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Hypoxia at night (Read 1119 times)
Jan 11
th
, 2010 at 9:46pm
SubZer0
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My instructor brought on an interesting question today during ground school:
Why are we more hypoxic at lower altitudes during the night?
Anyone care to give it a try?
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Reply #1 -
Jan 11
th
, 2010 at 10:33pm
DaveSims
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SubZer0 wrote
on Jan 11
th
, 2010 at 9:46pm:
My instructor brought on an interesting question today during ground school:
Why are we more hypoxic at lower altitudes during the night?
Anyone care to give it a try?
Been a while since the human factors class I once had to take. I am drawing a blank, other than true.
Dave
www.flymcw.com
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Reply #2 -
Jan 12
th
, 2010 at 12:06am
beaky
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This is either a case of not understanding what "hypoxia" means, or your instructor has got the facts mixed up...
Hypoxia is a physical condition which occurs when part or all of the body is deprived of adequate oxygen. For pilots, hypoxia usually results from the ambient pressure being too low for the lungs to take what oxygen there is and put it into the bloodstream.
We are not "more hypoxic at lower altitudes at night", because pressure is pressure regardless of day or night.
Hypoxia is not a problem for folks walking around on the surface (near sea level) at night, right? So... why would "lower altitudes" have less oxygen at night?
What you and/or your instructor might be thinking of is
reduced night vision
due to increase in altitude. The reduced oxygen level affects night vision much more than day vision.
According to the American Optometric Association:
The effect of altitudinal hypoxia on night vision is primarily one of an elevation of the rod and cone threshold. Although decreased cone function is clearly demonstrated by the loss of color vision at hypoxic altitudes, the decrement in central VA is usually insignificant. However, scotopic night vision at altitude can be significantly reduced. Scotopic vision has been reported to decrease by 5% at 3,500 feet, 20% at 10,000 feet, and 35% at 13,000 feet, if supplemental oxygen is not provided. Thus, the use of oxygen, even at low pressure altitudes, can be very important at night.
The rest of the article explains more:
http://www.aoa.org/x5352.xml
As far as breathing goes, pressure is pressure, whether it's dark or light outside. If anything, night temps being cooler (in general) allow for a lower lapse rate in ambient pressure (the colder it is, the denser the air is).
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Reply #3 -
Jan 12
th
, 2010 at 12:19am
SubZer0
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Thank you for your replies.
Rotty, perhaps I phrased the question wrongly. What I meant to ask was, "Why do we need oxygen masks at 5,000ft during the night but 8,000 during the day?"
I think you hit the spot right on with your explanation of vision during the night. Very cool information indeed
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Reply #4 -
Jan 12
th
, 2010 at 3:55am
C
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SubZer0 wrote
on Jan 12
th
, 2010 at 12:19am:
"Why do we need oxygen masks at 5,000ft during the night but 8,000 during the day?"
First I've heard of it...
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Reply #5 -
Jan 12
th
, 2010 at 4:38am
expat
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SubZer0 wrote
on Jan 12
th
, 2010 at 12:19am:
"Why do we need oxygen masks at 5,000ft during the night but 8,000 during the day?"
By that rational all airline pilots flying a night trip would require O2 as the standard cabin pressure once at altitude is 8,000 feet.
Matt
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Reply #6 -
Jan 12
th
, 2010 at 5:17am
Hagar
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Copied from FAA AIM dated February 14, 2008.
http://www.faa.gov/air_traffic/publications/ATPubs/AIM/AIM_Basic_2-14-08.pdf
Chapter 8. Medical Facts for Pilots
8-1-2. Effects of Altitude
a. Hypoxia.
6. Hypoxia is prevented by heeding factors that reduce tolerance to altitude, by enriching the inspired air with oxygen from an appropriate oxygen system, and by maintaining a comfortable, safe cabin pressure altitude.
For optimum protection, pilots are encouraged to use supplemental oxygen above 10,000 feet during the day, and above 5,000 feet at night.
The CFRs require that at the minimum, flight crew be provided with and use supplemental oxygen after 30 minutes of exposure to cabin pressure altitudes between 12,500 and 14,000 feet and immediately on exposure to cabin pressure altitudes above 14,000 feet. Every occupant of the aircraft must be provided with supplemental oxygen at cabin pressure altitudes above 15,000 feet.
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Reply #7 -
Jan 12
th
, 2010 at 6:39am
Mitch.
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Just to reiterate on Hagar's quoting, this is taken from my student manual:
"Use oxygen on all night flights above 4000-5000ft. The effect of hypoxia on
night vision
takes place at around 4000-5000ft and unless supplementry oxygen is used at this level,
night vision will be significantly reduced
"
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Reply #8 -
Jan 12
th
, 2010 at 7:01am
expat
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Just to clarify this thread, what sort of aircraft are we talking about here. Pressurised, unpressurised, commercial or private??
Matt
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B1 Boeing 737-800 and Dash8 Q-400
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Reply #9 -
Jan 12
th
, 2010 at 7:20am
Hagar
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expat wrote
on Jan 12
th
, 2010 at 7:01am:
Just to clarify this thread, what sort of aircraft are we talking about here. Pressurised, unpressurised, commercial or private??
Matt
I think SubZer0 was talking about unpressurised private GA aircraft. However, the way I read it, the FAA AIM I quoted from covers all types including pressurised commercial airliners.
