Effectiveness of 100% oxygen
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Effectiveness of 100% oxygen
I heard recently that if you are at high altitude such as FL 410 in a depressurized aircraft using 100% oxygen is not enough to keep you conscious. Is this true or could you stay at that altitude or say in the lower 30's for a long time with O2 masks on. I'm thinking of perhaps on a long overwater cargo flight.
The human body brings air into the lungs by the diaphragm creating differential pressure between the chest cavity and the outside atmosphere. However, even with pure O2, the incoming gas is only at the ambient pressure of the atmosphere around it. At 41 000' pure oxygen at ambient pressure is at the minimum density considered necessary for crew to remain functional. The only options available above this height are pressure breathing (forcing O2 at a higher density into the lungs) or a pressure suit (basically a space suit.).
In the "low thirties" useful consciousness on 100% O2 is unlimited, and in times gone by crews use to do long flights on oxygen at high altitudes (e.g WW2 bombers).
Modern pressurized aircraft oxygen systems are deigned as an emergency option, basically to keep crew and passengers alive until they can reach a safe altitude. They do not have the capacity to be used for a long range flight, and aircraft dispatched with inoperative pressurization systems are restricted to lower altitudes, so in reality your scenario of long flights on oxygen is somewhat a thing of the past.
It would also make getting up to go the the "Can" a challenge!!
Hope that helps
Wiz
In the "low thirties" useful consciousness on 100% O2 is unlimited, and in times gone by crews use to do long flights on oxygen at high altitudes (e.g WW2 bombers).
Modern pressurized aircraft oxygen systems are deigned as an emergency option, basically to keep crew and passengers alive until they can reach a safe altitude. They do not have the capacity to be used for a long range flight, and aircraft dispatched with inoperative pressurization systems are restricted to lower altitudes, so in reality your scenario of long flights on oxygen is somewhat a thing of the past.
It would also make getting up to go the the "Can" a challenge!!
Hope that helps
Wiz
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Above FL390 your lungs won't be able to suck the air,so the 100% won't help so much.You'll need oxygen under pressure,otherwise the oxygen from the mask will be of no use to you.In case of rapid depressurization,the TUC will be around 10 sec,so keep this in mind when flying above FL 390,and also to set your mask to 'emergency',in case of depressurization..
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The key as others have said is that usefull time conscious is of the order of ten seconds. Not much in the confusion of a rapid depressurisation, noise, mist, what the **** etc. So in my view the advice to select your regulator to the emergency position is good advice.
Its all to do with partial pressure of O2 in the lungs which drops the higher we go. As far as I can recall from my av med you can survive on normal air pretty unaffected upto around 8000 for prolonged periods. Above this you need an ever increasing percentage of O2 fed to you until at around 33000 you need pure O2. Around about 40000 you will then need this fed under pressure in order to keep the O2 levels up. If its not fed under pressure, depending on altitude, you will not immediately drop but will become hypoxic.
Hope that helps a bit
Cheers
Its all to do with partial pressure of O2 in the lungs which drops the higher we go. As far as I can recall from my av med you can survive on normal air pretty unaffected upto around 8000 for prolonged periods. Above this you need an ever increasing percentage of O2 fed to you until at around 33000 you need pure O2. Around about 40000 you will then need this fed under pressure in order to keep the O2 levels up. If its not fed under pressure, depending on altitude, you will not immediately drop but will become hypoxic.
Hope that helps a bit
Cheers
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Two points worth noting:
1) I understand the '10 seconds' TUC relates to NO supply of oxygen. With the mask at 100% (?and with pressure breathing?) this should improve.
BUT
2) an 'explosive' type decompression forces the air out of the lungs (don't try to hold it in at 41000') which REDUCES TUC.
1) I understand the '10 seconds' TUC relates to NO supply of oxygen. With the mask at 100% (?and with pressure breathing?) this should improve.
BUT
2) an 'explosive' type decompression forces the air out of the lungs (don't try to hold it in at 41000') which REDUCES TUC.
