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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 255
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.
· Gas in the gut can be vented from
both ends.
· Teeth may have small pockets of
air in them, if filled, together
with the gums. Although
dentists nowadays are aware of
people flying, and pack fillings
properly, the general public
don't fly every day, as you do, so
it's best to be sure. High altitude
balloonists actually take their
fillings out.
Motion Sickness
This is caused by a mismatch
between the information sent to the
brain by the eyes and ears.
Accelerating from straight and level
may give you the impression of
pitching up, because the sensors in
the inner ear perceive the body
weight as going rearwards and
downwards. As the most dependable
source of sensory information is
your eyes, believe your instruments.
Decompression Sickness
Where pressures are low, nitrogen in
the blood comes out of solution
(typically above 18,000', but more so
at 25,000'). Bubbles can form, and
are especially painful in the joints
(e.g. the bends, for the joints, the
creeps (skin), chokes (lungs) and the
staggers (brain).
Unfortunately, these bubbles do not
redissolve on descent, so if you are
affected you may need to go into a
decompression chamber. For this
reason, diving before flight should
be avoided, as extra nitrogen is
absorbed while breathing pressurised
gas and will dissolve out as you
surface again. Don't fly for 12 hours
if you have been underwater with
compressed air, and 24 hours if
you've been below 30 feet.
Circulatory System
Made up of the heart, arteries,
arterioles, capillaries, venules, veins
and blood.
The Heart
This item does not rest in the
same way as other muscles –
instead, it take a mini-rest for a
microsecond or two in between
beats (a normal pulse rate is
around 70 beats per minute).
The systolic blood pressure is the
peak pressure as blood is
256 Canadian Private Pilot Studies
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