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Oxygen Mask Pressure  
User currently offlineCricri From France, joined Oct 1999, 581 posts, RR: 7
Posted (14 years 9 months 4 days 2 hours ago) and read 2246 times:

Hi, can anyone tell me if there is an altitude restriction for an oxygen mask to work?
I heard that it can work only until a certain altitude and if the plane is flying too high, you won't have enough pressure to breathe the oxygen. Is this right? Thanks for your responses. Regards. Chris.

21 replies: All unread, jump to last
 
User currently offlineJETPILOT From United States of America, joined May 1999, 3130 posts, RR: 29
Reply 1, posted (14 years 9 months 3 days 15 hours ago) and read 2142 times:

The masks aboard airliners can work at any altitude the airplane is capable of flying at.

User currently offlineJim From United States of America, joined Jul 1999, 455 posts, RR: 1
Reply 2, posted (14 years 9 months 3 days 4 hours ago) and read 2133 times:

Just to point out that the masks for the passengers are diluter type. That means that the passenger is only getting about 15-20 % O2 with each breath. The Flight crew has the option of 100% or diluter, depending on circumstances.

jim


User currently offlineCricri From France, joined Oct 1999, 581 posts, RR: 7
Reply 3, posted (14 years 9 months 3 days 2 hours ago) and read 2123 times:

Jim and Jetpilot, thank you both for your clearly understanding respons. Regards, Cricri.

User currently offlinePerthWA From , joined Dec 1969, posts, RR:
Reply 4, posted (14 years 9 months 3 days 2 hours ago) and read 2123 times:

If oxygen masks require a certain a pressure to be workable, you can remember you can increase temo to increase pressusre or vice versa, also, If teh temo then becomes detremental to teh passangers, a secondry heating system can be had for teh cabin

Lastly, oxygen masks are used incase of pressurised problems, so If they dont work due to pressure prblems, how can they be used when the cabin is effetced by pressure problems!


User currently offlineJim From United States of America, joined Jul 1999, 455 posts, RR: 1
Reply 5, posted (14 years 9 months 2 days 5 hours ago) and read 2120 times:

Could you please re-post your question, as I'm not quite sure what you're asking.

jim


User currently offlineBuff From Australia, joined Mar 2007, 0 posts, RR: 1
Reply 6, posted (14 years 9 months 2 days 3 hours ago) and read 2115 times:

Cricri:

Could you please specify what kinds of O2 masks you are asking about? Are they the quick-don masks used by the pilots, the drop down masks used by passengers, the decompression/first aid oxygen kept on board pressurized aircraft, the oxygen in PBE's?

Each has its characteristics, and depending on how and when they are used, the answer to your question could change.

Regardless of the type though, if the oxygen is delivered to a person at high altitude (for instance FL350) in an unpressurized environment, the body will not absorb it.


Best Regards,

Buff


User currently offlineKoopas From United States of America, joined Oct 1999, 172 posts, RR: 1
Reply 7, posted (14 years 9 months 2 days 3 hours ago) and read 2108 times:

Buff,

Are you saying that if a sudden depressurization occurs at 35,000ft, donning the oxygen masks in the cabin by passengers will have no effect since the body cannot absorb it? If yes, why can't the body absorb it? and what is the point of having drop down masks?

Another question would be:
-In the case of rapid/explosive depressurization, i've read that almost everyone will die because of the cold temperature sweeping through the cabin before passengers will have a chance to don oxygen masks. Is this true?

Thanks for your input.
Alex


User currently offlineCricri From France, joined Oct 1999, 581 posts, RR: 7
Reply 8, posted (14 years 9 months 1 day 21 hours ago) and read 2106 times:

To Buff : this problem is in case of passengers oxygen masks that would drop down in the cabin and this is exactly what I wanted to say : that the body physically wouldn't be able to absorb it. Can you tell me more about this phenomen?
To Koopas : OK for the temperature but this is another point of the problem.


User currently offlineBuff From Australia, joined Mar 2007, 0 posts, RR: 1
Reply 9, posted (14 years 9 months 1 day 10 hours ago) and read 2106 times:

Buff, Are you saying that if a sudden depressurization occurs at 35,000ft, donning the oxygen masks in the cabin by passengers will have no effect since the body cannot absorb it?

