Alaska Airlines 737-900 MAX loses a door in-flight out of PDX
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Isn’t it a combination? ie the ratio of size (area) to strength of the door and frame structure that makes a blow out panel necessary. If a cockpit window blows out at max cabin diff a typical cockpit door gets roughly ~7tons applied to it directly towards the drivers. Surely a bit of a distraction for the emergency descent having the door ripped out of its frame and hitting you in the back of your head.
Last edited by Stagformation; 31st Jan 2024 at 22:35. Reason: edited to correct max load on door to 7tons
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Isn’t it a combination? ie the ratio of size (area) to strength of the door and frame structure that makes a blow out panel necessary. If a cockpit window blows out at max cabin diff a typical cockpit door gets roughly ~15tons applied to it directly towards the drivers. Surely a bit of a distraction for the emergency descent having the door ripped out of its frame and hitting you in the back of your head.
My calculation roughly 7tons at max diff; other than similar to the incorrect bolt length episode in a BAC1-11, the flight deck windows are perhaps stronger than the fuselage skin, they are able to withstand the chicken test; the common failure is the the outer layer which can be removed for a good weather ferry flight to suitable base for repairs.
A Marabou Stork buried itself in the side of one of my aircraft flight decks, but it was a bit more than a chicken, at least its beak was.
A Marabou Stork buried itself in the side of one of my aircraft flight decks, but it was a bit more than a chicken, at least its beak was.
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The door blows open. As here. And presumably as designed to do.
The door pulls open towards the main cabin. Any intruder trying to storm the door is pushing against the frame. That's the strong bit. To open it you only have the handle to pull, which is doubtless designed weaker than the latch. Despite this being a thread about an engineering failure, I still have confidence that Boeing, Airbus, and others have teams of design engineers who get all this stuff.
The door pulls open towards the main cabin. Any intruder trying to storm the door is pushing against the frame. That's the strong bit. To open it you only have the handle to pull, which is doubtless designed weaker than the latch. Despite this being a thread about an engineering failure, I still have confidence that Boeing, Airbus, and others have teams of design engineers who get all this stuff.
Re the decompression, things to consider:
Back in post 435 , Longtimer showed data from "The 737 Handbook" on facebook claiming that the door can withstand decompression pressures. But that could be based on nothing more than the observation that the cockpit blowout panels are one-way into the flight deck only. It's not Boeing saying that. In fact, Boeing said the cockpit door was designed to open in decompression.
But Boeing could be playing with words. The cockpit door could simply be designed to be weaker than the flight deck bulkhead as a general design principle, but not specifically designed to open under predicted decompression events. The exact decompression scenarios they designed to for cert aren't publicly known. The NTSB is hopefully looking into it.
When you have a small compartment adjacent to a large compartment, there will be higher delta p's on the partitions separating them when the breech is in the smaller compartment than when it's in the larger compartment. So that's kind of consistent with having one-way BOP's into the flight deck. But it doesn't really tell us if the door needed to open or if that was some kind of unexpected behavior.
It's not that easy to procure a latch that has a minimum AND a maximum strength. Post-911 we need cockpit doors that have a minimum strength, but going to a vendor and telling them you also want the latch to be guaranteed to break at a certain load for decompression will get you a lot of "can't meet that requirement" responses. I'm not saying they don't work that way, but it's something that would need custom engineering and assembly. I have no idea what that latch looks like, but to me, that's a big reason to wonder if the cockpit door really was designed to open. If you really need controlled venting, BOP's are the way to go.
Last thing I'll say is that N704AL will probably be out of service for some time while Boing verifies that the floor wasn't overloaded. There might have to be some repairs.
You also need to consider that the regulations for managing a decompression (without further structural damage) call out a maximum hole size that needs to be designed for. I don't recall what the hole size requirement was (it wasn't something I had to worry about in my area), but I'm reasonably sure it was significantly smaller than losing a door.
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You also need to consider that the regulations for managing a decompression (without further structural damage) call out a maximum hole size that needs to be designed for. I don't recall what the hole size requirement was (it wasn't something I had to worry about in my area), but I'm reasonably sure it was significantly smaller than losing a door.
Engine failures are typically covered by the formula in 25.365, but that's case by case, I wouldn't know if the 737 uses a bigger breech for fan blades or rotor burst. I would guess not but don't know for sure.
Notice that the FAR says "These loads may be considered as ultimate conditions" which means there could be permanent deformation or other damage after decompession. That's why I expect the incident aircraft will be out of service for a while.
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Were there ever really "true plugs" in commercial jets? The 737 main door looks like a plug, but the structural part that holds the door against pressure is the same stop and pin arrangement as the infamous door plug.
I found a video of someone demonstrating use of the old style overwing exit door that removes to the inside. It's functionally the same as the infamous plug, but the guides are on the frame side and the rollers on the door. The pressure is held by the same kind of pins and stops - you can see them clearly at 0:58. If they're misaligned, it's departing the same way as the AS1282 door:
https://www.youtube.com/watch?v=zhTrVEB0-W8
I found a video of someone demonstrating use of the old style overwing exit door that removes to the inside. It's functionally the same as the infamous plug, but the guides are on the frame side and the rollers on the door. The pressure is held by the same kind of pins and stops - you can see them clearly at 0:58. If they're misaligned, it's departing the same way as the AS1282 door:
https://www.youtube.com/watch?v=zhTrVEB0-W8
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The cockpit doors installed after 9/11 don't have blowout panels. That would defeat the purpose of the secure doors. The airline I worked for had decompression latches.
