Alaska Airlines 737-900 MAX loses a door in-flight out of PDX
1) I was referring to the plug. The weight of the plug door is obviously supported by its springs. The weight of the E/E is designed to be supported by its springs, according to Chris Brady.
2) Of course the E/E door weight is significantly greater than the plug
3) We don't know if the springs for the E/E door are the same as those used for the plug, or if they have different part numbers and compression forces fit for purpose. You have stated that previously and I agree.
But there is absolutely no doubt the springs used for the plug support the weight of the plug. And as I said, according to Chris Brady, the design intent for the spring hinges, their reason to exist, is to prevent re-closing of the E/E door in the event of evacuation.
2) Of course the E/E door weight is significantly greater than the plug
3) We don't know if the springs for the E/E door are the same as those used for the plug, or if they have different part numbers and compression forces fit for purpose. You have stated that previously and I agree.
But there is absolutely no doubt the springs used for the plug support the weight of the plug. And as I said, according to Chris Brady, the design intent for the spring hinges, their reason to exist, is to prevent re-closing of the E/E door in the event of evacuation.
So my understanding is that the people from the manufacturers that signed things off are DAR-Fs… these people have a long history with the company in manufacturing, but are not A&Ps or certainly not IAs. They often did 15+ years as riveters, but have no clue as to how an aircraft comes together otherwise.
I have no compunction about flying on Boeing Aircraft, and have no animosity toward them (except that they have had my 421 520-D cylinder set on hold for 17 months due to their production issues {happy to speak with somebody there}).
I have no compunction about flying on Boeing Aircraft, and have no animosity toward them (except that they have had my 421 520-D cylinder set on hold for 17 months due to their production issues {happy to speak with somebody there}).
Irrelevant discussion about door spring strength
All this to-and-fro about whether the lift-assist springs exert a force greater than or less than the weight of the door only applies on the ground or in level (more precisely 1g) flight. It wouldn't take much of an aerodynamic "bump" considerably to reduce the weight of the door, allowing it to translate upwards without the whole weight being supported by the springs.
I still don’t understand the design decision of keeping the lift-assist spring from the evacuation door in the door plug. Seems like an overly complex solution for a problem that, for this particular purpose, doesn’t even exist (preventing an opened door plug from closing). If any type of spring was required here to begin with, shouldn’t that have been a pulling string (preventing the door plug from opening)? Why lessen the force required to open a door plug?
All this to-and-fro about whether the lift-assist springs exert a force greater than or less than the weight of the door only applies on the ground or in level (more precisely 1g) flight. It wouldn't take much of an aerodynamic "bump" considerably to reduce the weight of the door, allowing it to translate upwards without the whole weight being supported by the springs.
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I still don’t understand the design decision of keeping the lift-assist spring from the evacuation door in the door plug. Seems like an overly complex solution for a problem that, for this particular purpose, doesn’t even exist (preventing an opened door plug from closing). If any type of spring was required here to begin with, shouldn’t that have been a pulling string (preventing the door plug from opening)? Why lessen the force required to open a door plug?
So we know that Boeing carefully selected the plug assist spring just for the task of keeping the stop fitting and pins from falling behind the stop pads when the plug is opened. So it’s logical to assume that the spring pressure is enough to lift the plug until the hinge fittings are pressed up against the stops in the end of the hinges when the door is open. The above photo demonstrates that the plug is held up by the springs and the stop pads and stop pins are not aligned when open.
Why? Probably to reduce the risk of the back of the stop pins from striking the back of the stop pads if someone tried to close the door in the down position. We don’t know how much force it takes to push the plug pack into place against the spring pressure, but I suspect it isn’t very much. But it may take a bit of force to fully seat the stop pins against the stop pads in the fully closed position. From the above photo, we can see evidence of the stop pins contacting the stop pads and maybe even grease or graphite transfer between the pads and pins. So there is contact and maybe a bit of friction between the two. Probably more friction from the seals. What we don’t know is how much force is needed to close or open the plug. I’ll bet the NTSB is looking into that.
Last edited by Old Ag; 12th Jan 2024 at 22:29.
I have not seen any images that support that assumption. In my opinion the image pair that you posted does not. Do you have a high resolution image of an OPEN emergency exit door showing the hinge arrangement?
My speculation is that, to the greatest extent possible, all components of the door plug and the active door are identical. Why else would locking bolt holes be provided in the guide fittings of active doors?
My speculation is that, to the greatest extent possible, all components of the door plug and the active door are identical. Why else would locking bolt holes be provided in the guide fittings of active doors?
