Alaska Airlines 737-900 MAX loses a door in-flight out of PDX
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Boeing plug history
Thanks aeromech3, I grant your memory accurate of course, but on this particular 727 (N7647U), the forward of wing door plug outline is clearly visible, also see this photograph: https://www.airport-data.com/aircraf...000658863.html
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Quote....
Originally Posted by roger4
What we really need is a nice clear video of the plug-door, and for contrast the E/E door, being opened from the inside!
My guess is that it is a two man job to remove the plug, as it has no outward constraint. If one person alone tried to open it, he would have difficulty holding on to it once it was partially open, and could be thrown out of the aircraft. The second person is required on the outside of the plane to lower the plug safely.
Originally Posted by roger4
What we really need is a nice clear video of the plug-door, and for contrast the E/E door, being opened from the inside!
My guess is that it is a two man job to remove the plug, as it has no outward constraint. If one person alone tried to open it, he would have difficulty holding on to it once it was partially open, and could be thrown out of the aircraft. The second person is required on the outside of the plane to lower the plug safely.
My guess is that it is a two man job to remove the plug, as it has no outward constraint. If one person alone tried to open it, he would have difficulty holding on to it once it was partially open, and could be thrown out of the aircraft. The second person is required on the outside of the plane to lower the plug safely.
My guess is that it is a two man job to remove the plug, as it has no outward constraint. If one person alone tried to open it, he would have difficulty holding on to it once it was partially open, and could be thrown out of the aircraft. The second person is required on the outside of the plane to lower the plug safely.
, I'd be surprised if there's any requirement to remove it during routine maintenance. Why is this an issue?
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If I were hiring independent inspectors to provide quality assurance in the construction of airliners, "literalist pedantic twits" are exactly the people I'd want. They are also the people future passengers would want, whether they know it or not.
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This, exactly. These plugs aren't intended to be routinely opened and closed. And, on the probably rare occasions when they are opened and closed, it shouldn't matter whether or not the job can be easily performed by a single person or require a second set of hands.
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Door plug springs
Perhaps I could be permitted to throw in a couple more rumour-like thoughts :-
There has been discussion about the amount of force provided by the lift springs. There is more than one photo of the open door showing the hinge guide fully against the stop washer at the top.
I was wondering about the logic for this and suggest that it is very much intentional and that the nuts on top are a height adjustment to align the upper guide roller and its door guide when the door is open.
Differential adjustment of the nuts on each guide also aligns the door vertically.
For this to be true, the springs must supply more than the weight of the door to be reliably solid at the top of travel.
The next point then is that the spring force will increase quite a bit as the door is pushed down and this force is only restrained by the rather slim "arrestor" bolts which have a shear force on them as there is nothing for them to clamp.
Any slight play in the sliding action of the hinge can also potentially cause wear at the bolt.
Having seen the construction I don't think I would feel happy sitting next to one of these "unscheduled exit" plugs - I would want big bolts directly between plug and frame!
There has been discussion about the amount of force provided by the lift springs. There is more than one photo of the open door showing the hinge guide fully against the stop washer at the top.
I was wondering about the logic for this and suggest that it is very much intentional and that the nuts on top are a height adjustment to align the upper guide roller and its door guide when the door is open.
Differential adjustment of the nuts on each guide also aligns the door vertically.
For this to be true, the springs must supply more than the weight of the door to be reliably solid at the top of travel.
The next point then is that the spring force will increase quite a bit as the door is pushed down and this force is only restrained by the rather slim "arrestor" bolts which have a shear force on them as there is nothing for them to clamp.
Any slight play in the sliding action of the hinge can also potentially cause wear at the bolt.
Having seen the construction I don't think I would feel happy sitting next to one of these "unscheduled exit" plugs - I would want big bolts directly between plug and frame!
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Maybe. But for that to be true, there would need to be something on the door surround, against which the door could lever itself upwards. And whatever the "something" is, it would presumably exist also on aircraft with the door plugs (albeit not used for that purpose).
I can't seen anything on any of the (admittedly poor) photos that resembles the above.
I can't seen anything on any of the (admittedly poor) photos that resembles the above.