That is two questions in one, so I will break it down with my answer:

First, for the cabin to go to 35,000 immediately, a catastrophic failure would have had to occur. An example on a small aircraft would be loss of a main door; on a large aircraft, a fuselage section (Aloha 737, or a bomb). Loss of a passenger window or even a door on a large aircraft would not result in an instantaneous cabin altitude of, in this example, 35,000.

Second, within seconds of the failure, the pilots will be starting an emergency descent to get the aircraft to 10,000 or a safe altitude, whichever is higher.

So the direct answer to your question is No I'm not saying that, because the passenger will not experience 35,000 cabin pressure. But given the worst case scenario of an explosive decompression (cabin altitude rising instantly to ambient altitude), the reason to get the masks on is you only have a few minutes of useful activity before you lose consciousness. Once the mask is on, during the descent your body will be able to respire the oxygen coming from the mask on your face. It will not "go without" for any length of time.

If the human being is exposed to 35,000 (or any altitude in excess of 30,000') for any length of time, no amount of oxygen supplied to a face mask will keep you alive for long, unless you're wearing a pressure suit (like a fighter pilot or an astronaut).

If yes, why can't the body absorb it? and what is the point of having drop down masks?

With regards the body's ability to absorb oxygen in a rarefied atmosphere, I can pass along my knowledge, but I would suggest you acquire a reliable text on aviation medicine with a good section on high altitude effects on the human body. In short, for oxygen to be absorbed into the cells of the body, the air pressure outside the cell has to be greater than the pressure inside the cell. If the pressure inside the cell is greater than outside, the cell will expel oxygen. Therefore with prolonged exposure to 35,000 foot pressuization, the more you breathe in from the oxygen mask, the faster you will be breathing out oxygen from within your body, including all the O2 from the mask. Reason being the oxygen entering your lungs from the mask is not absorbed, and is consequently expelled when you breathe out. Eventually, you will lose consciousness, and death will follow. Oxygen supplied to the pilots is under pressure and by a mask that seals itself around the mouth/nose. That oxygen is delivered to the lungs under positive pressure, so is more effective than the free flow oxygen provided by drop down passenger masks.

However, in the example of the drop down passenger masks of which you asked, the oxygen flow will be on your face providing pure oxygen, diluted by any air entering the mask from the cabin atmosphere, for you to breathe once the airplane gets down to the lower levels. And again, from above, the reason to get the masks on is you only have a few minutes of useful activity before you lose consciousness. Once the mask is on, during the descent your body will be able to respire the oxygen coming from the mask on your face. It will not "go without" for any length of time.

Another question would be:
-In the case of rapid/explosive depressurization, i've read that almost everyone will die because of the cold temperature sweeping through the cabin before passengers will have a chance to don oxygen masks. Is this true?


That is too hypothetical for me to answer. However, an answer may be forthcoming if the NTSB is able to determine what happened with last week's LearJet loss. I can speculate that if a cockpit window failed completely (another extremely to the extreme rarity, with their double pane construction), the resultant shock of 1) the window coming back in your face at 500 knots and 2) the inrushing of -55 to -65 degree celsius air at that same speed, then there's probably not much anyone this side of Heaven will be able to do to save the day. Best case scenario, a human being cannot survive -55 celsius temperatures for very long without protective clothing. However, from my reading of the press reports on that LearJet loss, all of the aircraft windows as observed by the F16's seemed to be intact. All speculation, I'm afraid.

Most people have never experienced -55 celsius (-67 fahrenheit). I have, but not in an airplane - I worked in the high Arctic for a couple of years in the early 80's. Anyone who has lived in northern Alberta or the NWT, or northern Ontario might also have experienced these temperatures.

I hope you're still reading, and I apologize for being lengthy, but I can't think of a short answer to a complex question like you've asked! As you can see, there are many variables involved.

Hope this is more clear then when you first asked your first question!!

