Hypoxia

[Connetts]

I have long wondered about this question, but before posing it I'd better explain who I am because I know enough to know that my question must be silly and yet I have no answer. I am an 87 year old retired university Law academic. I always wanted to be an airline pilot but my father made sure that I'd end up where I did. After my sons finished studies and the mortgage on the house was paid, I started on a PPL which a dissecting aortic aneurism put an end to. In the mean time, I specialised in research and publishing on the application of criminal justice to safety in aviation and have refereed papers to my name. The correctness of the "just culture" is something I feel strongly about. It was actually the G-AWNO incident which set me off. It was an outrageous prosecution and conviction, done for political reasons. I had two papers on it and my views are unshaken despite an angry letter from the CAA. I also read avidly on aviation and aircraft but am now a bit out of touch.

So I can distinguish between an engine and an elevator with some confidence and be correct most times. This is why I know my question must be silly even though it bothers me and I should know better.

When pressurisation is lost over 10,000 feet there is grave danger of hypoxia and oxygen is supplied. There is, I understand, no more than ten minutes for the pilots.

My question is this: an airplane needs air to fly and even over 10,000 feet there is a lot of it outside. Is it not possible, as a temporary and emergency matter, to augment the pilots' breathing by taking in outside air as an emergency supply? I appreciate the issues of drag and complexity, but under the circumstances would these not be acceptable penalties and possibly useful as an advantage by being designed to help to push the nose down? After all, the crew will be under desperate conditions and possibly becoming incapacitated. When safety has been restored, the emergency supply would naturally have to be cancelled.

If you've read this far, thank you. I respect this as a professional forum, and acknowledge that I am an outsider. Nothing that anyone writes in reply will be recorded and given in evidence!

[Bksmithca]

When pressurisation is lost over 10,000 feet there is grave danger of hypoxia and oxygen is supplied. There is, I understand, no more than ten minutes for the pilots.

My question is this: an airplane needs air to fly and even over 10,000 feet there is a lot of it outside. Is it not possible, as a temporary and emergency matter, to augment the pilots' breathing by taking in outside air as an emergency supply? I appreciate the issues of drag and complexity, but under the circumstances would these not be acceptable penalties and possibly useful as an advantage by being designed to help to push the nose down? After all, the crew will be under desperate conditions and possibly becoming incapacitated. When safety has been restored, the emergency supply would naturally have to be cancelled.

The problem is the higher you go the less usable oxygen you have available.

[gums]

Salute connetts!

Plenty of thread discussion around here and then there's basic wiki research and aviation websites for learning. I posted something just today, being an aviator and sometimes contributor here.

The main hypoxia disussion revolves about lack of oxygen as we go up. We pass oxygen to our blood when we breath out and the difference between internal and external pressures moves those molecules.

Never too old to learn something, man, and I am over 80 as well.

Gums sends...

[L1011effoh]

When pressurisation is lost over 10,000 feet there is grave danger of hypoxia and oxygen is supplied. There is, I understand, no more than ten minutes for the pilots.

Your estimate is a bit off for pilots in reasonable health. The Tine of Useful Consciousness at FL180 (18000’) is 10-15 minutes for a rapid decompression. See here for a useful article:
https://www.theairlinepilots.com/forumarchive/aeromedical/decompressionandhypoxia.php

[V_2]

I think Connetts means the crew oxygen only lasts 10 minutes. Most last much longer than that, on my aircraft it’s quoted as 22-90 minutes

air straight from outside might have ozone implications, although it’s possible and it’s what the Dreamliner essentially does. My aircraft does “scoop” air from outside to help pressurise the plane if the bleeds fail, but if there’s a big hole from rapid decompression than no scooping is going to be enough, just have to get down.

[Busbuoy]

The problem with breathing the outside air is that the percentage of oxygen, known as the partial pressure, is the same as the inside air and is insufficient to maintain consciousness and ultimately sustain life the higher you go. To restore the required partial pressure you can either compress the air, hence the advent of aircraft pressurization, or increase the percentage of oxygen in the breathing mix by supplying supplemental oxygen via a mask. You can't supply compressed air directly via a mask as the pressure in the lungs and chest would be well above the pressure of the surrounding cabin air and you would not be able to breathe out; you would literally be blown up like a balloon. You could extract the oxygen from the air and then administer it via a mask using a system such as OBOGS (onboard oxygen generating system) but these systems are heavier, costlier and require more technical support than a simple gaseous or chemical stored oxygen system.

[Equivocal]

This is not my area at all so I may be talking rubbish, but….I have always assumed that the problem is that altitude the air pressure is lower and so for each breath of air contains less of the constituent gasses. So hypoxia (in my mind, at least) is the consequence of not getting enough oxygen into the body. I presume that this could be overcome by feeding pressurised air through a mask, but the alternative of supplying supplemental oxygen, thereby increasing the amount of the critical gas, is easier, cheaper and requires less equipment to do. At more extreme altitudes I presume that human physiology comes into play and the rib cage volume and musculature still cannot get enough O2 into the body, hence the need for pressure suits.