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I have not seen any images that support that assumption. In my opinion the image pair that you posted does not. Do you have a high resolution image of an OPEN emergency exit door showing the hinge arrangement?
My speculation is that, to the greatest extent possible, all components of the door plug and the active door are identical. Why else would locking bolt holes be provided in the guide fittings of active doors?
My speculation is that, to the greatest extent possible, all components of the door plug and the active door are identical. Why else would locking bolt holes be provided in the guide fittings of active doors?
- The active door opens nearly 180 degrees, the maintenance crew actually put cushions on the fuselage so that it is not damaged by the door. The door plug would seem to only go 90 degrees, based on the hinges that remained on the accident aircraft, which appear to be fully extended.
-The active door would appear to have some sort of tension/torsion system to "slow" its opening. It appears to be going slower than what I would expect from just gravity. The door plug doesn't appear to have anything similar.
These are just assumptions of course.... nothing definitive
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I have not seen any images that support that assumption. In my opinion the image pair that you posted does not. Do you have a high resolution image of an OPEN emergency exit door showing the hinge arrangement?
My speculation is that, to the greatest extent possible, all components of the door plug and the active door are identical. Why else would locking bolt holes be provided in the guide fittings of active doors?
My speculation is that, to the greatest extent possible, all components of the door plug and the active door are identical. Why else would locking bolt holes be provided in the guide fittings of active doors?
Look at the video I posted earlier of the emergency exit in motion. It folds down about 140 degrees. The plug hinge does not have that range of motion.
In my opinion, its pretty obvious that the hinges are different between the plug and the door. And I can tell that from the exit door photo above and any of the dozens of mid exit plug photos like the one in post 843. In fact, stop pin fittings, guide fittings and the perimeter frame they attach to seem to be about the extent of common parts between the plugs and exit doors.
Does that prove they they use different springs. Nope. But it does prove that it cant be assumed that are even springs involved in the exit door hinges. And I haven't seen any photographs that prove the exit doors have similar hinges to the plugs.
Why are the guide fittings identical? Perhaps they didn't see a need to have different parts that serve the same purpose, that is guiding the door up on a bearing in the frame. Perhaps they didn't see a risk that someone would try to bolt an exit shut. Perhaps they do bolt the inactivated emergency exits shut. Reuse of one part doesn't mean they reused as many as possible. Weight and maintenance are much larger issues than maximizing parts commonality.
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Do we know that? Why would it matter where the fittings and pins fall when the plug is opened? And why would they fall back behind the pads after the plug is opened to its position leaning away from the fuselage at approx. 15 degrees, even if there were no springs at all?
The phrase "fall back behind the stops" may be referring to the case that if there were no assist springs, and you let go of the door while lifting it up, it would fall back closed. With the springs it wont. Maybe that avoids some type of damage the engineers don't like.
Last edited by Old Ag; 13th Jan 2024 at 02:00.
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The most likely scenario is that the bolts were not installed. There is broad agreement on that aspect. If the lower bolts were installed not only would the door not move up, but even if there were no nuts on bolts, the bolts would be trapped but the spring pressure and would not fall out. In the NTSB briefing they stated that the door moved up before it was ejected. As noted that cannot happen if the lower bolts were in place even if there were not nuts on them. If the upper bolts were in place the door could not move upward and would remain on the plug pads. That is the case even if all the lower hinges were not even installed. In that case the plug door would pivot around the upper rollers and it would be held in position by the pads, Based on these conditions there is only one conclusion, that is: None of the four locking bolts were present at the time the door was ejected.
According to the NTSB the tracks were damaged and the bolts were not recovered. Their initial analysis examination was that it was not clear if the bolts had been present. That does not take a rocket scientist to discern. If the bolts were in place both the inside and the outside of the track will be damaged. That is, if the bolt was in place the inside area could be broken by the pressure loads, but if the bolt is in place it there will be damage to the outside area of the track because the bolt will be transferring loads to the outboard side of the track. No damage to the outboard side of the track means that the bolts weren't there in the first place.
I don't believe that it is possible for this event to happen if any two of the safety bolts are installed. This provides a level of redundancy to the design. As has been noted this basic design has been employed on the previous generation 737 and flown for millions of hours with no known previous issues. That alone verifies the fundamental soundness of the basic design.
Calhoun understands all of this, but can't say anything until the NTSB makes its ruling, but note he's saying it was Boeing's responsibility and that's basically saying this was a failure in the process to properly rig the door. The issue here is one of a failure in either assembly or maintenance, most likely at Boeing but possibly at AAR during the WiFI installation. Based on those facts the remedy should be an AD to require an inspection to ensure the doors are rigged per specification and the planes returned to flight status.
