Alaska Airlines 737-900 MAX loses a door in-flight out of PDX
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).
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).
That is to say, perhaps the maintenance procedures are inadequate and/or incorrect.
Last edited by remi; 22nd Jan 2024 at 08:34.
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".
Boeing clearly doesn't get detailed documentation from every major check that operators carry out on their fleets, although it should be alerted on anything found that affects continuing airworthiness (as should the FAA, of course).
If Boeing says it hasn't received any such feedback from operators re door plug issues in the past, I'm inclined to take its word for that.
But equally, I don't believe the aircraft performed 151 sectors without any bolts having been installed.
Ok, let's shoot again
Ok, let’s have another go at this.
Spoiler: the attention drawn to the door frame seal by the NTSB Chair on 18 Jan, as reported by Reuters, may prove crucial.
In a static situation on the ground, without the 4 arrestor bolts in place, a relatively light human (certainly health and safety tolerant) force upwards, assisted by the springs, allows to vertically lift the door plug 1,5 inch to first free its 12 stop fittings from the corresponding stop pads on both sides of the door frame and almost immediately after to free the roller guides on the upper door plug from the rollers attached to the door frame. "Immediately after” and not “simultaneously”, since (as observed earlier in this thread) the stop pads seem to have a slightly smaller diameter than the vertical travel of the guides to free themselves from their rollers.
What I hadn’t realised before is that the shape of the inboard top door frame seal is to assist the almost simultaneous outward movement of the door plug from the top, as the seal’s large inward flange at the top is bent further inwards by the door plug in its flush/closed position and therefore exerts a certain amount of outward pressure.
So, as per design in a static and non-pressurised situation on the ground, only this will allow the door plug to move up and, once free from stop pads and rollers, also with almost no force to pivot outwards from the top, while the bottom hinges attached to the door frame with its spring pylons, which go through the guiding brackets attached to the door plug, will keep the bottom of the door plug close to and horizontally aligned with the bottom door frame. In this situation both restraining cables attached to the top sides of the door plug and door frame will prevent opening of the plug beyond 15 degrees, thereby also preventing further movement of the spring pylons beyond, that is to say below, the top of their guiding brackets.
As we very well know, this is not what happened here.
As suggested earlier in this thread, it could be argued that the door plug first moved outward at its bottom part. Evidence for this is suggested on the door frame where it appears that cracks have formed at both top corners where metal strips are located against the top sides of the large inward seal flange (as seen on the NTSB Flickr photos), suggesting inward and perhaps simultaneously upward pressure on the top door frame from the door plug. This is in line with the clear cracks seen along both top corners of the door plug itself, as seen on the photo of the door plug arriving at NTSB. However, the photo below of a closed door plug seen from the outside suggests that this “bottom door plug moved outward first” (without much vertical movement) scenario would be impossible without compromising the stop fittings and/or stop pads. And from what can be observed on several other photos, neither stop fittings nor stop pads seem to show any damage.
This brings us to the vertical movement tolerance of that inboard top of the door seal and the structural integrity of the door plug. What if in a dynamic (in-flight) situation, the top seal would allow the door plug, while the latter is loosing its structural integrity, in the sense that its upper part is slightly bent outwards, cracking it away from the top main door plug structure (as evidenced by cracks seen on the photo of the door plug arriving at NTSB), to move upwards to such an extent that the stop fittings would be lifted above the stop pads and leaving the top of the door plug momentarily only held back from moving outside by the roller guides hanging on the rollers for only a few millimetres, until the roller guides at their lower tips give way (bending evidenced on NTSB photos) and free the door plug to move outwards from the top and on its way out rip the lower guide hinge brackets from the door plug frame?
Credit: Reddit
Spoiler: the attention drawn to the door frame seal by the NTSB Chair on 18 Jan, as reported by Reuters, may prove crucial.
