Rapid Decompression

[VivaTheBeaver]

Can someone please explain why when a pressurised aircraft suffers a rapid decompression you only have 20 or so seconds of consciousness before hypoxia renders you unconscious when over 10,000 feet, when most fit people can easily hold their breath for over 1 minute. I can understand why a person would be immediately disabled if the aircraft should suffer an Explosive decompression such as the Payne Stewart accident but would like more information on the effects of rapid decompression on a person.

Regards

Beaver

[longarm]

I understood that in a rapid decompression all the air in the lungs is expelled (by various methods) and so you would have no air to work with in the first place.

[VivaTheBeaver]

So are you saying that your body would no longer be able to re-oxygenate the blood in the lungs as the lungs would be deflated/collapsed. As opposed to when you hold your breath with air in your lungs that can be used to oxygenate the blood for longer ?

[HugMonster]

The lungs would not be either deflated or collapsed. In a rapid decompression, they would, should you attempt to hold your breath, come close to exploding. Air would be forced out of your body. You would have no air.

Further, at low pressures the partial pressure of oxygen and nitrogen ensure that it is far more difficult to absorb oxygen than at lower altitudes, so although the partial volume of oxygen at altitude is the same at lower levels, approx 21%, you would absorb far less than that.

Another problem is with the "gasp reflex". In an explosive decompression, the temperature drops very rapidly. Your physiology with rapid chilling ensures that the body's first reflex is to gasp. Unfortunately, this is all wrong. You then immediately ensure that you can't hold your breath. There is a similar problem in immersion. If your aircraft hits the water, the cold water hits you and your body helps not a lot by immediately gasping, sucking in a load of water. Therefore, as you say, under non-stress conditions, although you can hold your breath for a minute or more, it is very very easy to drown in only a few seconds.

[Pilot1113]

I just have nit-pick here for a moment. Payne Stewart's aircraft didn't suffer an "explosive decompression."

It was more like a gradual decompression. Most likely the crew didn't realize they were suffering the symptoms of hypoxia and thus didn't take the action required.

At any rate, that's all speculation. What I want to point out is that the fog (that probably led to the explosive decompression theory) that formed was probably due to the fact that as the air escaped the pressure dropped and the moisture condensed. The -70 temperatures (I think that's the correct temp, but correct me if I'm wrong) then froze the condensation leading to the frosted windows.

The reason why I know that there wasn't a explosive decompression is rather simple: the structural integerty of the aircraft wasn't compromised. In an explosive decompression scenario, there would have been (most likely) a gaping hole in the aircraft.

Sorry, I have to nit-pick, it's a bad habit of mine!

- Neil Harrison

[SchmiteGoBust]

Isn't it something to do with the fact that at lower pressures the haemaglobin in the blood cannot absorb so much oxygen As a result it gives oxygen out into the lungs during a decompression rather than absorbing any more of the minimal amount there?

[HugMonster]

Yes, Schmite. That's the reason behind what I described in my second para. above, omitting your explanation (cos I couldn't remember exactly why! :)).

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Breeding Per Dementia Unto Something Jolly Big, Toodle-pip

[GotTheTshirt]

The Payne Stewart senario is exactly the same as the one we had in UK many years ago with a Beech King Air.
The Beech demonstrator pilot was checking a new owner pilot. Part of the drill was a de-compression at 30,000 feet.
In preparation the Oxygen was switched on ( manually by a Bowden cable on King Air) and masks were donned. the aircraft was then depressurised. Unfortunately the bowden cable had frozen ( as in iced up) and the crew slowly lost consiousness. The aircraft flew on autopilot across the Channel followed by both RAF and French Air Force aircraft until crashing in a field in France.

While flying on Photo Survey at 22,000 in a DC3 there were some very interesting situations when 02 ran low - the main thing being that people are just not aware of the problem and were unable to respond to control demands without even realising it.

[Barbers Pole]

Beaver

The time of usefull consiousness reduces with altitude.
18,000ft 10-15min
25,000ft 2-3 min.
35,000ft 45sec
45,000ft 12sec

Hypoxia: Reduced pressure in the alveloi rapidly reduces the transfer of oxygen into the bloodstream, first cells affected are those with the greatest oxygen requirement. (brain & eye)

Hence our companys procedure that if operating at or above FL 410 then 1 pilot must have a demand O2 mask on.