Like I say, might be rubbish and I defer to anyone properly qualified to answer!

[Busbuoy]

To add to the discussion, perhaps a description of the systems in a typical military aircraft might help. A typical fighter aircraft has a pressurized cockpit but the level of pressurization is less than that of an airliner. Less pressurization means less structure required therefore lighter weight. And you can get away with less pressurization by making the pilot wear an oxygen mask continuously. You need some pressurization to enable the aircraft to operate at higher altitudes because of the amount of oxygen required as you climb.
The oxygen is delivered by a demand flow regulator that senses cockpit pressure and increases the percentage of oxygen as the pressure decreases. At approximately 35000' cockpit pressure 100% oxygen is no longer sufficient and the oxygen flow begins to be delivered under pressure requiring a specific technique on the part of the pilot to breathe. The technique I was taught was "in for 2 seconds, hold for 2 seconds, exhale for 4 seconds" remembering that during the "in" phase you really just relax your chest and oxygen is forced in, and during the "exhale" phase you are forcing the exhaled air out against the mask pressure. This is tiring and makes carrying out complex tasks like piloting, etc difficult. So fighter aircraft pressurization schedules are designed so that at the maximum altitude capability of the aircraft, typically 50,000', the cockpit altitude is at least below 35,000' but is generally quite a bit lower due to other physiological problems associated with sustained exposure to altitudes above 20,000'. In the event of loss of cockpit pressure at high altitude, pressure breathing would be required immediately and the pilot's primary task becomes descending to where pressure breathing is no longer required and then below 25,000' to avoid the abovementioned problems. Above 50,000' feet cockpit altitude the pressure delivered by the regulator is higher than can be tolerated by the lungs and chest wall so a chest garment (called a pressure jerkin in my days) has to be worn to prevent the chest being "overinflated". If flight was planned above 50,000' a pressure jerkin would have to be worn in case of rapid depressurization.

[gums]

Salute!

Some good poop coming here.... OTOH, if I may add.

I flew over thousand hours in the A-37, and it was not presurized. And we used the origianal T-37 oxygen diluter-demand thing. We routinely came home above 20,000 feet and could feel the increase in pressure from our mask easily when climbing to 25,000.

My pressurized planes maintained a 5 psi differential, so we were not exactly like civilian transports.

With my personal problem I can tell you the partial pressure at 8,000 or 9,000 feet makes a difference, and looking at the atmosphere charts the ratio of oxygen there is somewhere around 75% of sea level, and I can tell it.

Oh well, good to see some discussion on the matter.

Gums sends...

[tdracer]

The other reason for a pressure suit at high altitudes is to avoid the "Bends" as dissolved nitrogen starts coming out of the blood.
During flight testing of the 777 back in the mid 1990's, there was a failure which resulted in a sudden depressurization of the aircraft. Although everyone quick got on O2 and the aircraft rapidly descended, a couple of the flight test crew were briefly hospitalized due to 'Bends' like symptoms.

[TheFiddler]

The B738 has a 114 cubic foot cylinder typically pressurised to between 800 and 1,200psi - it lasts way more than 10 minutes, even with 3 in the flight deck.

[Connetts]

I am grateful for the insights above. I ought to have made it clearer that I was interested in the crew especially on the flight deck in a decompression situation. Naturally the pilots and cabin crew are trained to skills which are maintained. Passengers must do what they're told, and national and international laws makes failure to comply with necessary orders a crime.

What I gather from the answers is that the problem has complexities which make the use of outside air to masks a doubtful measure. What is done -- oxygen to masks -- is simpler, effective, sufficient and frankly cheaper than anything suggested in my question.

It's been a long time since i submitted anything to Pprune, and as before I am grateful for the tolerance shown to an outsider, and the patience shown to my ignorance. I was troubled by the Greek crash many years ago when a maintenance engineer left a key control in a position which resulted in the lethal hypoxia of all on board. The prosecution that followed was unjustified, in my view, and did not enhance safety. I commented as lawyer interested in the just culture, and I was grateful then too for the patience shown to me.

Thank you all for your responses.

[PJD1]

What I gather from the answers is that the problem has complexities which make the use of outside air to masks a doubtful measure. What is done -- oxygen to masks -- is simpler, effective, sufficient and frankly cheaper than anything suggested in my question.

Not a doubtful measure, simply something that would not work. At normal atmospheric pressure we breathe air at 1 bar pressure, air is roughly 80% nitrogen and 20% oxygen so 0.8 bar of that pressure is nitrogen and 0.2 bar is oxygen (this is termed partial pressures). At 30,000ft the atmospheric pressure is only about 0.3 bar so the partial pressure of oxygen in air at that altitude would only be 0.06 bar, at that partial pressure there are simply not enough oxygen molecules getting in to our blood to keep us alive.

