Should depressurization be considered in the event of an inflight cabin fire?

Question

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?

Answer 1

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.

Answer 2

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
   
2. for Class E cargo compartments only, the pilots do depressurize the compartment.
   
3. 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.
   
4. Suppressant - Planes are equipped with Halon gas that suppresses fires until the plane lands.
   

P.S. Decompression will not eliminate all oxygen, as the plane is still in the atmosphere. It will only partly reduce the amount of oxygen in the cabin.

Answer 3

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.

Answer 4

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.

Answer 5

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.

Answer 6

You asked

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?

Not so much to extinguish the fire, but the "smoke evacuation" checklist on the Boeing 747 combi (and perhaps other models) calls for a descent to a breathable atmosphere and the cracking open of the front 2 cabin doors as described in the accident report for South African flight 295.

(emphasis mine)

At some point during the flight, believed to be during the beginning of its landing approach, a fire developed in the cargo section on the main deck which was probably not extinguished before impact. The 'smoke evacuation' checklist calls for the aircraft to be depressurised, and for two of the cabin doors to be opened. No evidence exists that the checklist was followed, or that the doors were opened.

Having watched the aircrash investigation tv show (Mayday in other juristictions) on this accident, the depressurization/opening of doors was to clear the smoke so the passengers could breath, not extinguish the fire.