In a static situation on the ground, without the 4 arrestor bolts in place, a relatively light human (certainly health and safety tolerant) force upwards, assisted by the springs, allows to vertically lift the door plug 1,5 inch to first free its 12 stop fittings from the corresponding stop pads on both sides of the door frame and almost immediately after to free the roller guides on the upper door plug from the rollers attached to the door frame. "Immediately after” and not “simultaneously”, since (as observed earlier in this thread) the stop pads seem to have a slightly smaller diameter than the vertical travel of the guides to free themselves from their rollers.
What I hadn’t realised before is that the shape of the inboard top door frame seal is to assist the almost simultaneous outward movement of the door plug from the top, as the seal’s large inward flange at the top is bent further inwards by the door plug in its flush/closed position and therefore exerts a certain amount of outward pressure.
So, as per design in a static and non-pressurised situation on the ground, only this will allow the door plug to move up and, once free from stop pads and rollers, also with almost no force to pivot outwards from the top, while the bottom hinges attached to the door frame with its spring pylons, which go through the guiding brackets attached to the door plug, will keep the bottom of the door plug close to and horizontally aligned with the bottom door frame. In this situation both restraining cables attached to the top sides of the door plug and door frame will prevent opening of the plug beyond 15 degrees, thereby also preventing further movement of the spring pylons beyond, that is to say below, the top of their guiding brackets.
As we very well know, this is not what happened here.
As suggested earlier in this thread, it could be argued that the door plug first moved outward at its bottom part. Evidence for this is suggested on the door frame where it appears that cracks have formed at both top corners where metal strips are located against the top sides of the large inward seal flange (as seen on the NTSB Flickr photos), suggesting inward and perhaps simultaneously upward pressure on the top door frame from the door plug. This is in line with the clear cracks seen along both top corners of the door plug itself, as seen on the photo of the door plug arriving at NTSB. However, the photo below of a closed door plug seen from the outside suggests that this “bottom door plug moved outward first” (without much vertical movement) scenario would be impossible without compromising the stop fittings and/or stop pads. And from what can be observed on several other photos, neither stop fittings nor stop pads seem to show any damage.
This brings us to the vertical movement tolerance of that inboard top of the door seal and the structural integrity of the door plug. What if in a dynamic (in-flight) situation, the top seal would allow the door plug, while the latter is loosing its structural integrity, in the sense that its upper part is slightly bent outwards, cracking it away from the top main door plug structure (as evidenced by cracks seen on the photo of the door plug arriving at NTSB), to move upwards to such an extent that the stop fittings would be lifted above the stop pads and leaving the top of the door plug momentarily only held back from moving outside by the roller guides hanging on the rollers for only a few millimetres, until the roller guides at their lower tips give way (bending evidenced on NTSB photos) and free the door plug to move outwards from the top and on its way out rip the lower guide hinge brackets from the door plug frame?
Credit: Reddit
Last edited by D Bru; 22nd Jan 2024 at 09:51. Reason: typo
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.
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.
It's perfectly possible for that statement to be true.
Boeing clearly doesn't get detailed documentation from every major check that operators carry out on their fleets, although it should be alerted on anything found that affects continuing airworthiness (as should the FAA, of course).
If Boeing says it hasn't received any such feedback from operators re door plug issues in the past, I'm inclined to take its word for that.
Boeing clearly doesn't get detailed documentation from every major check that operators carry out on their fleets, although it should be alerted on anything found that affects continuing airworthiness (as should the FAA, of course).
If Boeing says it hasn't received any such feedback from operators re door plug issues in the past, I'm inclined to take its word for that.
Chris Brady 737 Technical Aspects of AS1282 Updated Video
I’ve been looking again at Chris Brady’s 737 Technical Aspects of AS1282 Updated video of 17 Jan 2024.
For me it fully clarifies exactly how the plug door (I will call it a door) is opened and closed, and how the guide fittings and lift assist springs are designed to work. I’ve put his video into words:
Starting in the door “open” position i.e. hinged out and restrained by the upper straps.