We can only breathe gas at ambient pressure, our lungs are flexible membranes and if we tried to fill them with pressurised air to increase the partial pressure of oxygen available they would simply expand and burst like a balloon (the medical term for this is air embolism). The only way we can increase the partial pressure of oxygen to a level that will sustain us is to breathe a gas that has more oxygen in it. Breathing pure oxygen at 30,000 feet we have a partial oxygen pressure of 0.3 bar which is more than enough to keep us alive.

[Luc Lion]

I am surprised that oxygen concentrators or a mix of oxygen concentrator and oxygen cannister is not seriously considered by the aviation industry.
The mix is needed in case the power supply is interrupted.
Here is a study made by the US army on the use of such concentrators up to an altitude of 18000 ft.
https://apps.dtic.mil/sti/tr/pdf/ADA371747.pdf

[roger4]

People have climbed Everest without oxygen, so you don't necessarily die at 29000ft.

I remember talking a number of years ago to a researcher who was involved with studying the physiology of mountain climbers and (a) they were acknowledged as exceptional individuals, and (b) their oxygen saturation levels at altitude were at such low levels the medics couldn't understand how they were still alive, let alone functioning. Living and working at altitude causes the body to adapt to low oxygen levels, hence altitude training for athletes for a month or so before the Olympics etc. Not that any of that helps us mere mortals (flight crew or SLF) in the event of a sudden decompression.

I always understood the crew (and SLF) oxygen system only contained 10 minutes (or however long) supply as this was the time required to make an emergency descent to below 10000ft and back into oxygen levels that most of us can tolerate.

[gums]

Salute!

I can guarantee that health care oxygen concentrators work as PJ above has indicated. Have been on the things since 2014 due to lung disease and when up at my 8,000 foot cabin. Without any increase partial pressure, my measured oxygen concentration easily goes up to 94 or 95 %.

However, they lose their specified concentration level once above 10,000 feet or so unless built to a very high spec and cost much more than the ones that are readily available for health care. Go to high Colorado towns and see how many folks are walking about with the small units that only last 2 or so hours. Of course, there are small tanks of pure oxygen that work even in space! I have mine in the back of the car and the regulator costed me about $250.

Nevertheless, hypemic and histoxic hypoxia must be addressed, and my feeling is look at detection/warning systems then emergency systems to get down.

Gums sends...

[wiggy]

I always understood the crew (and SLF) oxygen system only contained 10 minutes (or however long) supply as this was the time required to make an emergency descent to below 10000ft and back into oxygen levels that most of us can tolerate.

As has been mentioned upthread how much oxygen is onboard is aircraft type and even airline dependent (because of their route structure),

Over many parts of the world you cannot simply dash down to 10,000 amsl in the event of depressurisation because the terrain gets in the way. If that's the case your supply may have to cater for extended cruise at an intermediate altitude above 10,000 feet, where all the crew and some/all passengers need supplementary oxygen for perhaps an extended period of time.

[tdracer]

I think some of the confusion is that 10 minutes of supplemental oxygen is the cert requirement. Hence the passenger O2 canisters/generators provide at least 10 minutes, and it's also a cert requirement that you demonstrate that - in an emergency - you can descend from the max certified aircraft cruise altitude to 10k in 10 minutes (although there are exceptions). Crew typically have much more than 10 minutes available - and on some freighter aircraft (e.g. 747F), SOP for a cargo fire is to depressurize the aircraft (to starve the fire of oxygen) and the crew go on supplemental O2 - these aircraft need to have several hours of crew O2 available.
As noted, 10k is a somewhat arbitrary number - people live at altitudes above that and I regularly ski at a Colorado ski resort where the town is over 9,000 ft. and most of the ski area is between 10k and 13k (granted, altitude sickness is a concern for those who come up from sea level).

[Luc Lion]

In EASA land, the article CAT.IDE.A.235 "Supplemental oxygen — pressurised aeroplanes" in AIR-OPS requires at least 2 hours of supplemental oxygen for the flight crew for pressurised aircrafts flying above 25,000 ft.
Supply for
1. Occupants of flight crew compartment seats on flight crew compartment duty
Duration and cabin pressure altitude
(a) The entire flying time when the cabin pressure altitude exceeds 13 000 ft.
(b) The remainder of the flying time when the cabin pressure altitude exceeds 10 000 ft but does not exceed 13 000 ft, after the initial 30 minutes at these altitudes, but in no case less than:
(1) 30 minutes’ supply for aeroplanes certified to fly at altitudes not exceeding 25 000 ft; and
(2) 2 hours’ supply for aeroplanes certified to fly at altitudes of more than 25 000 ft.