Hi there, I recently flew 1050NM at 11 going and 12,000 coming back,

my quesiton that I was really wanting to know even before take off was, 12,000 is very close to 12,500ft only 500ft difference,

Anyway I flew at 12,000 for 4 hours, and I was wondering what makes those oxygen requiremtns fixed? what happens if it's humid, or air density is less that day due to temperature,


Oxygen requirements: I know.

Question is: doesn't that 12,500 vary at all with temperature, humidty conditions just like it would for take off at high elevations

Oh by the way, I didn't get Hypoxic :D nor did I have blue fingertips extremeities


THANK YOU :D :D :D

I'd say the variations of air pressure due to the weather are just negligible next to that due to altitude (sea level pressure will vary about 5% over the usual range of weather conditions, while it's a 40% decrease at 12k ft).
If you really wanted to annoy the pilots with an extra calculation to make to have that extra "precision", you'd have to find a way to factor in the actual person's sensitivity to reduced oxygen levels, as that varies a lot between people, actually certainly more than the weather effects...
If you want to go that far, wouldn't you have to recalculate and adapt your altitude several times over a 1000NM course to account for the local climate variations?

So as it wouldn't really be possible to calculate the max altitude for a certain person with certain conditions - why complicate? Just choose something that will work for most, add some safety margin, and there we are. It would just be too complicated to gain maybe about 1500ft sometimes (about the variation that corresponds to the above mentioned 5%)...

Then you also have an advantage for ATC as you know that aircraft not equipped with masks will never be over 12500ft - and dont have to check everytime to see what that level is.

§ 91.211 Supplemental oxygen.
(a) General. No person may operate a
civil aircraft of U.S. registry—
(1) At cabin pressure altitudes above
12,500 feet (MSL) up to and including
14,000 feet (MSL) unless the required
minimum flight crew is provided with
and uses supplemental oxygen for that
part of the flight at those altitudes
that is of more than 30 minutes duration;
(2) At cabin pressure altitudes above
14,000 feet (MSL) unless the required
minimum flight crew is provided with
and uses supplemental oxygen during
the entire flight time at those altitudes;
and
(3) At cabin pressure altitudes above
15,000 feet (MSL) unless each occupant
of the aircraft is provided with supplemental
oxygen.

Above is the FAR 91 referrence for the use of supplemental oxygen.

The user friendly definition of pressure altitude is the indicated altitude with 29.92 or 1013.2 set on the altimeter. There is no temperature or humidity correction(s) involved with that definition. What makes the number fixed is the definition. If you stop and think about it, it kinda makes sense. When the reg was written my guess is that they wanted a fixed, simple value so they hung their hats on P.A.

Another factor that is always involved with this sort of decision making is common sense. An old, fat guy like me might think twice before cruising around at 12,000' without sucking on a hose. Regardless of what the regs say if it makes sense to do or not to do something, then do what makes sense as long as that isn't a clear violation of the FARs. And you can even violate them if you do it on your emergency authority. Just be ready to support your decision with a good explanation. You were perfectly legal to do what you did and it seems, from the outside, that it was safe.

Yes you did.

You didn't have any acute symptoms, but your body and mind were affected by the lack of O2.

Most people begin to be affected by hypoxia when they are 5,000 ft above the altitude they are aclimated to. This vary greatly based on your physical condition, and many other factors. The first symotom is a reduction in night vision, followed by a decrease in higer brain functions. These occur well bfore you have been abouve 12,500.

One pilot may need O2 well below 12,500, while a Sherpa from Nepal may be ok much higer.

The rules are arbitrary, black and white. The real world is grey. It is up to us to stay both leagal, and safe.