Best Regards,

Buff


User currently offlineExcelsior767 From , joined Dec 1969, posts, RR:
Reply 10, posted (14 years 9 months 1 day 9 hours ago) and read 2101 times:

Wouldn't a rapid decompression have a large impact on your ears as well? Wouldn't such a change in pressure not only hurt in your ears but also cause permanent damage, like blowing out your eardrum causing deafness? I have no idea what I'm talking about, that's why I'm asking. All I know is that your inner ear is affected by pressure and I can just imagine how painful a "pop" rapid decompression would cause in your ears. Can anyone help?

Jim in Boston


User currently offlineBuff From Australia, joined Mar 2007, 0 posts, RR: 1
Reply 11, posted (14 years 9 months 1 day 9 hours ago) and read 2102 times:

Hey Jim in Boston:

In my reply to Cricri, I mentioned one of the first symptoms of a decompression is felt in your ears. You are very correct that a rapid or explosive decompression can damage your ears, and even some other parts of your body. When I am flying, I depend very heavily on my ears and some other physiological reactions to be a first alert of a pressurization problem. In the cases where I have experienced loss of cabin pressure at 25,000 feet, I knew there was something wrong before the cockpit warning systems engaged.

Gases trapped within the body, when suddenly exposed to reduced pressure, try to get out. Usually, they get out with little difficulty. Like air escaping a balloon. If trapped gases leaves to quickly, ruptures can occur. The eardrum example is very good in illustrating this phenomenon.

A secondary problem occurs after the emergency descent, and that is the gases outside trying to get back into your body! Like trying to put the air back into the balloon. Anyone who has flown with a bad sinus cold will remember the climb to altitude not being that bad (the trapped gases have little difficulty leaving), but during descent for landing, because of blocked nasal and sinus passages, and eustachean tubes, the increased air pressure has trouble getting back in, causing discomfort or even extreme pain.

This whole discussion on loss of pressurization should serve to make people aware that the procedures that need to be followed are not a game, and that flying with physiological illnesses can complicate the already severe effects of high altitude exposure.

Hope that answers your question.

Best Regards,

Buff


User currently offlineKoopas From United States of America, joined Oct 1999, 172 posts, RR: 1
Reply 12, posted (14 years 9 months 1 day 5 hours ago) and read 2101 times:

I am still a little confused...

I don't see the point of drop down oxygen masks then. From your explanation, the pressure differential causes poor absorbtion of the oxygen by the body. Therefore, isn't the flow of oxygen pointless since the body cannot obsorb it? What is the purpose of 02 masks then?

Alex


User currently offlineJim From United States of America, joined Jul 1999, 455 posts, RR: 1
Reply 13, posted (14 years 9 months 1 day 5 hours ago) and read 2101 times:

The absorbtion question is not a problem IF you get back to 10,000 ' or less within three minutes or so. Think about mountain climbers. Mt Rainer is about 14,000' high, so people climb to a camp about 11,000' and stop until about 2am, then continue to the top. They don't arrive until about 2pm, and then they come right back down. They take twelve hours for that last 4000' due to the problems of breathing, as well as the ice

At 35,000 ' there isn't enough air pressure to allow you to absorb oxygen to keep you alive more than a few minutes. As you decend, there is more air, which contains more O2. As long as the pilot noses over, you'll be fine.

jim


User currently offlineBuff From Australia, joined Mar 2007, 0 posts, RR: 1
Reply 14, posted (14 years 9 months 1 day 4 hours ago) and read 2104 times:

Let me try again. The passengers will probably not experience a 35,000 foot cabin. The most they will experience will be in the region of 16,000 to 20,000 because as the depressurization event occurs (99 times out of 100, it is not going to happen instantaneously), the pilots are descending the airplane to lower levels. By the time the depressurization has allowed the cabin altitude to go to, say 18,000 feet, the aircraft has been in a rapid descent for over three minutes. Three minutes at 7,000 feet per minute down from 35,000 places the aircraft at about 14,000 feet ASL. Obviously in this scenario, as soon as the aircraft descended through the altitude that was the equivalent of the climbing cabin altitude, the cabin would no longer be depressurizing, but RE-pressurizing. (Negative pressurization is not possible: negative pressure relief valves)

If the aircraft stayed at 35,000 feet, the depressurization would continue until inside pressure was close to outside pressure. Only then would breathing the passenger oxygen be futile.