According to the NTSB the tracks were damaged and the bolts were not recovered. Their initial analysis examination was that it was not clear if the bolts had been present. That does not take a rocket scientist to discern. If the bolts were in place both the inside and the outside of the track will be damaged. That is, if the bolt was in place the inside area could be broken by the pressure loads, but if the bolt is in place it there will be damage to the outside area of the track because the bolt will be transferring loads to the outboard side of the track. No damage to the outboard side of the track means that the bolts weren't there in the first place.
I don't believe that it is possible for this event to happen if any two of the safety bolts are installed. This provides a level of redundancy to the design. As has been noted this basic design has been employed on the previous generation 737 and flown for millions of hours with no known previous issues. That alone verifies the fundamental soundness of the basic design.
Calhoun understands all of this, but can't say anything until the NTSB makes its ruling, but note he's saying it was Boeing's responsibility and that's basically saying this was a failure in the process to properly rig the door. The issue here is one of a failure in either assembly or maintenance, most likely at Boeing but possibly at AAR during the WiFI installation. Based on those facts the remedy should be an AD to require an inspection to ensure the doors are rigged per specification and the planes returned to flight status.
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Not really my point but I don't think it's proven one way or the other yet. 1 warning on the ground isn't exhaustive that it's not related. As a ga self driver interested if atpls often experience cabin pressure warnings and the sops ? I understand dual redundant CPM on 737m, do we know the exact warnings that came up, did both systems correlate (RA should ground it pending) or was it just discrepancy between the 2 ?
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I don't believe that it is possible for this event to happen if any two of the safety bolts are installed. This provides a level of redundancy to the design. As has been noted this basic design has been employed on the previous generation 737 and flown for millions of hours with no known previous issues. That alone verifies the fundamental soundness of the basic design.
Yes there is redundancy to physical failure of the bolts (and/or nuts), if they are installed. But in view of human factors there's no redundancy at all with respect to their installation, because they are certainly going to be installed (or forgotten) as a set. Thus there is only one level of redundancy to installation (inspection), which is also going to occur WRT the bolts/nuts as a single action. In other words, is very unlikely to fail to install or inspect the first bolt, but then proceed to the next, in sequence.
There are various types of redundancy. In "Practical Reliability Engineering" 4 bolts performing the same function would be termed "Like Redundancy".(identical hardware items performing the same function). The authors go on to state (section 6.9.2 "Common Mode Failures" p. 155) "In the design of redundant systems it is very important to identify and eliminate sources of common mode failures, or to reduce their probability of occurrence to levels an order or more below that of other failure modes." [2 potential bullets from a subsequent list of potential common mode failures are quoted below]:
5 Maintenance actions which are common to different paths, for example, an aircraft engine oil check after
which a maintenance technician omits to replace the oil seal on all engines. (This has actually happened
twice, very nearly causing a major disaster each time.)
6 Operating actions which are common to different paths, so that the same human error will lead to loss of both.
In my view this is missing from the design. What is necessary is a different type of redundancy, what the authors call "Active Redundancy", described as "[a system with] two independent parts with reliabilities R1 and R2, satisfactory operation occurs if either one or both parts function. Therefore, the reliability of the system, R, is equal to the probability of part 1 or part 2 surviving." Note that "parts" in this quote would mean dissimilar (distinct) parts, which would not be installed or inspected as a set, not just more identical bolts added to the design.
With respect to "no accidents in the past", keep in mind past performance is not an indication of sound engineering, as luck may play a part. Regardless of that point, the meaningful statistic to consider would not be number of miles flown, because once the bolts are properly installed, then the door/plug will subsequently be reliable indefinitely. The statistics to consider would be "how many planes have had the bolts forgotten by both installation and inspection?" Very likely 1 out of "N" where N is the number of planes with this design. But obviously, this is a statistically significant value, nowhere near the 99.9999999% reliability necessity for aviation that I believe was quoted earlier. It may have happened at installation time more frequently, but then caught by subsequent inspection.
reference: Patrick D. T. O’Connor, Andre Kleyner, "Practical Reliability Engineering" 5nd Edition, Wiley, 2012
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We have pressure vessels in subsea, with way higher loading, doesn't replace primary design ethos to minimise points of failure. Polyurethane bushes on the bolts would isolate the load cheaply and efficiently. Sure a couple of extra guide pins for ease of maintenance. 7-figure compliance cost is missing 2 to 3 zeros for fuselage design, assuming plug was integrated.
Say 50% more, around 100 lb ?
Don't you think some assistance to lift it up in order to open it, in an emergency, would be a good idea ? The initial design of the live door reportedly didn't have any assistance, and then Boeing thought better.