So exactly how this all works in not perfectly clear, but it is easy to imagine that pulling the handle up rotates the torque tube pressing the pins at the ends of the arms down towards the strikers, forcing or at least allowing the door to translate up. Where the assist to move the door upwards is unclear. It could be the visible springs and mechanisms in the middle of the door acting through the torque tube and strikers. There could be springs buried in the door that bear the doors weight, or it could be both.
Last edited by Old Ag; 13th Jan 2024 at 14:38.
IF the springs do support the total plug weight when extended to the stops, then they must exert significantly greater upthrust when compressed in situ.
In which case when dismantling, after removing the last bolt (when all correctly fitted), the plug would be ejected briskly upwards.
Seems both undesirable and unnecessary, unless the same springs are retained when a door is retrofitted?
In which case when dismantling, after removing the last bolt (when all correctly fitted), the plug would be ejected briskly upwards.
Seems both undesirable and unnecessary, unless the same springs are retained when a door is retrofitted?
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Looking at the above photo, you can see the handle attached to a round bar or torque tube that spans the door. On the side of the door, opposite the handle, you can see the bar pass through the door and attach to a lever arm that has some sort of pin on the other end. This pin seems to be engaged in some kind of striker or guide attached to the door frame between the second and third stop pad. This striker/guide is not shared with the plug configuration. The exact configuration is not clear but there are two torsion springs along the torque tube and a further mechanism buried in the door behind the handle with a large spring attached. You can’t see the door frame clearly on the handle side, but what you can see is consistent with the non handle side.
So exactly how this all works in not perfectly clear, but it is easy to imagine that pulling the handle up rotates the torque tube pressing the pins at the ends of the arms down towards the strikers, forcing or at least allowing the door to translate up. Where the assist to move the door upwards is unclear. It could be the visible springs and mechanisms in the middle of the door acting through the torque tube and strikers. There could be springs buried in the door that bear the doors weight, or it could be both.
So exactly how this all works in not perfectly clear, but it is easy to imagine that pulling the handle up rotates the torque tube pressing the pins at the ends of the arms down towards the strikers, forcing or at least allowing the door to translate up. Where the assist to move the door upwards is unclear. It could be the visible springs and mechanisms in the middle of the door acting through the torque tube and strikers. There could be springs buried in the door that bear the doors weight, or it could be both.
Some more from the Seattle Times, including this about Sen Maria Cantwell. Cantwell had asked to see the last 24 months of notices of FAA quality systems audits related to Boeing and one of its suppliers, Spirit AeroSystems.
Spirit, based in Wichita, Kan., builds the entirety of the fuselage for the 737 MAX 9, before sending it to Boeing’s Renton plant by train.
“Recent accidents and incidents … call into question Boeing’s quality control,” Cantwell wrote in the letter to the FAA Thursday. But, she continued, “it appears that the FAA’s oversight processes have not been effective” in ensuring Boeing’s planes are safe.
https://www.seattletimes.com/busines...so-is-the-faa/
Spirit, based in Wichita, Kan., builds the entirety of the fuselage for the 737 MAX 9, before sending it to Boeing’s Renton plant by train.
“Recent accidents and incidents … call into question Boeing’s quality control,” Cantwell wrote in the letter to the FAA Thursday. But, she continued, “it appears that the FAA’s oversight processes have not been effective” in ensuring Boeing’s planes are safe.
https://www.seattletimes.com/busines...so-is-the-faa/
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Spring force is a linear function of the distance it's been compressed.
Consider these number's I've pulled from my imagination:
- Free length of springs is 6" longer than installed length with the door in the open/up position (at the stops)
- Springs together exert 64lb when open to fully support the weight of the door
Subtract the weight of the door, and we're talking about less than 20lb to contend with when installing/removing the arrestor bolts. Seems manageable.
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Perhaps I could be permitted to throw in a couple more rumour-like thoughts :-
There has been discussion about the amount of force provided by the lift springs. There is more than one photo of the open door showing the hinge guide fully against the stop washer at the top.