"Hartwell developed a rapid decompression pressure relief latch after the September 11th attack as additional security measures mandated by the FAA. Latch used to lock the blowout panels on the Cockpit Door and can only be functioned during a pressure event. When Latch senses a rapid change in cabin pressure, the latch automatically unlocks allowing it to function, releasing the panels in the Cockpit Door within milliseconds. This prevents structural failure of the door or bulkhead due to sudden, unequal pressure loads.
Features and Benefits:
"Hartwell developed a rapid decompression pressure relief latch after the September 11th attack as additional security measures mandated by the FAA. Latch used to lock the blowout panels on the Cockpit Door and can only be functioned during a pressure event. When Latch senses a rapid change in cabin pressure, the latch automatically unlocks allowing it to function, releasing the panels in the Cockpit Door within milliseconds. This prevents structural failure of the door or bulkhead due to sudden, unequal pressure loads.
Features and Benefits:
- Incorporate into door structure with minimal engineering effort
- Quick and cost-effective due to the mechanical system as compared to solenoid actuated system
- Unique design allows for cabin/flight deck pressure equalization during rapid decompression event"
Latch
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It's a highly transient millisecond-scale process. The faster the breached compatment evacuates the higher the deltas. For a given size of breach, a smaller compartment will vent faster than a large one, that's why breaches in smaller compartments are worse.
It's a pretty standard analysis that's done all the time. Google "aircraft decompression analysis software" to get a feeling how it's done.
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There have just been many posts and comments here (and many other places) about how the plug should be like "other" plug doors that are wedge shaped and can't be blown out because of their shape. I spent some time looking, and I haven't actually found any on aircraft, which isn't really surprising, either. All the "other" doors, including 737 main doors and old overwing "pull inside and set it on the seats" 737 classic exits also use stop fittings and pins and are displaced to get around them. I don't really have issues with the basic design, except maybe for the springs, depending on their neutral position. It really comes across as a QA/process error rather than anything in particular about the design. Any aircraft that's already been through a heavy maintenance cycle has already been through a thorough inspection of those things (as long as the shop doing that is trusted...)
chrisl137
Well for info: the L1011 pax doors are plug they open inwards and up into the ceiling void.
The problem with this action in an accident scenario, is that a pile of bodies against the inside might prevent the retract spring (cartridge) from lifting the door.
Well for info: the L1011 pax doors are plug they open inwards and up into the ceiling void.
The problem with this action in an accident scenario, is that a pile of bodies against the inside might prevent the retract spring (cartridge) from lifting the door.
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Much later I also read the reason why: most of the times they get opened in emergencies in emergencies that haven't done a lot of damage to the plane (or aren't even emergencies) and throwing them out the window tends to do a lot of damage to the wing and some to the door. If you watch the video I linked earlier, they don't look all that light, either, so most people in the overwing exit rows aren't really going to be able to toss them clear anyway.
ETA: some aircraft also have slides that deploy to get people off the wings, and you wouldn't want to toss a door onto the slide and mess it up.
Last edited by chrisl137; 1st Feb 2024 at 06:58.
If you'd ever seen the spring that powered the TriStar doors, that wouldn't be a concern.
I used to see emergency cards that said to put them on the seats. For real. For years.
Much later I also read the reason why: most of the times they get opened in emergencies in emergencies that haven't done a lot of damage to the plane (or aren't even emergencies) and throwing them out the window tends to do a lot of damage to the wing and some to the door. If you watch the video I linked earlier, they don't look all that light, either, so most people in the overwing exit rows aren't really going to be able to toss them clear anyway.
ETA: some aircraft also have slides that deploy to get people off the wings, and you wouldn't want to toss a door onto the slide and mess it up.
Much later I also read the reason why: most of the times they get opened in emergencies in emergencies that haven't done a lot of damage to the plane (or aren't even emergencies) and throwing them out the window tends to do a lot of damage to the wing and some to the door. If you watch the video I linked earlier, they don't look all that light, either, so most people in the overwing exit rows aren't really going to be able to toss them clear anyway.
ETA: some aircraft also have slides that deploy to get people off the wings, and you wouldn't want to toss a door onto the slide and mess it up.
Old designs had outward opening doors. Easy and simple.
Continued with the arrival of pressure cabins.
Events and accidents with outward opening doors (example HS-748 many events then a fatal) and increasing pressure differentials caused a design change toward “PLUG” designs. A “PLUG” design being defined as primary being held close by pressure differential on a circumferential ‘seating’ (in my book the 737 pin-pad design being more secondary)
The escape hatches mentioned above were real plugs. Which meant you had to pull them inward and deposit them on the seats before people could exit the aircraft. Re the circumferential ‘seating’ (would require rotating the hatch say 60-90 degrees to properly get them out).
Then over time .. events and accidents…( including influence by cargo door events DC10 Turkish Paris killing all (including floor collapse adding vent and blow out knowledge), 747 large fwd cargo flipping open at altitude killing nine. ).
In one case a young girl/woman, sitting next to the escape hatch, did not have the (know your card) and physical strength to open the hatch quickly and then deposit the hatch on hers or a free seat, when she finally got the hatch out with some help, the hatch landed on her, and passengers rushing out scrambled over the hatch thereby pinning her in her seat…
That led to - first guidelines and rules to put a physically stronger person in escape seats, to instruct and question them on opening - second to designs rethinking outward opening with minimal obstruction and damage, hence outward then upward operating hatches … which requires extra sub-functionalities solved by a mix of means (structure, mechanical, sensors, procedures, etc..).
Last edited by A0283; 1st Feb 2024 at 09:21.