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height is the same as breathing air at
sea level. At 40,000' it is the
equivalent of breathing air at 10,000
feet). Above 40,000 the oxygen
needs pressure (also, exposure to 03
becomes significant). Having said all
that, your learning ability can be
compromised as low as 6000 feet
(Source: RAF).
The normal rate of breathing is
around 18 times a minute,
exchanging .35-.65 litres of air.
Oxygen
Pure oxygen is a colourless, tasteless,
odourless and non-combustible gas
that takes up about 21% of the air
we breathe. Although it doesn't burn
itself, it does support combustion,
which is why we need it, because the
body turns food into heat. As we
can't store oxygen, we survive from
breath to breath.
How much you use depends on your
physical activity and/or mental
stress—for example, you need 4
times more for walking than sitting
quietly. The proportion of oxygen to
air (21%) actually remains constant,
but as the air gets less dense, each
lungful contains less oxygen in
Human Factors 253
proportion (that is, the partial pressure
becomes less), which is why high
altitude flight requires extra supplies.
Nothing more is required below
5000 feet, as 95% of what you would
find on the ground can be expected
there. However, at over 8000 feet,
you may find measurable changes in
blood pressure and respiration,
although healthy individuals should
perform satisfactorily.
Lack of oxygen leads to...
Hypoxia
A condition where you don't have
enough oxygen in the tissues,
resulting from inefficient transfer of
it into the blood, but anaemia can
produce the same effect, as can
alcohol (there are actually several
types of hypoxia, but we won’t
bother with that here). In other
words, there may really be too little
oxygen, or you don’t have enough
blood to carry what you need around
the body—you may have donated
some, or have an ulcer. You might
also be a smoker, with your
haemoglobin blocked by carbon
monoxide (anaemic hypoxia). A
blockage of 5-8%, typical for a heavy
smoker, gives you the equivalent
altitude of 5-7000 feet before you
even start!
The effects of hypoxia are similar to
alcohol but the classic signs are:
·  Personality changes. You get jolly,
aggressive and less inhibited.
·  Judgement changes. Your abilities
are impaired; you think you are
capable of anything and have
much less self-criticism.
·  Muscle movement. Becomes
sluggish, not in tune with your
mind.
·  Short-term memory loss. This leads
to reliance on training, or
procedures established in longterm
memory.
·  Sensory loss. Blindness occurs
(colour first), then touch,
orientation and hearing are
affected.
·  Loss of consciousness. You get
confused first, then semiconscious,
then unconscious.
·  Blueness.
The above are subjective signs, in that
they need to be recognised by the
person actually suffering from
hypoxia, who is actually in the wrong
state to recognise anything. External
observers may notice some of them,
but especially lips and fingertips
turning blue and possible
hyperventilation (see below) as the
victim tries to get more oxygen.
All are aggravated by:
·  Altitude. Less oxygen available,
and less pressure to keep it
there.
·  Time. The more exposure, the
greater the effect.
·  Exercise. Increases energy usage
and hence oxygen requirement.
·  Cold. Increases energy usage and
hence oxygen requirement.
·  Illness. Increases energy usage
and hence oxygen requirement.
·  Fatigue. Symptoms arise earlier.
254 Canadian Private Pilot Studies
·  Drugs or alcohol. Reduced
tolerance.
·  Smoking. Carbon monoxide
binds to blood cells better than
oxygen does.
The times of useful consciousness (that is,
from the interruption of the oxygen
supply to when you can do nothing
about it) are actually quite short:
Height Time
18,000' 20-30 mins
22,000' 5-10 mins
25,000' 2-3 mins
28,000' 1-1 ½ mins
30,000' 45-75 secs
35,000' 25-35 secs
45,000' 12-20 secs
Oxygen Requirements
The oxygen to be carried, and the
people to whom masks should be
made available, varies with altitude,
rate of descent and MSA. The latter
　
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