I was wondering about the logic for this and suggest that it is very much intentional and that the nuts on top are a height adjustment to align the upper guide roller and its door guide when the door is open.
Differential adjustment of the nuts on each guide also aligns the door vertically.
For this to be true, the springs must supply more than the weight of the door to be reliably solid at the top of travel.
The next point then is that the spring force will increase quite a bit as the door is pushed down and this force is only restrained by the rather slim "arrestor" bolts which have a shear force on them as there is nothing for them to clamp.
Any slight play in the sliding action of the hinge can also potentially cause wear at the bolt.
Having seen the construction I don't think I would feel happy sitting next to one of these "unscheduled exit" plugs - I would want big bolts directly between plug and frame!
There has been discussion about the amount of force provided by the lift springs. There is more than one photo of the open door showing the hinge guide fully against the stop washer at the top.
I was wondering about the logic for this and suggest that it is very much intentional and that the nuts on top are a height adjustment to align the upper guide roller and its door guide when the door is open.
Differential adjustment of the nuts on each guide also aligns the door vertically.
For this to be true, the springs must supply more than the weight of the door to be reliably solid at the top of travel.
The next point then is that the spring force will increase quite a bit as the door is pushed down and this force is only restrained by the rather slim "arrestor" bolts which have a shear force on them as there is nothing for them to clamp.
Any slight play in the sliding action of the hinge can also potentially cause wear at the bolt.
Having seen the construction I don't think I would feel happy sitting next to one of these "unscheduled exit" plugs - I would want big bolts directly between plug and frame!
Progressive springs tend to have some coils close to decrease the number of open/active coils. In the closed position the coils are still uniformly spaced, the exceptions being the last turn that is used to make a 90 degree end to seat the spring.
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I won't make things too confused by talking about other failure modes like bearing, because the order of magnitude is what's interesting.
The ones at the top work in bending but still, each one will be able to resist any spring force with ease.
All they have to do is keep the door on its stops. An engineer will look at this and see a quadruple-redundant design that's almost over-designed and will feel pretty good about it.
The problem is, it's not quad-redundant. If we go with the prevailing theory, one mistake prevented the installation of all 4 bolts, instead of 4 independent mistakes.
But to me as a stress guy, I'm thinking if production can't reliably build and install this door, they can't do anything. It's a pretty basic aerostructure. How are we supposed to engineer around not being able to build and install basic things?
Apropos of not this topic, I'll also add they were so lucky the door let go at 16,000 and not 35,000. I suspect the blowout was bigger than what the airplane was designed for, and had the delta-p been higher something very very bad could have happened.
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'
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
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
Reuters Article
An interesting article from Reuters about Boeing workers involvement with the plug door.
Reuter’s source has told them that, contrary to Boeing standard factory procedures, (Renton) workers “usually remove the panel, and use the gap to install pieces of cabin equipment, before replacing the panel and finishing the installation”.
But the source familiar with Boeing's industrial process said on Friday that according to its standard factory procedures, the company only removes or adjusts the panel if there are signs it was installed incorrectly
Boeing does perform certain checks and conducts pressurization tests before delivering the plane to the airline, the source said, adding that interiors are loaded elsewhere in the plane.
Well, if true we have here unauthorised work being carried out, presumably without the normal checks, paperwork, inspections and signatures, leading almost inevitably – in time – to incorrect reassembly and/or missing parts.
https://www.reuters.com/business/aer...ce-2024-01-13/
Reuter’s source has told them that, contrary to Boeing standard factory procedures, (Renton) workers “usually remove the panel, and use the gap to install pieces of cabin equipment, before replacing the panel and finishing the installation”.
But the source familiar with Boeing's industrial process said on Friday that according to its standard factory procedures, the company only removes or adjusts the panel if there are signs it was installed incorrectly
Boeing does perform certain checks and conducts pressurization tests before delivering the plane to the airline, the source said, adding that interiors are loaded elsewhere in the plane.
Well, if true we have here unauthorised work being carried out, presumably without the normal checks, paperwork, inspections and signatures, leading almost inevitably – in time – to incorrect reassembly and/or missing parts.
https://www.reuters.com/business/aer...ce-2024-01-13/