View Full Version : Extinguishing onboard fire by de-pressurising

13th Sep 2007, 16:53
I was having a discussion with a mate regarding the possibility of extinguishing a fire by depressurising the aircraft. In theory, if you would have an open cabin fire spreading quickly with no chance of getting down in time would opening the outflow valve to try to depressurise be possible in order to kill the fire? Again, very theoretical question...just curios about your inputs!


13th Sep 2007, 17:52
The newer 737NGs have an automatic system whereby if smoke is detected in either cargo hold the cabin automatically descends at 750fpm. This is not to extinguish the fire but to help prevent the smoke from reaching the cabin.

I Just Drive
13th Sep 2007, 18:37
I didn't know that. So why doesn't the cargo fire checklist tell you this?

13th Sep 2007, 19:06
Just a thought but, if you depressurise, then the cabin air will move (towards the outflow valve) and is it not then likely that the fore will move, and therefore spread in that direction?

Ndicho Moja
13th Sep 2007, 21:17
Remember the three requirements for a fire: Fuel, Heat and Oxygen. With the amount of oxygen, as a percentage of volume, still much the same at altitude as it is at sea level, the chances of extinguishing a fire by depressurisation is pretty slim. It is only earthly creatures that have difficlty at altitude and that is because of a lack of pressure.

Don Coyote
13th Sep 2007, 21:36
If you depressurise then the cabin masks will drop. Loads of passengers will then pull them and activate the masks thereby filling the cabin with oxygen and making the situation worse!

13th Sep 2007, 21:45
The only time I had had this on a check list to depress cabin (and starve the fire of air) was when in non-pax mode ATR in freight mode
I think it was called C class cabin because there was no cabin crew to extinguish a fire in the cabin

Clarence Oveur
13th Sep 2007, 21:57
With the amount of oxygen, as a percentage of volume, still much the same at altitude as it is at sea level, the chances of extinguishing a fire by depressurisation is pretty slim.

Oxygen content as a percentage of volume might be constant as you ascend, but the molecular concentration certainly is not. It's the very reason why pressure drops with altitude.

Depressurisation is often the procedure on freighters in case of smoke or fire.

Ndicho Moja
13th Sep 2007, 22:31
Clarence, thank you. I am afraid my in depth knowledge on this topic has come to an end, so no more from me.

13th Sep 2007, 22:32
If you depressurise then the cabin masks will drop. Loads of passengers will then pull them and activate the masks thereby filling the cabin with oxygen and making the situation worse!

Could you supply some verification for this statement?, perhaps some numbers by volumn or percentage change in O2 content assuming of course the passengers aren't dumb enough to inhale the pure oxygen and change it to CO2 and water so it can't keep the fire going :p

14th Sep 2007, 02:38
An aircraft "full" of oxygen because the masks have dropped. That blows Avogadro's hypothesis out of the window faster than that fat red-haired bloke in the Bond movie. Actually may I remind you of the Boeing advisory of 01 Apr in such situations that depressurisation to extinguish a fire must be accompanied by jettisoning ALL fuel including contingency and landing reserves so as to deprive the seat of the fire of both oxygen and fuel. Under the circumstances CFIT Regulations apply and it is not necessary to land at the nearest or any airport.
Raoul Lufbery (WW1) said that if his aircraft caught fire, he would put it into a steep dive and if that did not work, he'd jump. It didn't and he did. Clarence A Maaske (WW2) said that depressurisation didn't lose enough oxygen to suppress the fire but it stopped the screaming. (Lufbery flew a single seater, Maaske a Superfortress).
Lovely idea to depress but i don't think it is going to happen fast enough to make ten cents difference. Just point the nose at Johnnie Howard's house and hope he is in.

14th Sep 2007, 07:50
The second photo on this page (http://www.b737.org.uk/pressurisation.htm)shows a 737 cargo version pressurisation panel. If you compare it to the standard pax aircraft panel above you can see the "Smoke clearance" switch. This also works by slowly depressurising the cabin / hold.

BTW the aircraft was c1969, so not a new idea.

Spooky 2
14th Sep 2007, 16:12
Interesting concept that is in fact used in the GV/550 baggage compartment just aft of the main cabin. The door between these two compartments is a pressure door and if smoke or fire is detected in this compartment, one would of course make sure the door is closed, then deactivate the pressure seal around the external baggage door, by pulling a handle just above the internal door, thus allowing the baggage compartment to equalize with the outside pressure. Since most GV/550's operate at altitudes in the range of 410 to 510 this would appear to be a fairly effective procedure. Don't know if anyone has used this procdure yet, but if someone here has more inof please post the results. This feature is exclusive to the GV/550 and not any other GAC products.

