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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.
246 JAR Private Pilot Studies
· Drugs or alcohol. Reduced
tolerance.
· Smoking. Carbon monoxide
binds to blood cells better than
oxygen.
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
two are dependent on each other, in
that it's no good having a good rate
of descent if the MSA stops you. It
may well be that, although you're
flying at a level that requires fewer
masks, the MSA may demand that
you equip everybody.
Preflight stuff includes ensuring that
oxygen masks are accessible for the
crew, and that passengers are aware
of where their own masks are. Check
the security of the circular dilution
valve filter (a foam disc) on all of
them, together with the pressure.
Beards will naturally reduce their
efficiency. Briefings should include
the importance of not smoking and
monitoring the flow indicator. All
NO SMOKING signs should be on
when using it.
If you know you will need oxygen at
night, it's best to start using it from
takeoff.
There are three types of oxygen
supply, continuous flow, diluter demand
and pressure demand. Refer to the Air
Law chapter for legal requirements.
Hyperventilation
This is simply overbreathing, where
too much oxygen causes carbon
dioxide to be washed out of the
bloodstream, which then gets too
alkaline (oxygen is actually quite
corrosive – it belongs to the same
chemical family as chlorine and
fluorine, so too much is toxic).
Unconsciousness slows the
breathing down so that the CO2
balance is restored, but falling asleep
is not often practical! The usual
cause is worry, fright or sudden
shock, but hypoxia can be a factor.
Symptoms include:
· Dizziness
· Pins and needles, tingling
· Blurred sight
· Hot/Cold feelings
· Anxiety
· Impaired performance
· Loss of consciousness
Pressure Changes
Aside from oxygen, the body
contains gases of varying
descriptions in many places; some
occur naturally, and some are created
by the body's normal working
processes. The problem is that these
gases expand and contract as the
aircraft climbs and descends. Some
Human Factors 247
need a way out, and some need a
way back as well.
· Gas in the ears is normally
vented via the Eustachian
tubes. If these are blocked
(say with a cold), the pressure
on either side of the eardrum
is not balanced, which could
lead (at the very least) to
considerable pain, and (at
worst) a ruptured eardrum.
· Sinus cavities are also
vulnerable to imbalances of
pressure, and are affected in
the same way as eardrums.
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