘ funny face ‘ … ?
Opening wider would also trigger the question if the seal could or shall be replaced … which may have a relation with why it stuck 151 flights…
Opening wider would also trigger the question what kind of pressure testing should or shall follow … which also may have a relation with why it stuck 151 flights…
You would expect NTSB to have sketched an outline of the basic procedure by now and indicate which part was covered by evidence at this stage.
Normally it would now stay quiet for one year to final or interim report. But you wonder if in this case earlier findings should be reported full width to Boeing and Spirit and airlines and (international) industry. Cultural deficiencies should be handled differently from more focussed cases.
Lift Assist Spring Force
What do you suppose holds the door up with the hinge slides extended to their stops when the plug is open? Much earlier in this discussion there seemed to be general agreement that it was held up by the springs.
There is still no doubt in my mind that a single temporary pin was keeping the plug closed. Doesn't anyone else wonder what was hidden under the insulation blanket covering the aft upper corner of the plug?
There is still no doubt in my mind that a single temporary pin was keeping the plug closed. Doesn't anyone else wonder what was hidden under the insulation blanket covering the aft upper corner of the plug?
Overall, the observed damage patterns and absence of contact damage or deformation around holes associated with the vertical movement arrestor bolts and upper guide track bolts in the upper guide fittings, hinge fittings, and recovered aft lower hinge guide fitting indicate that the four bolts that prevent upward movement of the MED plug were missing before the MED plug moved upward off the stop pads.
If there were no bolts then the ‘Boeing email’ photo shows that the springs do not exert enough force to lift the door/plug (at 1g) even depressurised. Although we not have any figures, my guess is that ‘lift assist’ means just that and with the door/plug open they are exerting just about the force balanced condition. I believe they are only claimed to stop the door from falling back.
Looking at the NTSB door on the bench photo there seems to be damage to the forward floor hinge bracket - spring pillar over-rotation during ejection? and there seems to be damage to the aft lower stop pad. Views?
If Tex Johnston was the acceptance flight pilot there wouldn't have been the same bother.
Last edited by Thrust Augmentation; 7th Feb 2024 at 13:23.
That's the accident aircraft and, if I'm not mistaken, that looks to me (and my source) like a zip tie around one of the stops on the forward edge of the door. If so, that could have been what kept the door in place until either the tie snapped or it worked its way off one or other of the elements of the stop.
I'll admit to being very surprised that there's no reference to it in the NTSB report.
Lift Assist Springs
I agree - your spring calcs must be in the right ball park. Now we can see that the lift assist force is not enough to lift the door/plug even depressurised. Is the likely reason that closing the door to the point where the pins / pads engage compresses the seal sufficiently to generate an outward force which keeps them engaged? Photos show that the pins are convex and the pads are concave giving a spherical type seating so this might play a part.
In my earlier post #1211 of an analysis based on Chris Brady's video of the mechanics of how the door closes and opens, I'd said this:
"The door is pulled inwards until the hinges are closed – the door is now more or less flush with, and just inside, the line of the fuselage.
All the weight of the door is resting on the lift assist springs. The springs are fully extended with the hinge guide fittings in their maximum “up” position, hard up against the 2 locked nuts at the top of the hinge and spring assembly."
I now refer to the NTSB Report Fig 15 showing a photo of a door in a partly open state. If you zoom in to the visible bottom right of the door you can see the hinge assembly and the hinge guide fittings - they are hard up against the double lock nuts and washer. I accept that because of the angle of the door the entire door weight isn't being taken by the springs, but for me the likelihood is that the door would indeed be held at its maximum "up" position.
I give another reason for this to be the case. With the door held fully up by the springs there is a certainty that as the door is pulled closed the 12 stop fittings will pass over the top of their mating stop pads. This surely is an important feature, because if during closing the stop fittings impacted their stop pads (because the springs only held the door partly up its travel) there is obviously a possibility of damage to one or the other or both. Should this happen, but went unnoticed, unreported or uninspected there is a chance of such damage to critical primary structure causing door problems in the future.
I’m quoting a quote here but the NTSB preliminary says:-
Overall, the observed damage patterns and absence of contact damage or deformation around holes associated with the vertical movement arrestor bolts and upper guide track bolts in the upper guide fittings, hinge fittings, and recovered aft lower hinge guide fitting indicate that the four bolts that prevent upward movement of the MED plug were missing before the MED plug moved upward off the stop pads.
Overall, the observed damage patterns and absence of contact damage or deformation around holes associated with the vertical movement arrestor bolts and upper guide track bolts in the upper guide fittings, hinge fittings, and recovered aft lower hinge guide fitting indicate that the four bolts that prevent upward movement of the MED plug were missing before the MED plug moved upward off the stop pads.
Last edited by EXDAC; 7th Feb 2024 at 14:24.
Just a thought.
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That cable tie must surely be an improvised way of keeping the door down while the bolts were ftitted. I can see why it might be used. Would be OK if it had been official, numbered and brightly coloured with a large tag to make sure it was never left on. It would certainly explain how the door stayed secure for so long.
