Q about the Alaska door blow out incident
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Q about the Alaska door blow out incident
Posting this here so as not to get in the way of the discussion in the professionals' forum but an aspect of this I still don't understand is, is there an actual plug door, albeit deactivated and panelled over internally, (because if there is, I don't understand how that could have blown out), or is the hole in the fuselage to take a door if required filled with some other kind of panel short of an actual plug door as such and it's the fixings of that panel which have failed?
Thanks in advance to anyone who can clear that up for me.
Thanks in advance to anyone who can clear that up for me.
The term being used - "plug door" - is slightly misleading, as it's not a door at all. It can, however, be removed and replaced by one at some later stage in the aircraft's life, if desired.
There are two means to retain hole blocking items in a pressure vessel, such as a fuselage. Either the use of bolts that hold a cover against the force from the pressure, such as you see on the heads of car engines - a bunch of so-called "head bolts" that resist the force of combustion and compression stroke pressures. A typical failure was seen when a BAC111 cockpit window escaped. It was installed from the outside of the fuselage with undersized and under-length retaining bolts. Eventually the limited engagement of the threads was overcome and the window left; the air inside the balloon pushed one pilot partly out the window opening; fortunately he was captured by his fellow crew members before he too could complete an escape. Unfortunately he was severely beaten by turbulence because he was not aerodynamically smooth.
The other means of sealing is that the item is inside the pressure vessel is situated such that the retaining features directly resist the pressure without adding load to fasteners. For example, patching a tubeless car tire with a rubber flap stuck to the inside. The higher the pressure, the harder the patch is pressed against the hole.
The conventional idea of the plug is that it is shaped so that it is larger than the hole so as pressure increases the force on a sealing gasket increases. This can be a problem because the gasket can take a set and not seal properly.
In this case they used a mutual finger arrangement. There are fingers on the door that match fingers on the opening. The higher the pressure the more force is on the fingers, preventing sliding into the disengaged position. Since the fingers are acting as the stop for the plug, there is no increased load from that force on the seals, so the seals cannot get smashed and therefore leak. This mutual finger arrangement is seen in millions of home use pressure cookers.
In earlier 737s they have true plug doors for the emergency exits. This poses a problem as to use them they have to be pulled fully into the interior of the plane. They can be turned edgewise and chucked out, but lifting and turning of a heavy door when seated in an emergency is a big problem that the mutual finger arrangement eliminates.
The other means of sealing is that the item is inside the pressure vessel is situated such that the retaining features directly resist the pressure without adding load to fasteners. For example, patching a tubeless car tire with a rubber flap stuck to the inside. The higher the pressure, the harder the patch is pressed against the hole.
The conventional idea of the plug is that it is shaped so that it is larger than the hole so as pressure increases the force on a sealing gasket increases. This can be a problem because the gasket can take a set and not seal properly.
In this case they used a mutual finger arrangement. There are fingers on the door that match fingers on the opening. The higher the pressure the more force is on the fingers, preventing sliding into the disengaged position. Since the fingers are acting as the stop for the plug, there is no increased load from that force on the seals, so the seals cannot get smashed and therefore leak. This mutual finger arrangement is seen in millions of home use pressure cookers.
In earlier 737s they have true plug doors for the emergency exits. This poses a problem as to use them they have to be pulled fully into the interior of the plane. They can be turned edgewise and chucked out, but lifting and turning of a heavy door when seated in an emergency is a big problem that the mutual finger arrangement eliminates.
Last edited by MechEngr; 7th Jan 2024 at 08:59.
Simple answer.
No, it’s not a plug.
It’s a panel that slides downwards and inwards onto retainers from outside.
It gets retained at multiple points.
That sliding motion happens to usually be spring loaded too, so it can be ejected upwards and outwards when the retaining bolts are removed.
Until they publish what was missing/wrong about the installation, all that’s known is that the retaining bolts didn’t work (if indeed they were all there).
The strange thing is the lack of damage, it’s almost as if it was held in by sticky tape till the eject mechanism overcame it.
They need the door, some of the retaining bolts are through fittings in the door. The lack of damage should make it obvious once they have the door.
No, it’s not a plug.
It’s a panel that slides downwards and inwards onto retainers from outside.
It gets retained at multiple points.
That sliding motion happens to usually be spring loaded too, so it can be ejected upwards and outwards when the retaining bolts are removed.
Until they publish what was missing/wrong about the installation, all that’s known is that the retaining bolts didn’t work (if indeed they were all there).
The strange thing is the lack of damage, it’s almost as if it was held in by sticky tape till the eject mechanism overcame it.
They need the door, some of the retaining bolts are through fittings in the door. The lack of damage should make it obvious once they have the door.
A typical failure was seen when an Airbus cockpit window escaped. It was installed from the outside of the fuselage with undersized and under-length retaining bolts. Eventually the limited engagement of the threads was overcome and the window left; the air inside the balloon pushed one pilot partly out the window opening
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Thanks to those who replied. When I posted, I was assuming, wrongly, the plug was held against the pressure around its complete circumference (which is why I couldn’t understand how such a thing could blow out) but I now understand it’s only held against the pressure at a few points so the question is, how did the plug manage to wriggle free of these points? Thanks again.
As far back as 1963 Boeing offered the 707-320C with a similar aft of the wing exit door/hatch (called the hatrack door).
Same or similar design/drop down type hatch as seen today -
plus the option of using a door plug if not required by the airline (one of which blew out 2 days ago on the 737M-9)
On the 707C deactivating the door and using a door plug, allowed and limited 189 pax, but with the door activated it allowed up to 219 pax.
These pax figures are the same for today's stretched 737's ie: 189 without the extra 2 exits, and up to 219 with the doors active (or 235 is possible on the larger 737M-10)
and to simplify more, the 737's with Door Plugs (exit unused) has a Window, and those with an active exit door - these have a small porthole.
Same or similar design/drop down type hatch as seen today -
plus the option of using a door plug if not required by the airline (one of which blew out 2 days ago on the 737M-9)
On the 707C deactivating the door and using a door plug, allowed and limited 189 pax, but with the door activated it allowed up to 219 pax.
These pax figures are the same for today's stretched 737's ie: 189 without the extra 2 exits, and up to 219 with the doors active (or 235 is possible on the larger 737M-10)
and to simplify more, the 737's with Door Plugs (exit unused) has a Window, and those with an active exit door - these have a small porthole.
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I don't think we would object to such a well asked question.
And you might like to know, that I too was unsure about this mechanism, and how it gave way, but your side thread has answered that for me as well. Thanks
And you might like to know, that I too was unsure about this mechanism, and how it gave way, but your side thread has answered that for me as well. Thanks
Thanks to those who replied. When I posted, I was assuming, wrongly, the plug was held against the pressure around its complete circumference (which is why I couldn’t understand how such a thing could blow out) but I now understand it’s only held against the pressure at a few points so the question is, how did the plug manage to wriggle free of these points? Thanks again.
Incidentally, having followed the "professional" thread since it started, I can assure you that your original question is far from the dumbest.