Hope that clears it up. It is most important to understand that the depressurization to 35,000 feet is not instantaneous. If 1 time in 100 it is, then you have BIG problems.

Best Regards,

Buff


User currently offlineKoopas From United States of America, joined Oct 1999, 172 posts, RR: 1
Reply 15, posted (14 years 9 months 1 day 3 hours ago) and read 2101 times:

Buff,

Thanks for your reply. Are you saying that while the pilot attempts to decend quickly towards a more clement altitude (~10,000 ft), the 02 masks are only dropped to HELP the passengers breathe? I am still wondering if the 02 masks in the cabin (which from what you've told me are not delivering pressurized oxygen) are actually of help since it appears the primary contributing factor to a passenger's survival during depressurization is the altitude and pressure of the aircraft, REGARDLESS if additional oxygen is provided. Perhaps you can clarify this again, please.

Thanks for your input.
Alex


User currently offlineTriStar From Belgium, joined Oct 1999, 848 posts, RR: 1
Reply 16, posted (14 years 9 months 1 day 1 hour ago) and read 2100 times:

Hello all, and pardon me for dropping in on you.

I've been following this thread with great interest, and I must say Buff's posts are -as always- very interesting and enlightening.

Koopas, from what I understand of Buff's answers, the thing to do in case of depressurization is always: DESCEND. It's basically as simple as that. IMHO, his explanation about the way the oxygen is or isn't absorbed by the body on different altitudes, says it all. Passengers may lose consciousness for a while until on lower altitudes, but by the same token, they will be *on* that lower altitude very quickly. An emergency descent is *pretty* fast and *pretty* steep.
The thing is, people don't realize a commercial aircraft can do lots of things that seem out of this world and which probably wouldn't do bad in an airshow. At least that's what I've learned from talking to pilots -maybe Buff can explore the matter further...? In any case, the reason why people aren't aware of this, is an airliner doesn't usually perform that many "tricks".
Maybe the awareness could motivate more people to actually keep their seatbelts fastened while seated throughout the flight, as is being suggested during safety briefings.

I would say the oxygen masks are quite vital. I'm not sure as from which altitude they actually start working accordingly. As Buff implied, it might be wise to consult texts on aviation medicine regarding the subject. Still, if the oxygen masks were of no use in the cabin, I see absolutely no reason as to why all those airlines around the world would invest in them. Other than them being obliged to do so, that is...

Looking forward to reading more about this highly interesting topic,

TriStar


User currently offlineBuff From Australia, joined Mar 2007, 0 posts, RR: 1
Reply 17, posted (14 years 9 months 15 hours ago) and read 2097 times:

Well said, Tristar. Yes, an emergency descent from altitude is quite dramatic. There is an international standard for aircraft certification that it be able to descend from its highest cruising altitude to below 14,000 feet in (I believe) 4 minutes.

Lately, we've been hearing lots of stories in the media about airliners "plummeting at supersonic speeds". Well, the procedure we follow during training has us descending at nearly 10,000 feet per minute with a deck angle of -15 to -20 degrees. The average negative deck angle experience by an airline traveller is -2. An easy way to envision this very steep deck angle is to recall the positive deck angle after takeoff, and reverse it.

When properly executed, the push over to initiate the emergency descent produces no negative G's. But in real situations, sometimes things aren't executed perfectly. During the initiation of an emergency descent, negative G might be experienced throughout the airplane. This would in all likelihood result in injuries, especially if seatbelts were not fastened per the advisories.

TriStar also hit the mark with the drop down oxygen. One thing I hinted at in one of the earlier posts in this thread was the requirement during high altitude training to use 100% oxygen for at least 1/2 hour prior to the chamber ride. This is to dissolve/purge as much of the free nitrogen from your system as possible. In a worst case decompression in the airplane, once the O2 is on your face, even if you lose consciousness, your body will still be breathing in the O2. Your body will absorb it as soon as it is able, even if you are not aware. If nitrogen bubbles have formed, the pure (albeit a little diluted) O2 will accelerate reabsorbtion of these bubbles more quickly.

