faro From Egypt, joined Aug 2007, 1754 posts, RR: 0 Posted (4 years 7 months 3 weeks 14 hours ago) and read 7526 times:
If one has a fuselage-borne fire situation in the cruise where the suppressants are not enough to extinguish the fire, what would happen if you (somehow...) provoked depressurisation? Would it help extinguish/retard the fire given the paucity of oxygen in cruise-level air or would it worsen things? Can you voluntarily provoke full depressurisation in the first place on modern airliners? I assume you could not do this with fully-opened pressure relief valves...
The study (to my understanding) suggests that an intentional decompression of the aircraft does not necessarily yield great benefits in fighting a fire.
A second but related topic is smoke evacuation. This is typically used when there is a great deal of smoke in the cabin but (in general) the PIC is reasonably assured the fire is extinguished (introducing more oxygen to an unextinguished fire could be catastrophic). The B-747 in some older cabin crew manuals had a fairly unique procedure of having the flight crew descend the aircraft, depressurize the cabin and the cabin crew disarmed a main cabin door (usually two) and open it a "crack", allowing for air flow. The door handle was apparently also secured during this procedure. (Note: the cabin door is not fully opened.) Other aircraft, like the Gulfstream 550 has a dedicated "smoke evacuation" button, which in the case of the Gulfstream depressurizes a seal around the aft baggage door to allow for smoke to leave the aircraft.
The effectiveness of these techniques is somewhat questionable for smoke evacuation. A few airlines rather rely on an inflatable bag to assure clear vision in the cockpit called EVAS. You can see a video of a simulated smoke evacuation in this video (you can see the EVAS system as well) : http://youtu.be/cgIT6pkp6LQ
In summary, yes an aircraft can be manually depressurized. But doing would be exceptionally rare and for highly unusual circumstances.
[Edited 2011-10-07 01:40:36]
Release your seat-belts and get out! Leave everything!
lowrider From United States of America, joined Jun 2004, 3220 posts, RR: 9
Reply 3, posted (4 years 7 months 3 weeks 12 hours ago) and read 7485 times:
Quoting Markhkg (Reply 1): However, it has been a subject for cargo aircraft fires, which has been been the subject of at least one FAA research study which you can see here
Its more than a subject, it is in the QRH. The operating theory is that depressurizing the cabin up to FL250 will reduce the available oxygen enough to slow, if not extinguish most fires. I don't care to test this personally, but I see it mostly as measure to buy a little more time to get to an airport.
Quoting Markhkg (Reply 1): The B-747 in some older cabin crew manuals had a fairly unique procedure of having the flight crew descend the aircraft, depressurize the cabin and the cabin crew disarmed a main cabin door (usually two) and open it a "crack", allowing for air flow.
On the freighter version, you slow to under 200 knots and open the escape hatch in the cockpit.
fr8mech From United States of America, joined Sep 2005, 6699 posts, RR: 16
Reply 4, posted (4 years 7 months 3 weeks 10 hours ago) and read 7442 times:
One of the issues with reducing the oxygen available to the the fire is that the fire will continue to smolder and generate heat. A re-introduction of oxygen as the aircraft descends will get the fire going again.
If the fire was in a confined area and the fire load gets really hot while it's smoldering and oxygen is quickly re-introduced it could lead to an explosive reignition (backdraft type scenario). Not likely, but very possible.
lowrider From United States of America, joined Jun 2004, 3220 posts, RR: 9
Reply 8, posted (4 years 7 months 3 weeks 2 hours ago) and read 7236 times:
Quoting CosmicCruiser (Reply 7): I should've added some of our jets now have a fire suppression system for the main cargo deck
Is it a halon based system? I have been toying around with an idea for a main deck suppression design on my own time, that could be retrofitted, but have run into some limitations in materials technology.
francoflier From France, joined Oct 2001, 4399 posts, RR: 11
Reply 10, posted (4 years 7 months 3 weeks 2 hours ago) and read 7213 times:
Quoting Markhkg (Reply 1): The B-747 in some older cabin crew manuals had a fairly unique procedure
It's still on the manuals today, up to the 744. The procedure is still taught to cabin and cockpit crew alike. There is a specially designed strap that holds the door handle in a certain position to keep it ajar. One or two doors are then opened (depending on the location of the smoke) to create a draft of air to evacuate said smoke.
The procedure is only designed to evacuate smoke, and would obviously do no good in case of fire.
It is very much a last resorts measure, and I've never heard it ever being used in real life. I don't know whether the -8 offers that procedure as well.
Note that the 747 also has a 'vent' behind the overhead C/B panel in the cockpit which can be opened by the flight crew to help evacuate cockpit smoke. ...The escape hatch technique is a lot more radical and effective, if unofficial, provided you're depressurized.
As for the 744 cargo, main deck fire fighting procedure includes depressurizing the cabin to 25000 ft. As said above, it slows the fire rather than smother it. Every minute helps when you're possibly hours away from a suitable field in the middle of the Pacific ocean...
I've often wondered what would happen to a cargo fire at 35000 ft cabin altitude, but depending on what's burning, I think it still wouldn't be enough to extinguish it. And the crew wouldn't have enough O2 to stay that high long enough.
Looks like I picked the wrong week to quit posting...
HAWK21M From India, joined Jan 2001, 31875 posts, RR: 54
Reply 11, posted (4 years 7 months 2 weeks 6 days 12 hours ago) and read 7059 times:
Quoting CosmicCruiser (Reply 5): Actually it's part of the checklist on the cargo jets I've flown. It worked as advertised for one of our crew a few years ago and probably saved their lives.
Part of smoke evacuation process on B757 Freighters though one does not depressurize the aircraft but the aeromatic valve moves the smoke laded air out though the valve by differential pressure when the Recirculation fans are switched off.
CosmicCruiser From United States of America, joined Feb 2005, 2268 posts, RR: 6
Reply 15, posted (4 years 7 months 2 weeks 5 days 4 hours ago) and read 6753 times:
Well don't forget your comparing apples and oranges. One is a wellcontrolled pressure and a sophisticated fuel and the other isn't. I can say that the procedure worked as advertised for out DC-10 crew yrs ago that landed in Newburg NY. They raised the cabin alt and the fire was subdued long enough for a ldg and exit from the jet. When the fire dept decided to manually pump open the cargo door in lieu off chopping thru the fuselage the fire flourished and the jet burned to the ground.
Markhkg From United States of America, joined Dec 2005, 960 posts, RR: 1
Reply 16, posted (4 years 7 months 2 weeks 5 days 1 hour ago) and read 6695 times:
Quoting CosmicCruiser (Reply 15): I can say that the procedure worked as advertised for out DC-10 crew yrs ago that landed in Newburg NY
CosmicCruiser, was this N68055? It is a very interesting case study, but I was surprised that the effectiveness of raising the cabin altitude wasn't really explored by the NTSB. (It would have seemed to be the perfect case study for this.)
KingairTA From United States of America, joined Feb 2009, 458 posts, RR: 0
Reply 18, posted (4 years 7 months 2 weeks 3 days ago) and read 6386 times:
On the C-130 Emergency depresuriztion was part of Fuselage fire/Smoke and Fume elimination. If the out flow valve and safety valve couldn't depress fast enough one could pull a handle and have the center overhead emergency hatch pop out dumping the cabin pressure almost instantaniously.