14th Sep 2007, 17:25
The QRH for a main deck cargo fire on the 744F involves de-pressurising the aircraft

14th Sep 2007, 18:29
727-100 Freighter/Combi QRH included descent to and maintain 20,000ft then depressurize (plus several other critical reconfiguration items all completed by the S/O). The lack of oxygen at 20,000 would starve but not fully extinguish the fire and give you an opportunity to divert. S/O would enter main deck compartment with full face oxygen/smoke mask and asbestos gloves and operate a very large extinguisher with a long metal extension tube.

737-200 Freighter/Combi "Smoke Clearance" switch completed similar S/O items automatically as you descended to and maintained 20,000. Switch selection determined by whether you were in full freight or freight/pax configuration. F/O would enter the main deck compartment and action per S/O on 727.

Not pretty but when you are in the High Arctic at night 2 hours from any useable airport, well................

Don Coyote
14th Sep 2007, 19:28
Sorry but I did not make myself clear as I was talking about the A320/321 when carrying passengers. On the A320/321 the oxygen masks are connected to chemical oxygen generators, there is a generator each side of the cabin above each row of seats with 3 masks fitted to each generator. Only 1 mask has to be pulled to activate the generator which then provides pure oxygen for around 15 minutes.

The masks drop automatically when the cabin altitude goes above 14,500ft and when in operation the generators also give off a lot of smoke and heat.

Given that the safety brief states that passengers should pull and then don the mask when they drop then a number, if not all, of the oxygen generators will be activated and then start giving off pure oxygen which is not what you want when there is a fire in the cabin.

The emergency procedures do suggest depressurising the aircraft as a means of clearing the smoke but only "if the smoke/fumes become the greatest threat to the safety of the aircraft."

That is the case with the A320/321 when carrying passengers. It is a different situation with an empty aircraft or a freighter or if the oxygen is supplied by demand regulators.

15th Sep 2007, 01:52
Ok DC. It's only my opinion but I think if you do some technical analysis you will find the amount of oxygen released is quite small. A hint is the size of the tube carrying the oxygen to the mask. If you are depressurizing at a cabin climb rate of 2000fpm or more you can probably guess where most of the cabin air and oxygen is going! If you are on an older type with bottled oxygen and one of the lines burns through then all of the above is irrelevant.
As usual-Caveat emptor.

15th Sep 2007, 02:20
I can understand this if in freight config, but as stated so eloquently above, depressurisation to such an extent that it causes any kind of slowing in burn rate is going to make breathing rather difficult!

In freight config, not so important, but with pax on board I'd say it's rather a big point to consider!

17th Sep 2007, 09:43
One thing to remember is that O2 masks are of diluter type so when breathing, one is inhalating O2 and ambiant air with toxic fumes, that is why we ask pax to use a tissue and go down to the floor level where there is less fumes to be inhalated.
Now if you depressurize when having a fire in a Pax version a/c, you have the masks falling and the pax reaching for them, breathing a lot of toxic fumes and having problems...unless you disable the masks!
That is why the difference in dealing with fire with pax vs freighter.

19th Sep 2007, 16:00
the equivalent altitude (as defined by the partial pressure of oxygen of inspired gas) as a function of the fractional oxygen enrichment. Figure 3 shows that at 5000 m, for each percentage point increase in oxygen concentration (increasing from 21% to 22% oxygen concentration is an increase of one percentage point), the effective altitude experienced by the body decreases by 300 m. Thus, if the oxygen concentration in the buildings on the MMA site is increased from its natural 21% up to 26%, the altitude effects on the workers in the buildings will be the effects expected for 3500 m

Note: 3500m is equivalent to about 12,000 feet.

The problems of fire hazard in a low pressure, oxygen enriched atmosphere such as that proposed for the MMA buildings are addressed by the National Fire Protection Association (NFPA) codes18,19. The NFPA19 defines an oxygen enriched atmosphere to have increased fire hazard, in the sense that it will support increased burning rates of materials, if the percentage concentration of oxygen is greater than 23.45/(TPatmos)0.5 , where TPatmos is the total barometric pressure expressed as a fraction of sea-level pressure. For the MMA site TPatmos=0.55 so if the oxygen concentration is greater than 31.6% it would exceed the NFPA threshold. Our proposed oxygen concentration of 26% is well below this threshold. Tests show that the burning rate of paper in 26% oxygen at an altitude of 5000 m is about 70% of the sea level burning rate