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Above FL390 your lungs won't be able to suck the air,so the 100% won't help so much.You'll need oxygen under pressure,otherwise the oxygen from the mask will be of no use to you.In case of rapid depressurization,the TUC will be around 10 sec,so keep this in mind when flying above FL 390,and also to set your mask to 'emergency',in case of depressurization..
Are your lungs able to suck in the oxygen in that case?
Thanks for the info.
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Emergency O2 is exactly what you need selected at these altitudes. When you have the mask on it won't be a continuous flow anymore, but it just assists you breathing with a small positive pressure.
Back to the original question - also consider decompression sickness before you plan to continue with a high (>F250) cabin altitude. Ideally you should breathe 100% oxygen before going to those altitudes to get the nitrogen out of your blood. The risk increases significantly above F250, and also increases with the time of exposure, age and obesity of the person, and the amount of activity at altitude.
Be careful up there!
Joe
Back to the original question - also consider decompression sickness before you plan to continue with a high (>F250) cabin altitude. Ideally you should breathe 100% oxygen before going to those altitudes to get the nitrogen out of your blood. The risk increases significantly above F250, and also increases with the time of exposure, age and obesity of the person, and the amount of activity at altitude.
Be careful up there!
Joe
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Can't find the precise source but I understand from our training dept that NASA have done some new TUC tests and the age old figures are only valid if sitting still, calmly. Reduces by up to half with increased heart rate / effort being expended.
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"Time of useful consciousness" varies greatly among individuals, and even for a single individual under different circumstances. A long-distance runner, mountain climber, or a person who normally lives at a high altitude will have a longer TUC than the average Joe. A cold, fatigue, or other factors can significantly reduce TUC.
For those not used to flying at higher cabin altitudes (e.g., 8,000'), even 100% O2 at a cabin altitude of 30,000' will not necessarily keep you lucid.
Test your personal limits under controlled conditions (e.g., in an altitude chamber) -- NOT as the PIC of an airplane!
For those not used to flying at higher cabin altitudes (e.g., 8,000'), even 100% O2 at a cabin altitude of 30,000' will not necessarily keep you lucid.
Test your personal limits under controlled conditions (e.g., in an altitude chamber) -- NOT as the PIC of an airplane!
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O2
Gentlemen:
It's been a long time since my basic, high-altitude physiological training, but I seem to recall the figure FL250...
Yes, it's true, at high altitudes, due to the partial pressure of gasses, you have to be "force fed" oxygen in order to get the lungs to absorb what they need. This is what the "Emergency" selector can be used for...among other things (e.g. clearing a mask in the case of smoke, etc.). If I remember correctly, at FL250, this phenomenon begins to manifest...but, really becomes critical around the low to mid thirties.
Fortunately, the new-style masks do this for you. Please refer to Airbus FCOM 1.35.20 P7. With regard to the "Emergency" pressure selector:
"Note: Overpressure supply is automatically started, when cabin pressure exceeds 30,000 feet."
For those aircraft equipped with the older-style masks, it is necessary to manually force-feed yourself until you can get below at least the low thirties...preferably maybe FL250 (in my humble opinion).
In any case, without pressure feed at high altitudes, you can suck like crazy...just before you pass out....
Pantload
It's been a long time since my basic, high-altitude physiological training, but I seem to recall the figure FL250...
Yes, it's true, at high altitudes, due to the partial pressure of gasses, you have to be "force fed" oxygen in order to get the lungs to absorb what they need. This is what the "Emergency" selector can be used for...among other things (e.g. clearing a mask in the case of smoke, etc.). If I remember correctly, at FL250, this phenomenon begins to manifest...but, really becomes critical around the low to mid thirties.
Fortunately, the new-style masks do this for you. Please refer to Airbus FCOM 1.35.20 P7. With regard to the "Emergency" pressure selector:
"Note: Overpressure supply is automatically started, when cabin pressure exceeds 30,000 feet."
For those aircraft equipped with the older-style masks, it is necessary to manually force-feed yourself until you can get below at least the low thirties...preferably maybe FL250 (in my humble opinion).
In any case, without pressure feed at high altitudes, you can suck like crazy...just before you pass out....
Pantload
FL250 is correct. Anything above that you'll be needing positive pressure breathing, even on 100% oxy. TUC above this is improved on 100% 02 but you won't last forever! Best to get yourself below ASAP!
