Hypoxia at night

[SubZer0]

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?

[DaveSims]

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.

[beaky]

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

[SubZer0]

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

[C]

"Why do we need oxygen masks at 5,000ft during the night but 8,000 during the day?"

First I've heard of it...

[expat]

"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

[Hagar]

Copied from FAA AIM dated February 14, 2008. http://www.faa.gov/air_traffic/publications/ATPubs/AIM/AIM_Basic_2-14-08.pdf

[color=#0000ff]Chapter 8. Medical Facts for Pilots

8-1-2. Effects of Altitude

    a. Hypoxia.

[Mitch.]

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"

[expat]

Just to clarify this thread, what sort of aircraft are we talking about here. Pressurised, unpressurised, commercial or private??

Matt

[Hagar]

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.

[SubZer0]

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.

[C]

The only aeroplanes I've flown at night have either used oxygen masks all the time, or have been pressurised!

[DaveSims]

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. 

[C]

[quote]There is some basis for this, although it is not every night if my memory serves right (been 8 years since that class).

[SubZer0]

C, that is very interesting. My teacher now wants to know WHY are eyes more susceptible to the effects of hypoxia at night. :-?