[L J R]

Got the Teeshirt.


Interesting news. Recently a King Air in Oz flew most the way across the GAFFA after failing to descend at his planned point. Crew LOC due autopilot climb and ran out of fuel [1800 miles down track].

Shows Beechcraft learned nothing. [or failed to make a Corporate D on saving more lives after a blunder with a TP.]

Kinda Sad really.

GAFFA [Great Australian F... All] - Abbreviation explained JIC [just in case]

[GotTheTshirt]

Barbers Pole

Just as matter if interest hiw do these people live high in the Andes or climb Eversest with out O2 ?

Just a thought

LJR
There was a Beech SB to remove the O2 operating cable and fill with a non-freezing grease. Why they didnt do this originally as the cable is clipped to the outer frames on the skin. Also the O2 cylinder is in the adft of the aircraft outside the pressurise area. The standard procedure was to put it on on preflight check but they put it off to stop cylinder leakage.
The procedure now is to put it on at preflight and leave it on.

[Lord Lucan]

VivaTheBeaver

Here’s how I understand the situation I’m a SCUBA diver (increased pressure breathing) and also a pilot (reduced pressure breathing)

The amount of oxygen the body can use from whatever gas fills the lungs depends on the partial pressure of the oxygen contained in that gas.

Under normal circumstances this lung-filling gas is air. Which is, to a first approximation, 20% oxygen and 80% nitrogen. For most of us this air fills our lungs at 1 atmosphere (1 bar) pressure. At this (sea level) pressure, the partial pressure of oxygen is around 0.2 bar. (ie. 20% - the percentage of oxygen in air, times the total pressure, 1 bar). That’s the partial pressure of oxygen most of us humans are accustomed to breath.

Obviously, if we live at 16,000 feet up in the Andes Alte Plano or in the Himalaya, we are accustomed to breathing a pressure somewhat less than this. The total air pressure, will be reduced and hence the oxygen partial pressure also. The body will acclimatise/adapt to this if we stay at this altitude for long enough.

But most of us live at sea level. When we ascend to (say) 10,000 ft, the total pressure is reduced to about 0.7 bar. The partial pressure of oxygen is about 0.14 bar. (0.7 x 0.2). This remains just enough for most of us (acclimatised to sea level pressures) to continue to function, Enough to just get by. If the partial pressure of oxygen falls much lower than this we get problems.

There is one other fact to consider. The human body can only breath in/out at the ambient pressure. There is no “holding your breath”, it cannot be done. The only pressure our lungs can work with is the ambient (cabin) pressure. For a scuba diver this “pressurisation” may be several bars pressure, for a pilot, it may be a fraction of a bar. This product of “cabin” (ambient) pressure, times the percentage oxygen in the gas (air) mixture gives the critical partial pressure of oxygen. The partial pressure is what keeps you alert and active.

If we humans want to continue to function adequately at altitudes much above 10,000ft ambient, we need to maintain a partial pressure of oxygen of (at least) 0.14 bar.

We get two choices:

We can either increase the “total pressure”, while continuing to breath air. This is called pressurisation.

Or we can breath gas mixtures enriched with extra oxygen (to a total of 100% oxygen) and still allow ambient pressure to fall. The sums are the same as far as the body is concerned.. We need to maintain a partial pressure of oxygen of a minimum of about 0.14 bar. The body does not care how it gets it. Total pressure and/or percentage oxygen in the gas mixture.

Looking at my Met notes, atmospheric pressure is about these values at various altitudes:
100 mb – 53,000’
200 mb – 38,000’
300 mb – 30,000’
400 mb – 24,000’
500 mb – 18,000’
700 mb – 10,000’

So you can see that when breathing 100% oxygen, one can go to somewhere between 30 to 45,000 ft and still maintain a partial pressure of around 0.14 mb oxygen. More or less enough to continue to function.

Current airliners cruise somewhere around 30,000 to 42,000 ft. You can do the sums yourself to see what is required.

Breathing 100% oxygen at 40.000ft cabin altitude may not be enough to remain conscious. (A cabin depressurisation without an emergency descent, even while breathing 100% oxygen)

However, a depressurisation at (say) 28,000 ft, while breathing 100% oxygen is not that bigger deal.

But you must also remember that there are large individual differences in reactions to reduced oxygen partial pressures. Caution is advised.