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Some good avmed stuff here, and some not so good.
Alexban is correct about needing oxygen supplied at a positive pressure above about 40,000 ft. Osaka Joe gets the prize for being the first to remember the very real hazards of decompression sickness (or illness) but is not correct when he says, “ . . .emergency oxygen is exactly what you need at these altitudes.” If he’s referring to altitudes above 40,000 ft then “emergency” oxygen, (ie 100% oxygen with a slight overpressure), is not enough. We need oxygen under a higher pressure than that.
We must be careful not to confuse an oxygen mask’s ability to provide “overpressure” with a mask’s ability to provide positive pressure breathing. The mask’s “overpressure” facility, (which we normally have selected as a precaution prior to donning the mask), provides a slight overpressure to make sure we cannot breathe any fumes in from the cabin in the event of a less than perfect mask seal. This overpressure will also be provided automatically, regardless of the switch selection made by the pilot, in the event the cabin climbs above a certain altitude. Again, this is to prevent the pre-determined oxygen/air mix from being diluted with ambient cabin air due to a poor mask seal. This overpressure is not the same as the positive pressure breathing that is required for flight above 40,000 ft.
Above about 40,000 ft positive pressure has to be applied to the O2 to provide the necessary partial pressure to push it into the blood stream. This positive pressure is in a different magnitude to the slight overpressure used for fume prevention. (Mask overpressure to protect against leakage is in the order of 2 mmHg: positive pressure breathing will be in the order of 10-30 mmHg. That’s quite a difference - ask any military pilot who has practised positive pressure breathing in an altitude chamber or custom built training rig).
The 100% or emergency selection on the masks will not, therefore, protect us above 40,000 ft. (Consider this: if it could, the military wouldn’t add an additional positive pressure delivery system in their masks, would they?) If, however, your aircraft is certified to fly above 40,000 ft then there must be a facility in the mask and regulator assembly to provide you with oxygen at sufficient partial pressure to keep you functional until you have time to carry out your drills. These drills invariably involve a descent to a safe altitude. You’ll be out of positive pressure breathing territory very quickly provided, of course, you don’t screw up the drills.
You will not find any aircraft drills that recommend, under any circumstances (except temporarily for safety altitude), that you stay above 10,000 ft cabin altitude following a decompression. Even the military fighter drills tell the pilots to get below 25,000 ft. This is to protect against decompression illness - "the bends”. The military (and civil) masks can easily protect against hypoxia at 25,000 ft but they cannot protect against the bends.
(Note: it used to be 25,000 ft for the bends, then it was dropped to 22,000 ft because studies showed there is still a low risk there). We’re considered safe below 18,000 ft. Safe, that is, from the bends, but not from hypoxia if you are not wearing a functioning mask (despite contrary advice from some non-experts on other threads who think that flight above 10,000 without oxygen may be OK in some circumstances. It isn’t. Never. This is why cabin altitudes are limited to 8,000 ft and the military provide expensive bottled oxygen free-of-charge above this altitude).
Intruder wrote: “A long-distance runner, mountain climber, or a person who normally lives at a high altitude will have a longer TUC than the average Joe.”
Not necessarily. A person from Denver may not, someone from Peru might. You cannot predict a person’s TUC.
“A cold, fatigue, or other factors can significantly reduce TUC.” Well said.
In summary:
Supplementary oxygen is required above 10,000 ft (in fact it’s 8,000 ft for safety – refer to haemoglobin’s oxygen dissociation curve in your old avmed notes).
Mask oxygen concentration (and hence partial pressure) progressively increases above 10,000 ft to provide 100% O2 at 33,700 ft (100% O2 at 33k provides the equivalent O2 partial pressure of sea level).
Between 33,700 ft and 40,000 ft the partial pressure of the 100% oxygen in the mask drops to the same partial pressure of ambient oxygen at 10,000 ft (that's 103 mmHg if you’re still with me).
Above 40,000 ft positive pressure breathing is required to maintain this 103 mmHg partial pressure of oxygen. (ie Proper positive pressure breathing – the mask overpressure provided by the 100%/EMERG selections is not good enough).
