Crew oxygen is, as I understand it, a very high percentage of purity.

Oxygen-enriched air acts as an accelerant to a fire, causing things to ignite which wouldn't in ordinary air, and making fire more difficult to put out. Nice illustration of what oxygen-enriched atmospheres can do to shirts: here, p. 5. This also says that hair and clothes can trap oxygen, and this can be very dangerous. A spill from a mask might well accumulate in hair and clothes.

Also, although I'm not a chemist, it does appear that the margin between normal air and dangerous oxygen-enriched air is quite a slim one: this document says that at over 23% concentration "mixtures must be handled with all the precautions and care of pure oxygen as they start to change fire chemistry and enhance combustion."

In the event of flight deck depressurisation occurring simultaneously with a fire, and the crew using their bottled oxygen, could the following events pose a serious danger:

  1. leak in one of the tubes involved in supplying the oxygen
  2. crew member taking off their mask (and using a portable oxygen supply instead)
  3. crew member inadvertently pulling an oxygen tube out of its attachment in the course of some bodily manoeuvre
  4. crew member switching their mask to "emergency mode" i.e. where constant pressure is maintained to prevent inhalation of noxious substances, but also perhaps resulting in leaks of oxygen from the mask? (here)

Not an aviation person so don't know how often emergency mode actually gets used, but if the FD were filling with smoke wouldn't you switch to emergency mode to try not to be breathing it in? But could this exacerbate the fire?

Unless my thinking is flawed in some way, wouldn't that mean that descent to an altitude where you no longer have to use oxygen might be quite an important priority in the event of a FD fire? Or maybe blowing an inert gas like argon into the FD*?

Just a final thought: oxygen-enriched fires also burn at particularly high temperatures (Apollo 1, EgyptAir 667). So the danger of such a fire actually destroying navigational or communication equipment and effectively killing the plane might be greater.

* High-concentration argon poses its own danger but for this specific situation, where the crew are on bottled oxygen, it might be OK until the fire was out.

  • $\begingroup$ Don't forget that oxygen leaking from the crew emergency supply will be diluted by the ambient atmosphere. If there's a fire on the flight deck the crew will have more to worry about than the effect the relatively small amount of oxygen from their emergency kit will have. $\endgroup$ Jul 7, 2019 at 23:01
  • $\begingroup$ Thanks, yes, I was wondering about relative volumes and dilution. OTOH, because the air in the FD is depressurised in this scenario mightn't the atmosphere become quite oxygen-rich quite quickly? Particularly in the case of a leaking or ripped-out hose. Also check out the "shirt" link above: specific warnings about how oxygen can accumulate in clothes and hair = pretty likely if oxygen is spilling from a mask. $\endgroup$ Jul 7, 2019 at 23:34
  • $\begingroup$ Hmm, the shirt is stuffed with paper and filled with pure ("saturated") oxygen. No wonder it explodes. Though, it would also burn quite fast in normal atmosphere, and I'm pretty sure, 23% oxygen wouldn't make it explode. And... A person with oxygen in the clothes should go to fresh air and pat their clothes? If oxygen is such a hazard, patting clothes should be the last thing to do, since it could cause electrostatic discharge, which could start a fire... $\endgroup$
    – sweber
    Jul 8, 2019 at 5:51
  • $\begingroup$ @sweber Fair points. Wd be nice to know how dangerous oxygen-enriched air really is. $\endgroup$ Jul 8, 2019 at 8:14

1 Answer 1


Wouldn't [a] descent to an altitude where you no longer have to use oxygen might be quite an important priority in the event of a [flight deck] fire?

It is an important priority. Below is the first page of the procedure for the Airbus A320:

enter image description here

Going through the procedure, if things got out of hand, there is:

At ANY TIME of the procedure, if situation becomes UNMANAGEABLE :
IMMEDIATE LANDING............................CONSIDER

And even opening the cockpit window is an extreme option for smoke that is too much to be ventilated.

A note on the depressurized cabin/cockpit scenario: Since there is depressurization, any scenario where a hose leaks, the supply will quickly dissipate towards the pressurization leak source (and not saturate the cabin/cockpit). (It will also help dissipate any smoke.)

So it's the opposite, one would link pressurization with oxygen saturation.

One of the items on the smoke checklist is turning off the air supply systems (called PACKs) and using the manual system to deliberately depressurize the plane:

PACK 1+2............................................................OFF
MODE SEL..........................................................MAN
MAN V/S CTL.............................................. FULL UP

"MAN V/S CTL" means "Manual Vertical Speed Control", basically how the cabin altitude (a pressurization term) is set. Full up means max positive rate, which will equalize the pressure with the outside – dumping pressurized air so to speak.

  • $\begingroup$ Thanks. Just a slight query, perhaps: if depressurisation has already occurred and sucked air out, equalising the pressures between the FD and the outside, I don't see that there would be any massive movement of air (whether or not oxygen-enriched) towards the hole where the leak had occurred. Opening the window though - at 35000 ft. Far out. $\endgroup$ Jul 8, 2019 at 0:00
  • $\begingroup$ @mikerodent: Any additional air that is added (offsetting the equilibrium) will go to the leak. As for the window, it's not at 35,000 feet, it requires low altitude and slow speed, but we've already established descending is a priority. Let me know if something else is still unclear. $\endgroup$
    – user14897
    Jul 8, 2019 at 0:03
  • $\begingroup$ Oh, not at 35,000 ft. What a shame. Adding extra gas to a certain volume of gas would increase the overall pressure of course, but any bit of that overall volume might then be pushed out of the depressurisation hole. Note the point about 23%+ concentration being dangerous, accumulation in hair and clothes, and the high combustion temperature (potential instrument destruction). I'm learning as I go along, glad to hear descent is a priority. $\endgroup$ Jul 8, 2019 at 0:11

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