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Uncontrolled cabin fires only need a few minutes to spread throughout the cabin of an aircraft, killing everyone onboard and destroying the airframe.

Now, we all know that depressurization and fire do not combine; the lack of oxygen will eliminate one of the elements necessary to keep the fire alive. So if an aircraft had a way to depressurize the cabin and eliminate oxygen in the event of a fire, the fire would be extinguished, saving lives. Of course this applies to flights above 14,000 ft. I know that depressurization is dangerous but there is a better chance of surviving a depressurization than surviving flames.

Does a mechanism exist to depressurize the aircraft in case of uncontrolled fire in the cabin? If so, under what circumstances would it be used?

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    $\begingroup$ I disagree that fire is the biggest enemy for aircraft. A quick web search shows that its been more than 30 years since a major fire in the air. Even for small planes (which usually don't fly pressurized), the leading danger is fuel starvation, not fire. $\endgroup$ – abelenky Dec 26 '13 at 2:37
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    $\begingroup$ Fire itself is not the main problem. Toxic fumes and smoke are - as with most fires. One important factor in interior design and material choice are FST properties (fire smoke toxicity). $\endgroup$ – yankeekilo Dec 26 '13 at 10:37
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    $\begingroup$ There are emergency procedures for every airplane for fire response. They determine when and if the plane should be depressurized in the event of smoke or fire. $\endgroup$ – xpda Dec 30 '13 at 23:16
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    $\begingroup$ "So if an aircraft had a way to depressurize the cabin and eliminate oxygen in the event of a fire, " Most of us have a hard time living when the oxygen is eliminated. $\endgroup$ – Skip Miller Jan 29 '14 at 18:24
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    $\begingroup$ Just wanted to point out that Swissair 111 was in 1998, and UPS 6 was in 2010. Both are cases of large planes brought down by fire, so it has certainly been much less than "30 years since a major fire in the air." There may be more but those are the main two that came to mind. $\endgroup$ – fooot Mar 26 '14 at 17:00
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Some aircraft do have a manual decompression button which can be used by the pilot in a situation like this. Keep in mind however that it will only help until the pilot descends (which you will want to do asap in order to land) and the air isn't as thin.

It is very important for the crew to get on the ground as rapidly as possible, and simultaneously attempt to put out the fire in order to minimize the amount of damage that can be done. The single biggest factor in an aircraft fire is how much time elapses before they land and can evacuate.

Also, if the cabin depressurizes, typically the oxygen masks deploy automatically which when combined with a fire can have even more disastourous results.

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  • $\begingroup$ Not so sure about "it will only help until the pilot descends", do you mean that the fire will start again once descended? $\endgroup$ – user197 Dec 26 '13 at 0:13
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    $\begingroup$ @HaLaBi Basically, yes: If the materials are still hot and the partial-pressure of oxygen rises enough to support combustion again the materials could re-ignite, putting the crew right back in firefighting mode. (Of course assuming the decompression was both rapid & extreme enough to put out the fire in the first place - which isn't guaranteed.) $\endgroup$ – voretaq7 Dec 26 '13 at 0:20
  • $\begingroup$ Ok, I still think it has better chances, in the worst case scenario a few more minutes to live. Thanks for the answer 😊 $\endgroup$ – user197 Dec 26 '13 at 0:21
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    $\begingroup$ @HaLaBi well in the worst scenario you have a rapid decompression from 8000 feet to say 40,000 feet, the whole cabin fogs up so nobody can see anything (including the pilots), and then things get "interesting". (Not sure on the likelihood of such an event, I'm no expert on pressurized aircraft, but there is that old engineering axiom: "It could always be worse - Just try 'fixing' the problem!" :-) $\endgroup$ – voretaq7 Dec 26 '13 at 6:22
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    $\begingroup$ @voretaq7 I've been in a cockpit that was IMC due to a poor ECS system that fogged frequently during rapid descents. It sounds scary, but its not as bad in real life. I waved off an approach when I couldn't see my instruments, but, usually, the fog dissipates rather quickly. I'd say much worse than fog would be the major risk of DCS and instantaneous hypoxia (single digit seconds of useful consciousness). $\endgroup$ – Rhino Driver Feb 11 '15 at 2:26
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It takes three things to make combustion - heat, fuel, oxidizer. If you eliminate air, you may not have eliminated the oxidizer if the fire has access to some other oxidizer. While decompression does eliminate some heat (probably not enough), if it's an electrical, self-oxidizing, or metal fire, you're not going to put it out with decompression.

Oxygen-fueled small fires can be put out with a wet cloth (like a wet cotton t-shirt). If the fire is in the engine, most (if not all) commercial planes have built in extinguishers and fuel shut-off, if it's in the baggage compartment, then there's a chance that you're not going to land fast enough to avoid structural failure on or before landing, and decompression won't help because the baggage compartment is already depressurized.

Update: David Richerby brought to my attention that passenger planes have pressurized cargo areas. In my research, I came across this page on Boeing's site. Apparently, Boeing decided to solve the issue by:

  1. Starve the fire of oxygen
    1. for Class E cargo compartments only, the pilots do depressurize the compartment.
    2. for other types of cargo compartments, the pilots close the vents, to essentially allow for the fire to burn out all the existing oxygen, and preventing the flow of additional oxygen to the compartment.
  2. Suppressant - Planes are equipped with Halon gas that suppresses fires until the plane lands.
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    $\begingroup$ Baggage compartments are pressurized on most large planes. The only ones that aren't pressurized are the ones behind the rear pressure bulkhead. $\endgroup$ – David Richerby Jan 1 '15 at 17:50
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It really depends on the jet and what type of fire it is and where it's located. It is my understanding you are not necessarily RAM DUMPing (depressurizing) to starve the fire, but remove the smoke and fumes from the aircraft.