[This message has been edited by Lord Lucan (edited 28 January 2001).]

[This message has been edited by Lord Lucan (edited 28 January 2001).]

[Corporate Yank]

Some years back, I took FAA’s high altitude course (hyperbaric chamber). During the classroom portion, the instructor told us that the partial pressure of oxygen in the atmosphere at approx fl330 equals the pressure required for O2 molecules to pass through the thin walls of your alveoli (sp?), hence no transfer above this altitude without pressure breathing equipment. Keep this in mind the next time you’re a passenger up nice and high in cruise. The mask that is provided for you is just an ornament to satisfy the regulation.-CY

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*deep inhale* I LOVE the smell of jet-a in the morning..

[BEagle]

Having done quite a few sessions in the 'Ruddles Rocket' at Luffenham, I remember that we were told that you start to suffer from significant hypoxia when in an environment at 'half your accustomed atmospheric pressure'. Hence the chaps who live at higher levels can cope with even higher levels before becoming hypoxic than those of us who live at sea level. We were also given a whole load of rubbish about coping with jet lag - we told them the aircrew trick. Get drunk when you get to the hotel and wake up when the alarm call goes off!! They didn't like that one!!

Why the 'Ruddles Rocket'? Because we were always told to stay off the beer (the local beer in the mess was Ruddles County) and eat sensibly the night before the chamber run. Being aircrew - and the place being run by the arch-enemy (doctors), we always ignored their advice and went to Stamford for a curry and a gallon or two of Ruddles. The next day, the release of intestinal gas in the chamber was probably sufficient to propel a rocket to the moon.

[Code Blue]

<font face="Verdana, Arial, Helvetica" size="2">Just as matter if interest hiw do these people live high in the Andes or climb Eversest with out O2</font>


These folk become polycythaemic - a posh way of saying they have more haemoglobin than dwellers at sea level. This enables them to carry more oxygen/ml of blood. It takes several weeks or even months to produce the extra red cells to house all this Haemoglobin. Everest climbers go and live on the tops of mountains before they try the climb.

A greater concentration of red cells also increases the risk of DVT but that's another thread altogether.... ;)

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edited for the customary appalling typing

[This message has been edited by Code Blue (edited 28 January 2001).]

[Barbers Pole]

Got the T shirt

code blue has nailed it for the high altitude living, I think from memory that base camp on Everest is 18,000ft?

Some Athletes also train at altitude before a major event to increase there bodies abilitly to hold O2 in the blood = better preformance.(sprinters)
I heard that some athletes drain a pint of blood off a few weeks before a event and then stick it back in there body before competeting so they can absorb more O2 to enhance their preformance, not sure whether this is true or not?

Anyone know??

[Luftwaffle]

Barber Pole, yes, the practice is called blood doping. It's not exactly allowed, but it's pretty much indetectable, being the athlete's own blood. I think there was an Eastern bloc athlete who died in connection with improperly stored blood, and some US athletes who contracted hepatitis because they didn't use their own blood.

http://www.tc.cc.tx.us/~mstorey/beckham.html

Maybe if you do a whole lot of flogging around at 10,000' in unpresurized aircraft before you get that jet job your body will be acclimatised to altitude and you'd have more useful consciousness in the even of a depressurization.

[Code Blue]

<font face="Verdana, Arial, Helvetica" size="2">Maybe if you do a whole lot of flogging around at 10,000' in unpresurized aircraft before you get that jet job your body will be acclimatised to altitude and you'd have more useful consciousness in the even of a depressurization</font>

The adaptive changes from chronic hypoxia are usually reversible. Thus if you move from 18000' to sea level your physiology will slowly return (adapt) to its new altitude. I think some of the indigenous peoples of the Andes have a genetically higher haemoglobin concentration than 'Westerners'.


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[earnest]

I work in this field as well as fly and Lord Lucan has it just about spot on. Oxygen getting into your lungs isn’t the main issue, it is whether there is enough pressure to drive it across the thin walls of the alveoli into the blood stream. There it’s picked up by the haemoglobin in the red blood cells and transported. When you are holding your breath under water there is oodles of pressure , but at altitude the pressure is insufficient.

I’m a mmHg (millimetres of mercury) person, I’m afraid.