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.
As the door closes the 12 stop fittings pass over the top of their mating stop pads.
At the top of the door the upper guide track fittings are now in line with the roller pins. To clarify, the roller pins (on the fuselage frame) are now in line with the bottom opening of the guide track fittings on the door.
To close the door into its final position it now needs to be pulled inwards until the rollers pins are inside and against the back face of the guide track fitting. Now the door is pulled downwards by around 4cm until the track guides are up against the fixed guide pins.
The lift assist springs are resisting this downwards movement, and so the door needs to be held in place to allow the guide fitting locking bolts to be inserted under the guide pins, and the vertical movement arrestor bolts inserted through the hinge guide fittings.
At this point the 12 stop fittings are aligned with and close to fixed stop pads. The small gap will be taken up as the aircraft pressurises and transfers the door load to the fuselage.
For me it fully clarifies exactly how the plug door (I will call it a door) is opened and closed, and how the guide fittings and lift assist springs are designed to work. I’ve put his video into words:
Starting in the door “open” position i.e. hinged out and restrained by the upper straps.
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.
As the door closes the 12 stop fittings pass over the top of their mating stop pads.
At the top of the door the upper guide track fittings are now in line with the roller pins. To clarify, the roller pins (on the fuselage frame) are now in line with the bottom opening of the guide track fittings on the door.
To close the door into its final position it now needs to be pulled inwards until the rollers pins are inside and against the back face of the guide track fitting. Now the door is pulled downwards by around 4cm until the track guides are up against the fixed guide pins.
The lift assist springs are resisting this downwards movement, and so the door needs to be held in place to allow the guide fitting locking bolts to be inserted under the guide pins, and the vertical movement arrestor bolts inserted through the hinge guide fittings.
At this point the 12 stop fittings are aligned with and close to fixed stop pads. The small gap will be taken up as the aircraft pressurises and transfers the door load to the fuselage.
Last edited by SRMman; 23rd Jan 2024 at 08:54. Reason: correction
A madrel, being tapered, could slowly eject if g loads offset the spring force.
Last edited by EXDAC; 22nd Jan 2024 at 11:37.
It apppears that related issues have encountered with 737-900ER models and the FAA have issued a safety alert
The FAA issued a “safety alert for operators” disclosing some airlines have conducted additional inspections on the 737-900ER mid-exit door plugs “and have noted findings with bolts during the maintenance inspections”.
US authorities say more Boeing 737 planes should get checks after door plug blowout | US news | The Guardian
The FAA issued a “safety alert for operators” disclosing some airlines have conducted additional inspections on the 737-900ER mid-exit door plugs “and have noted findings with bolts during the maintenance inspections”.
US authorities say more Boeing 737 planes should get checks after door plug blowout | US news | The Guardian
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%.
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%.
Excel also silently defaults to radians.
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The only argument I see against that is if the springs are strong enough to lift the plug over the stop fittings. If that's true, it should have launched on its first flight.
My working hypothesis is that the bolts were installed but the cotter pins either absent or not bent to lock the castle nuts. The cotters (if present but not bent) eventually vibrated out, then the nuts off, then the bolts out. But then NTSB should be seeing some signs if the bolts having been there at some point. You'd also think there would be marks on the paint around the bolt holes on the upper alignment channels on the plug from the nuts/washers pressing in.
So it's a mystery.
My working hypothesis is that the bolts were installed but the cotter pins either absent or not bent to lock the castle nuts. The cotters (if present but not bent) eventually vibrated out, then the nuts off, then the bolts out. But then NTSB should be seeing some signs if the bolts having been there at some point. You'd also think there would be marks on the paint around the bolt holes on the upper alignment channels on the plug from the nuts/washers pressing in.
So it's a mystery.
Good summary, although I think you have a couple of steps slightly out of sync.