At any time: slight mask overpressure is required to prevent ingress of ambient air in the case of suspected fumes or smoke. This usually needs to be selected by pilot during pre-flight checks and protects against poor mask fit.
Above about 8-10,000 ft: slight mask overpressure is automatically supplied by a barostat in the regulator to prevent mask dilution by ambient air (protects against poor mask fit).
Finally, never be complacent about the oxygen drills or your TUC. The book guidelines are exactly that – rough guidelines. That super fit, mountain climbing FO in Mensa may fare considerably worse than his fat, weedy, intellectually retarded, chain-smoking Captain.
Many posters start with, “. . . it’s been a while since my avmed training but I think I recall . . .”
In the wake of the Helios crash and some other near misses, why not get those books out now or Google the subject. But only read those reputable sources who reference their facts.
Ref: Aviation Medicine, 2nd Ed. Ernsting and King (1988) ISBN 0 7506 2275 X. Part I, Chapters 1-9.
Airbus A320/321 FCOM
Alexban is correct about needing oxygen supplied at a positive pressure above about 40,000 ft. Osaka Joe gets the prize for being the first to remember the very real hazards of decompression sickness (or illness) but is not correct when he says, “ . . .emergency oxygen is exactly what you need at these altitudes.” If he’s referring to altitudes above 40,000 ft then “emergency” oxygen, (ie 100% oxygen with a slight overpressure), is not enough. We need oxygen under a higher pressure than that.
We must be careful not to confuse an oxygen mask’s ability to provide “overpressure” with a mask’s ability to provide positive pressure breathing. The mask’s “overpressure” facility, (which we normally have selected as a precaution prior to donning the mask), provides a slight overpressure to make sure we cannot breathe any fumes in from the cabin in the event of a less than perfect mask seal. This overpressure will also be provided automatically, regardless of the switch selection made by the pilot, in the event the cabin climbs above a certain altitude. Again, this is to prevent the pre-determined oxygen/air mix from being diluted with ambient cabin air due to a poor mask seal. This overpressure is not the same as the positive pressure breathing that is required for flight above 40,000 ft.
Above about 40,000 ft positive pressure has to be applied to the O2 to provide the necessary partial pressure to push it into the blood stream. This positive pressure is in a different magnitude to the slight overpressure used for fume prevention. (Mask overpressure to protect against leakage is in the order of 2 mmHg: positive pressure breathing will be in the order of 10-30 mmHg. That’s quite a difference - ask any military pilot who has practised positive pressure breathing in an altitude chamber or custom built training rig).
The 100% or emergency selection on the masks will not, therefore, protect us above 40,000 ft. (Consider this: if it could, the military wouldn’t add an additional positive pressure delivery system in their masks, would they?) If, however, your aircraft is certified to fly above 40,000 ft then there must be a facility in the mask and regulator assembly to provide you with oxygen at sufficient partial pressure to keep you functional until you have time to carry out your drills. These drills invariably involve a descent to a safe altitude. You’ll be out of positive pressure breathing territory very quickly provided, of course, you don’t screw up the drills.
You will not find any aircraft drills that recommend, under any circumstances (except temporarily for safety altitude), that you stay above 10,000 ft cabin altitude following a decompression. Even the military fighter drills tell the pilots to get below 25,000 ft. This is to protect against decompression illness - "the bends”. The military (and civil) masks can easily protect against hypoxia at 25,000 ft but they cannot protect against the bends.
(Note: it used to be 25,000 ft for the bends, then it was dropped to 22,000 ft because studies showed there is still a low risk there). We’re considered safe below 18,000 ft. Safe, that is, from the bends, but not from hypoxia if you are not wearing a functioning mask (despite contrary advice from some non-experts on other threads who think that flight above 10,000 without oxygen may be OK in some circumstances. It isn’t. Never. This is why cabin altitudes are limited to 8,000 ft and the military provide expensive bottled oxygen free-of-charge above this altitude).
Intruder wrote: “A long-distance runner, mountain climber, or a person who normally lives at a high altitude will have a longer TUC than the average Joe.”
Not necessarily. A person from Denver may not, someone from Peru might. You cannot predict a person’s TUC.