That cable tie must surely be an improvised way of keeping the door down while the bolts were ftitted. I can see why it might be used. Would be OK if it had been official, numbered and brightly coloured with a large tag to make sure it was never left on. It would certainly explain how the door stayed secure for so long.
I can't take any credit for this, as it's been pointed out to me privately by a forumite, but this is an interesting photo (P17, Figure 16) from the NTSB report (my blow-up):
That's the accident aircraft and, if I'm not mistaken, that looks to me (and my source) like a zip tie around one of the stops on the forward edge of the door. If so, that could have been what kept the door in place until either the tie snapped or it worked its way off one or other of the elements of the stop.
I'll admit to being very surprised that there's no reference to it in the NTSB report.
That's the accident aircraft and, if I'm not mistaken, that looks to me (and my source) like a zip tie around one of the stops on the forward edge of the door. If so, that could have been what kept the door in place until either the tie snapped or it worked its way off one or other of the elements of the stop.
I'll admit to being very surprised that there's no reference to it in the NTSB report.
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Looking on Fig.13 from the NTSB report, then the wear of the green paint in the vertical hole in the hinge guide indicates that the door plug must have been moving up and down quite a lot of times.
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Did trim secure the plug door?
As non-aviator, non-engineer SLF, I am reluctant suggest a significant issue has eluded us all, but ...
There has been copious discussion of the relative merits of plug door weight, spring strength, g-force variations, seal rigidity/stickiness, and pressure differential as the force(s) that kept the plug door in place for as long as it stayed (mostly) put. I have not seen any suggestion that interior trim might provide significant (perhaps decisive) force preventing the plug door from rising from its correct position under normal circumstances.
Comparing the two windows visible in fig. 16, p. 17, suggests to me that the edge of the outer lip around the window trim is sufficiently square that, if the interior trim panel has inverse geometry and the panel is engineered to press outward around the window trim (to avoid unsightly and dirt-catching gaps between interior and window trim), and presuming upward movement of interior panels is constrained (a longitudinal bracket retaining panel top edge and/or overhead compartment floor?), could that not provide sufficient force to almost always hold the plug door down without any other retention mechanism? Then, when the aircraft hits a heck of a downward bump at the same time it has a Goldilocks ("just right") pressure differential -- not enough to strongly press stop fittings to stop pads, but enough to bulge the plug window outboard just a bit -- the trim lips disengage and the plug door with its window slides upward and ... we know the rest.
There has been copious discussion of the relative merits of plug door weight, spring strength, g-force variations, seal rigidity/stickiness, and pressure differential as the force(s) that kept the plug door in place for as long as it stayed (mostly) put. I have not seen any suggestion that interior trim might provide significant (perhaps decisive) force preventing the plug door from rising from its correct position under normal circumstances.
Comparing the two windows visible in fig. 16, p. 17, suggests to me that the edge of the outer lip around the window trim is sufficiently square that, if the interior trim panel has inverse geometry and the panel is engineered to press outward around the window trim (to avoid unsightly and dirt-catching gaps between interior and window trim), and presuming upward movement of interior panels is constrained (a longitudinal bracket retaining panel top edge and/or overhead compartment floor?), could that not provide sufficient force to almost always hold the plug door down without any other retention mechanism? Then, when the aircraft hits a heck of a downward bump at the same time it has a Goldilocks ("just right") pressure differential -- not enough to strongly press stop fittings to stop pads, but enough to bulge the plug window outboard just a bit -- the trim lips disengage and the plug door with its window slides upward and ... we know the rest.
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Part of the Figure 16 caption: "These Boeing personnel were discussing interior restoration after the rivet rework was completed during second shift operations that day."
The Figure 16 combined with the Leeham commenter's account paint a compelling sequence.
Spirit does the second rivet fix, the seal fix, etc., but most of the details are only in SAT. A shift turnover happens, and then Boeing employees come to see an apparently seated door ready for closing up the paneling. They don't look for bolts, because why would they? They're not QA, and there's no record in CMES that the door was even opened.
The Figure 16 combined with the Leeham commenter's account paint a compelling sequence.
Spirit does the second rivet fix, the seal fix, etc., but most of the details are only in SAT. A shift turnover happens, and then Boeing employees come to see an apparently seated door ready for closing up the paneling. They don't look for bolts, because why would they? They're not QA, and there's no record in CMES that the door was even opened.
In this case the stop washer at the top of the hinge guide was pulled through the hinge guide bracket and tore out the bushing. It was probably responsible for the paint damage.
Cable Tie
I can’t agree - surely Boeing’s quality assurance required another cable tie on the other side of the door.
Seriously, if this turns out to be the case you can only imagine the press headlines.
Seriously, if this turns out to be the case you can only imagine the press headlines.
Last edited by Europa01; 7th Feb 2024 at 16:29. Reason: Tweak