In our checklist for an engine fire, you just want to get the engine shutdown as fast as possible to prevent burning through hydraulic lines, fuel lines, etc; your next step is to land the jet. It makes no mention of depressurizing the jet.

However, for an electrical fire the first steps are to gangload your regulator (100% O2/EMERGENCY) and RAM DUMP the cockpit and then start getting the battery, generators, and each individual component shut off to isolate the problem.

So, standard pilot answer, IT DEPENDS... It depends on what the checklist calls for your specific jet and in your specific EP.

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    $\begingroup$ This. Ram dump is a separate emergency procedure to clear smoke and/or fumes in the cockpit which may be due to the fire, but is not connected to the normal fire procedures. $\endgroup$ – Rhino Driver Feb 10 '15 at 23:10
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What you're asking about is essentially a Class E compartment. See this other question/answer for more info: Is the life of pets considered when dealing with a cargo fire indication?

Short answer, yes you can manually depressurize the cabin to try to suppress the fire. But it's a matter of effectiveness. The Class E compartments are used for freighters where the normal "cabin" area is carrying cargo, not passengers. There are three problems with using a Class E method for suppressing a fire in the cabin of a passenger airplane.

First, the passengers need oxygen. The oxygen masks given to passengers were only designed to give them oxygen in a concentration and duration long enough to keep them alive in the event of a depressurization, and the several minutes it'll take to make an emergency descent to 10,000' where supplemental oxygen isn't needed. The supply of oxygen isn't sufficient for a hours long, or even 30 minute long diversion in the event of a fire/smoke situation. Even if the decision was made to allow the passengers to become hypoxic by not providing them any oxygen because of the fire, that's not possible because the masks will drop automatically once the cabin altitude exceeds 14,000'.

Second, those cheap "dixie cup" masks are not fully sealing. If you look at any oxygen mask that the flight crew uses for example (http://aviationoxygen.com/aviation-masks.html), you'll see the difference between those and the yellow passenger masks. The flight crew ones are designed to fully seal the mask around the pilot's face so that the pure oxygen being supplied to the pilot doesn't leak into the surrounding environment where there may be a fire. Fire + pure oxygen = bigger fire. So the last thing you would want is the passenger oxygen system to be activated if there's an active fire in the cabin. This, by the way, is the reason why you won't see commercial pilots with beards or extremely long mustaches. Significant amounts of facial hair interfere with the sealing capability of those masks.

Third, and probably biggest reason, is that the Class E method of depressurizing does not extinguish the fire, it simply suppresses it. This has been proven in a couple recent accidents, most notably the UPS6 747-400 that crashed in Dubai (https://en.wikipedia.org/wiki/UPS_Airlines_Flight_6). Boeing and the industry as a whole, has been re-evaluating the (lack of) effectiveness of depressurization as a fire protection method, not to mention the carriage of lithium ion batteries.

The most effective way to deal with a fire in the cabin is to attack it directly with fire extinguishers, even if that means cutting into the interior structure and paneling to access the fire. In general, flight crew are provided with the tools and training to do that. The quality and extent of the training provided obviously varies widely depending on the airline, regulatory authority, country, culture, etc.

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Uncontrolled cabin fires only need a few minutes to spread throughout the cabin of an aircraft, killing everyone onboard and destroying the airframe.

Well, how do you get an uncontrolled cabin fire? It starts with a cabin fire that the crew notice and try to control, and then ultimately fail to control after many minutes of trying. In those minutes the pilot has hopefully descended from altitude and is nearing an emergency landing site.

When there's a fire on a passenger plane, pilots are expected to land ASAP so everyone can get away from the blazing inferno. That makes good sense to me.

So if an aircraft had a way to depressurize the cabin and eliminate oxygen in the event of a fire, the fire would be extinguished, saving lives.

Except this "solution" requires the pilot to stay at altitude when there is a cabin fire, so that de-pressurization will be effective. Contrasted with the alternative of descending to have a chance at making an emergency landing, I'd have to say that the latter seems much more likely to save lives to me.

I know that depressurization is dangerous but there is a better chance of surviving a depressurization than surviving flames.

I would say it's probably a more pleasant way to die, but doesn't have much going for it apart from that. The fire is much more likely to be slowed rather than stopped, and you can't survive at high altitude forever (even with those silly oxygen mask things). The fire would be back at full strength once you descended.

Also, once people are dependent on the oxygen masks for consciousness, it prevents them from simply reshuffling to areas of the plane that the flames haven't reached yet. Besides, by the time the fire got bad enough that you'd even consider a measure as extreme as de-pressurizing the cabin at altitude, many people would probably already have re-shuffled away from the flames and there wouldn't be enough oxygen masks for everyone in the non-flaming section of the plane.

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  • $\begingroup$ The pilots can't land just anywhere. On transatlantic flights, the nearest airport could be 3 or 4 hours away. Of course, you can always try ditching, but even if that worked perfectly, I'm not sure I'd rather be stuck in a little orange dinghy in the middle of the Atlantic than a perfectly functional plane with a little bit of fire trouble $\endgroup$ – raptortech97 May 4 '15 at 11:04
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    $\begingroup$ @raptortech97 Of course, if the fire can be fought effectively (without de-pressurizing anyone), that's a better solution. But I'd -much- rather be stuck in a little orange dinghy in the middle of the Atlantic than be de-pressurized at 30k feet. $\endgroup$ – Atsby May 5 '15 at 6:26

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