The CFRs require that at the minimum, flight crew be provided with and use supplemental oxygen after 30 minutes of exposure to
cabin pressure altitudes
between 12,500 and 14,000 feet and immediately on exposure to cabin pressure altitudes above 14,000 feet. Every occupant of the aircraft must be provided with supplemental oxygen at cabin pressure altitudes above 15,000 feet.
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Reply #10 -
Jan 12
th
, 2010 at 9:04am
SubZer0
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Hagar and Mitch hit the target with their replies. Hagar, I took that information from the same place
Matt, in this case, we are talking about an unpressurized GA aircraft, such as a C-152 in my case.
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Reply #11 -
Jan 12
th
, 2010 at 11:54am
C
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The only aeroplanes I've flown at night have either used oxygen masks all the time, or have been pressurised!
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Reply #12 -
Jan 12
th
, 2010 at 1:00pm
DaveSims
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There is some basis for this, although it is not every night if my memory serves right (been 8 years since that class). I can not remember the exact reason, but hypoxia can occur at lower altitudes at night. We were usually advised to avoid flight above 8000MSL at night at our 141 school for just such reason.
Dave
www.flymcw.com
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Reply #13 -
Jan 12
th
, 2010 at 1:27pm
C
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DaveSims wrote
on Jan 12
th
, 2010 at 1:00pm:
There is some basis for this, although it is not every night if my memory serves right (been 8 years since that class). I can not remember the exact reason, but hypoxia can occur at lower altitudes at night. We were usually advised to avoid flight above 8000MSL at night at our 141 school for just such reason.
I suspect its to do with diurnal (the recurring cycle between day and night for those unfamiliar) variation of the depth of the atmosphere of some sort.
*I may well be wrong!
«
Last Edit: Jan 13
th
, 2010 at 3:50am by C
»
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Reply #14 -
Jan 12
th
, 2010 at 10:21pm
SubZer0
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C, that is very interesting. My teacher now wants to know WHY are eyes more susceptible to the effects of hypoxia at night.
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Reply #15 -
Jan 13
th
, 2010 at 1:32am
Splinter562
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You should check out the Pilot's Handbook of Aeronautical Knowledge, Chapter 16: Aeromedical Factors
http://www.faa.gov/library/manuals/aviation/pilot_handbook/
The last part of the chapter deals with night vision nicely.
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Reply #16 -
Jan 13
th
, 2010 at 6:25pm
beaky
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SubZer0 wrote
on Jan 12
th
, 2010 at 10:21pm:
C, that is very interesting. My teacher now wants to know WHY are eyes more susceptible to the effects of hypoxia at night.
Again:
http://www.aoa.org/x5352.xml
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Reply #17 -
Jan 13
th
, 2010 at 6:28pm
beaky
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C wrote
on Jan 12
th
, 2010 at 1:27pm:
DaveSims wrote
on Jan 12
th
, 2010 at 1:00pm:
There is some basis for this, although it is not every night if my memory serves right (been 8 years since that class). I can not remember the exact reason, but hypoxia can occur at lower altitudes at night. We were usually advised to avoid flight above 8000MSL at night at our 141 school for just such reason.
I suspect its to do with diurnal (the recurring cycle between day and night for those unfamiliar) variation of the depth of the atmosphere of some sort.
*I may well be wrong!
I think the effects of diurnal variation, including nighttime temp. inversions, are pretty negligible as a risk factor for hypoxia at unusually low altitudes. I, too, could be wrong... but I doubt it.
The big concern (and rightly so) about night flight is the pronounced effect that decreasing pressure has on
night vision.
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Reply #18 -
Jan 14
th
, 2010 at 12:26am
SubZer0
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Thank you guys for the interest
The answer is as follows:
During the night, the eyes use rods much more than cones. These rods are 10,000 times more sensitive than cones and therefore much more susceptible to hypoxia.
It's definitely in here, where Splinter said to look. Just read the entire section about night vision -- it's very interesting.
http://www.faa.gov/library/manuals/aviation/pilot_handbook/
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Reply #19 -
Jan 14
th
, 2010 at 3:24am
Mitch.
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Some great information here. I trust we all learnt something, I know I have. Very helpful guys.
---------->Currently building a native FSX Beechcraft Starship 2000<----------
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Reply #20 -
Jan 14
th
, 2010 at 6:19am
C
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beaky wrote
on Jan 13
th
, 2010 at 6:28pm:
C wrote
on Jan 12
th
, 2010 at 1:27pm:
DaveSims wrote
on Jan 12
th
, 2010 at 1:00pm:
There is some basis for this, although it is not every night if my memory serves right (been 8 years since that class). I can not remember the exact reason, but hypoxia can occur at lower altitudes at night. We were usually advised to avoid flight above 8000MSL at night at our 141 school for just such reason.
I suspect its to do with diurnal (the recurring cycle between day and night for those unfamiliar) variation of the depth of the atmosphere of some sort.
*I may well be wrong!
I think the effects of diurnal variation, including nighttime temp. inversions, are pretty negligible as a risk factor for hypoxia at unusually low altitudes. I, too, could be wrong... but I doubt it.
Quite! As I say, not something we really come across over here. Just thinking outside the box.
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