Hope its more clear...

Best Regards,

Buff


User currently offlineKoopas From United States of America, joined Oct 1999, 172 posts, RR: 1
Reply 18, posted (14 years 9 months 11 hours ago) and read 2098 times:

Buff, Tristar,

are we saying that the donning of oxygen masks at 35,000 ft is useless if oxygen is breathed at THAT same altitude without an emergency decent being performed?

are we also saying that the donning of oxygen masks is ONLY useful in the recovery of passengers from unconsciousness once at lower (and more breathable) altitudes? In other words, do the masks ONLY help at altitudes of less than 14,000?

Alex


User currently offlineKoopas From United States of America, joined Oct 1999, 172 posts, RR: 1
Reply 19, posted (14 years 8 months 4 weeks 1 day 18 hours ago) and read 2094 times:

Could somebody jump in and answer that?

User currently offlineTriStar From Belgium, joined Oct 1999, 848 posts, RR: 1
Reply 20, posted (14 years 8 months 4 weeks 1 day 16 hours ago) and read 2091 times:

Koopas, I'm sorry for not having noticed your earlier post. It must have hit the forum at a particularly busy time, hence being pushed back before I picked up on it.

Basically, I was trying to elaborate on Buff's explanation. I'm not all that familiar with aviation medicine, myself. By consequence, I can't answer your question to the full, I'm afraid. Maybe Buff can, after he gets back from work...? :-)

In any case, the topic is mindarousing, and it certainly motivates me to look deeper into it. I'll be talking to some people about this, for sure.

Hope to be back with you to either hear or give more information, soon.

TriStar


User currently offlineCedarjet From United Kingdom, joined May 1999, 8060 posts, RR: 54
Reply 21, posted (14 years 8 months 4 weeks 1 day 12 hours ago) and read 2088 times:

I think the masks would work below altitudes of about 18,000 feet, and have some limited effect even a little higher. If there was a loss of cabin pressure with a significant rate of pressure loss, the flight would descend quickly but there would be a portion of the descent where the cabin altitude had risen significantly but the aircraft was still at altitude, and you'd probably feel quite flaky indeed. Other factors to bear in mind are the sudden noise, ear pain and the obvious fact that the plane is plummeting like a stone with lots of buffeting (spoilers up, maybe gear down too). I think the fear and disorientation would make hypoxia just one part of a whole mix of unpleasant sensations.

By the way, most BAC111s don't have passenger oxygen, and this may also be the case with early Caravelles (before the 6R was built for US export). I doubt there would be much use for passenger oxygen on Concorde either, although I think they are on there. Presumably in case something happens during the climb or descent, because if the plane gets a hole in it at 62,000 feet and twice the speed of sound, it's going to be all over very very quickly.

For more info on depressurisation, there is a chapter in a book called "The Rise And Fall Of The DC10" (mangled wreck in the forest in France splashed across the cover, classy). It details what happened when the pilots were experimenting with the autothrottle on a National DC10 flying over Arizona. No 3 engine exploded and shrapnel damaged the aircraft quite badly, and a piece of engine smashed a window and sucked the passenger next to it through the hole. The cabin lost all pressure very quickly and an emergency descent ensued. The oxygen in the DC10 is supplied by using canisters of chemicals which, when activated, trigger a chemical reaction between two elements that creates oxygen. And a lot of heat. Passengers who yanked too hard on the mask pulled the canister out of it's place and suffered serious burns, and smouldered carpeting and seats. Some passengers lost consciousness and most suffered some degree of loss of awareness. The pilots levelled the plane at 11,000 ft after a spectacular emergency descent and after some radio malfunctions and other comms problems managed to make an emergency landing at (I think) Phoenix. Other than the man who was sucked out the window, there were no fatalities and only minor injuries, mostly caused by flying debris and the evacuation.



fly Saha Air 707s daily from Tehran's downtown Mehrabad to Mashhad, Kish Island and Ahwaz
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