“A cold, fatigue, or other factors can significantly reduce TUC.” Well said.
In summary:
Supplementary oxygen is required above 10,000 ft (in fact it’s 8,000 ft for safety – refer to haemoglobin’s oxygen dissociation curve in your old avmed notes).
Mask oxygen concentration (and hence partial pressure) progressively increases above 10,000 ft to provide 100% O2 at 33,700 ft (100% O2 at 33k provides the equivalent O2 partial pressure of sea level).
Between 33,700 ft and 40,000 ft the partial pressure of the 100% oxygen in the mask drops to the same partial pressure of ambient oxygen at 10,000 ft (that's 103 mmHg if you’re still with me).
Above 40,000 ft positive pressure breathing is required to maintain this 103 mmHg partial pressure of oxygen. (ie Proper positive pressure breathing – the mask overpressure provided by the 100%/EMERG selections is not good enough).
At any time: slight mask overpressure is required to prevent ingress of ambient air in the case of suspected fumes or smoke. This usually needs to be selected by pilot during pre-flight checks and protects against poor mask fit.
Above about 8-10,000 ft: slight mask overpressure is automatically supplied by a barostat in the regulator to prevent mask dilution by ambient air (protects against poor mask fit).
Finally, never be complacent about the oxygen drills or your TUC. The book guidelines are exactly that – rough guidelines. That super fit, mountain climbing FO in Mensa may fare considerably worse than his fat, weedy, intellectually retarded, chain-smoking Captain.
Many posters start with, “. . . it’s been a while since my avmed training but I think I recall . . .”
In the wake of the Helios crash and some other near misses, why not get those books out now or Google the subject. But only read those reputable sources who reference their facts.
Ref: Aviation Medicine, 2nd Ed. Ernsting and King (1988) ISBN 0 7506 2275 X. Part I, Chapters 1-9.
Airbus A320/321 FCOM
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I remember a discussion about combi aircraft and a possible A380 Combi with whole main deck cargo, some alleged SOPs regarding cargo fires.
Namely, that unlike passenger and cargo plane belly holds which are often equipped with fire suppressant bottles, the main deck freighters often lack fire bottles on main deck. And that SOP for uncontrolled cargo fire was supposed to be descending to around FL200, decompressing the plane and using crew oxygen.
Can anyone confirm or deny?
Even the military fighter drills tell the pilots to get below 25,000 ft. This is to protect against decompression illness - "the bends”. The military (and civil) masks can easily protect against hypoxia at 25,000 ft but they cannot protect against the bends.
(Note: it used to be 25,000 ft for the bends, then it was dropped to 22,000 ft because studies showed there is still a low risk there). We’re considered safe below 18,000 ft. Safe, that is, from the bends, but not from hypoxia if you are not wearing a functioning mask (despite contrary advice from some non-experts on other threads who think that flight above 10,000 without oxygen may be OK in some circumstances. It isn’t. Never. This is why cabin altitudes are limited to 8,000 ft and the military provide expensive bottled oxygen free-of-charge above this altitude).
Intruder wrote: “A long-distance runner, mountain climber, or a person who normally lives at a high altitude will have a longer TUC than the average Joe.”
Not necessarily. A person from Denver may not, someone from Peru might. You cannot predict a person’s TUC.
(Note: it used to be 25,000 ft for the bends, then it was dropped to 22,000 ft because studies showed there is still a low risk there). We’re considered safe below 18,000 ft. Safe, that is, from the bends, but not from hypoxia if you are not wearing a functioning mask (despite contrary advice from some non-experts on other threads who think that flight above 10,000 without oxygen may be OK in some circumstances. It isn’t. Never. This is why cabin altitudes are limited to 8,000 ft and the military provide expensive bottled oxygen free-of-charge above this altitude).
Intruder wrote: “A long-distance runner, mountain climber, or a person who normally lives at a high altitude will have a longer TUC than the average Joe.”
Not necessarily. A person from Denver may not, someone from Peru might. You cannot predict a person’s TUC.
What is the TUC at 0 feet?
Sure. Then the fatigue might reduce TUC to zero even on sea level...