 Sea Level partial pressure of Oxygen is about 103 mmHg
 At 10,000 ft this pressure is only 55 mmHg, but is enough for normal fit people to get by on.
 Above 10,000 ft the oxygen concentration breathed has to be increased, ideally to maintain 103 mmHg, ie more oxygen is added to the air mix in the mask.
 At 33,700 ft breathing 100% oxygen still gives you 103 mmHg.
 Between 33,700 ft and 40,000 ft the partial pressure of your 100% oxygen drops to 55 mmHg. A normal, fit person is still ok, as he is at the equivalent altitude of about 10,000 ft.
 Above 40,000 ft you need positive pressure added to your 100% oxygen.

We are ok up to 10,000 ft because haemoglobin has cleverly adapted its properties with respect to the absorption and release of oxygen. This allows us to live up at these altitudes. Above these altitudes the local population have adapted as previously described, and the climbers have adapted, trained, and are super fit (and usually carry oxygen for the higher bits). Cabin altitudes are kept below 10,000 ft to add a bit of safety factor.

Corporate Yank’s comment is possibly misleading. You are going to get hypoxic in a depressurisation anywhere above 10,000 ft, only the length of time and severity will vary. Above 33,700 ft the drop down masks should enrich your oxygen supply, but they can’t deliver 100% (plus they don’t fit as snugly as the crew masks). They should, however, provide sufficient oxygen for the short time it takes for the emergency descent to start and get you down to safer altitudes. Passengers with respiratory or cardiac problems may suffer though.

[manuel ortiz]

I went to fly A-319's to south america for a few months and had the following experience at Cusco , a nice tawn in Peru with 11,300 feet elev. in the andes.
By the way , if you ever go there ask for the jump seat to see the landing .

So we where as part of a visiting group watching an old church, my wife to my left and a very young beautifull woman to my right , well suddenly this last one starts leaning on my right shoulder ....WOW hmmmm sh...t this had not happened to me before and now my wife being right here ...NO!!! maybe this beautifull girl has not realized abaut my wife ? hmmmm maybe if I turn a bit and wisper to see us a bit later ?
She starts now leaning her head onto my right and well "developed pectoralis"
:-0 continuing dawn !!!!!!
Uuups , UUUUPPSS I better stop alucinating ! she is falling unconscious .......
Still prevented her from hitting ground but took abaut a minute for us to wake her up (unfortunatelly no CPR was needed) http://www.pprune.org/ubb/NonCGI/frown.gif

I was really interested in asking if she had felt any disconfort or sign of what was coming , she said no , one moment she was there and ok and suddenly she woked up on the ground.
High altitude created this sort of event that day also for another visiting person, both of them during their SECOND day there.
Interesting , I had shortness of breath but only on my second night and while in bed ,
( No , not with her ... http://www.pprune.org/ubb/NonCGI/frown.gif
You know why shortness of breath while in bed ? Serious now , looks like your lungs get kind of crushed and get less air capacity I guess.

So , food for thoughts folks !!!

Ah, if you go there , be carefull with the teas you drink , the most comon is the coca tea , it comes in regular nice tea bags and you could have throubles with your medical,
read the label first.

By the way , the andes is a very large region with high MEA's , escape routes are needed , we use special emergency 02 generators for pax in case of depress. this to have more time at high alt to reach lower terrain.

Nuf said folks , Cheers


Manuel

[Corporate Yank]

Check out this article on oxygen usage. Very well written.
&lt;http://www.avweb.com/articles/highalt/&gt;


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*deep inhale* I LOVE the smell of jet-a in the morning..

[VivaTheBeaver]

In response to Barbers Pole’s comment that oxygen mask have to be worn continuously in his company at FL410 and above, and earnest’s comment that the Crew mask’s have a better fit than the drop down masks for the pax. Can anyone tell me what the rapid descent procedure is for Concord at mach 2, has this ever been carried out with pax on board on a revenue flight ? And are the paz masks in Concord more robust to cope with the lower pressure ?

[Code Blue]

manuel ortiz:

High altitude can cause pulmonary oedema. This is a reaction to the low partial pressure of oxygen in which some people get 'leaky' capillaries in their lungs. This produces shortness of breath and can be severe enough to be life threatening. Classically pulmonary oedema is worsened by lying flat and improved somewhat by sitting upright.

Perhaps you had a mild dose thereof.

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