It's probably easier to imagine the relevant steps in the opening, rather than closing, sequence:
The door rises so that the pads on the door lift clear of the stops on the door frame - but the stops disengage while the roller pins are still in their channels (near the bottom of them, obviously). At that point, given sufficient cabin pressure, the door will open with the rollers still engaged in the guide channel, with the result that the latter will fracture (as indeed was the case with AS1282).
Closing would be the opposite sequence: the rollers would enter and start to move up the guide channel before the stops engage.
Corrections to my understanding, as ever, welcomed.
As the door closes the 12 stop fittings pass over the top of their mating stop pads.
At the top of the door the upper guide track fittings are now in line with the roller pins. To clarify, the roller pins (on the fuselage frame) are now in line with the bottom opening of the guide track fittings on the door.
At the top of the door the upper guide track fittings are now in line with the roller pins. To clarify, the roller pins (on the fuselage frame) are now in line with the bottom opening of the guide track fittings on the door.
The door rises so that the pads on the door lift clear of the stops on the door frame - but the stops disengage while the roller pins are still in their channels (near the bottom of them, obviously). At that point, given sufficient cabin pressure, the door will open with the rollers still engaged in the guide channel, with the result that the latter will fracture (as indeed was the case with AS1282).
Closing would be the opposite sequence: the rollers would enter and start to move up the guide channel before the stops engage.
Corrections to my understanding, as ever, welcomed.
Good summary, although I think you have a couple of steps slightly out of sync.
It's probably easier to imagine the relevant steps in the opening, rather than closing, sequence:
The door rises so that the pads on the door lift clear of the stops on the door frame - but the stops disengage while the roller pins are still in their channels (near the bottom of them, obviously). At that point, given sufficient cabin pressure, the door will open with the rollers still engaged in the guide channel, with the result that the latter will fracture (as indeed was the case with AS1282).
Closing would be the opposite sequence: the rollers would enter and start to move up the guide channel before the stops engage.
Corrections to my understanding, as ever, welcomed.
It's probably easier to imagine the relevant steps in the opening, rather than closing, sequence:
The door rises so that the pads on the door lift clear of the stops on the door frame - but the stops disengage while the roller pins are still in their channels (near the bottom of them, obviously). At that point, given sufficient cabin pressure, the door will open with the rollers still engaged in the guide channel, with the result that the latter will fracture (as indeed was the case with AS1282).
Closing would be the opposite sequence: the rollers would enter and start to move up the guide channel before the stops engage.
Corrections to my understanding, as ever, welcomed.
The FAA's investigation into Boeing is getting wider as it flags another plane model for door-plug inspections
FAA Widens Boeing Investigation, Flags Another Jet for Inspections (businessinsider.com)- The FAA told airlines to inspect the door plugs of another Boeing jet, the 737-900ER.
- It uses the same door-plug design as the 737 Max 9 involved in the Alaska Airlines blowout.
- United, Alaska, and Delta Air Lines operate the 737-900ER with a door plug.
FAA Statement re: -900ER
From FAA.gov.
Link to the SAFO is included:
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.
Link to the SAFO is included:
FAA Statement on Recommending Visual Inspections of Boeing 737-900ER Mid-Exit Door Plugs
Sunday, January 21, 2024This 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.
From the SAFO:
"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."...(emphasis added)
Is this "do it again" instruction a cause for (or an additional cause for) alarm about the fuselage plug problem; is such an instruction in some sense typical for inspections in a situation like the present?
"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."...(emphasis added)
Is this "do it again" instruction a cause for (or an additional cause for) alarm about the fuselage plug problem; is such an instruction in some sense typical for inspections in a situation like the present?
From the SAFO:
"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."...(emphasis added)
Is this "do it again" instruction a cause for (or an additional cause for) alarm about the fuselage plug problem; is such an instruction in some sense typical for inspections in a situation like the present?
"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."...(emphasis added)
Is this "do it again" instruction a cause for (or an additional cause for) alarm about the fuselage plug problem; is such an instruction in some sense typical for inspections in a situation like the present?