70
GA
/ Vol. 5 / No.9 / SEPTEMBER 2013
I
don't know much about you
and what you are doing,
Reader, but I'm going to
hazard a guess about one thing
that you're doing right now. I
think you are breathing maybe
15 times per minute, in and out,
as you've done since you were
born, and will carry on right up
until you die (pretty much by
definition). Aircraft designers
breath too, but they also need
to think about how you and I
breath and this means having
a good understanding of how
the lungs and body work.
So, let's start about the point
where we've just breathed out.
The gas mix in your lungs is
about at atmospheric pressure
and a mix of oxygen, nitrogen,
water vapour and carbon
dioxide. If you hold your
breath out for a little bit, the
oxygen will exchange across the
tissue of the lungs with carbon
dioxide in the blood so that
the proportion of oxygen goes
down a bit and the CO
2
goes
up. Now, breath in, and the
proportion of oxygen goes up.
This affects the balance across
the lung tissue, because there's
always a fair bit of carbon
dioxide within the blood the
other side of that tissue. To try
and restore the balance, much
more CO
2
crosses to the lungs,
passing oxygen going the other
way into the red blood cells.
When you then breath out, the
body gets rid of lots of that CO
2
and it all starts again. Without
fail, for as long as you live.
What's all this got to do
with aeroplanes? Well, the body
has an internal pressure that
by Dr Guy Gratton
Aircraft Technical
doesn't change much and we've
evolved to get the right amount
of oxygen around sea level in
fact up to about 8,000ft which
is about as high as the human
race went until we invented the
aeroplane. Above that, we start
to hit problems the important
figure is the partial pressure
of oxygen which reduced with
altitude at about the same rate
as absolute air pressure. Past
about 8,000ft the body starts to
get less oxygen than it needs to
be at full efficiency and things
stop working - night vision
degrades, decision making gets
poorer, and eventually we lose
consciousness. In fact aviation
medics refer to this in terms of
time of useful consciousness,
the higher we fly, the shorter that
time is. I've shown a graph here
for time of useful consciousness
of a typical young fit adult. As
you can see it starts to be an
issue around 16,000ft where our
hypothetical fit adult would flake
out at about 16 hours hardly
an issue for anybody but a high
altitude balloonist; on the other
hand at 30,000ft we're down
around a minute - not enough
time to get much done. Clearly
not all humans are the same:
a 60 year old sedentary chain
smoker will not handle altitude
anything like as well as a 25
year old clean-living athlete. But
most of us will handle that basic
8,000ft for as long as we need to.
Which is why most aircraft
designed to fly routinely
higher than about 12,000ft will
normally be fitted with some
system to bring the partial
pressure of oxygen in the
occupants' lungs to at least the
level they'd see at 8,000ft.
The simplest way to do this is
personal oxygen masks. Medium
performance military aeroplanes
such as turboprop trainers or
high performance helicopters, or
even high performance gliders
will normally have a system of
oxygen masks which typically
will have two or three modes of
use. The most commonly used
mode will be a system which
mixes cabin air with pure oxygen
from a bottle or (particularly on
fighters) "OBOGS" On-Board
Oxygen Generator. So as the
aeroplane climbs, the oxygen
levels are increased so that whilst
the wearer keeps breathing at
local atmospheric pressure, they
also keep breathing in enough
partial pressure of oxygen to
achieve full performance from
their bodies. These systems will
also usually have a 100% oxygen
mode which can be used if the
wearer feels unwell or there are
fumes in the cockpit that they
don't want to breath in. A few
very high performance aeroplanes
primarily high performance
fighters, will also have a pressure
breathing mode which puts
oxygen into the mask at a higher
pressure than the cabin pressure.
Pressure breathing feels very
unnatural as you reverse the
normal breathing behaviour of
relax to breath out then work to
breath in with a pressurised
mask you relax to breath in,
then have to force the air out.
Quite odd and uncomfortable,
but in a fighter at 60,000ft
about the only way to do it.
In a passenger airliner
however, this really isn't an
acceptable option other than as
an emergency system it's just
too difficult to sip your G&T
in first class whilst wearing an
oxygen mask, and never going
to catch on. So, airliner cabins
are leaky pressure vessels with
high pressure air being constantly
bled into them from one or more
of the engines. Most of them
are designed with a constant
flow into the cabin, usually
from high pressure compressor
bleed-air, and then two or more
outflow valves (I mentioned
these last month when I was
talking about cabin cooling)
that can be controlled to set the
cabin altitude. Most jet aeroplane
systems are fairly automatic:
once pressurised they'll usually
run to a cabin altitude about
half the actual altitude, until
after the cabin altitude reaches
8,000ft or so, it'll stay there
until the aeroplane descends
again. (Hence, incidentally,
why mid-Atlantic, you can't
get a decent cup of tea whilst
the human body can handle
8,000ft the boiling point of
water is a rather low 73°C, and
tea leaves don't brew properly
at that low temperature).
The emergency oxygen masks
on a passenger airliner at-least
those in the cabin, are similar to
the air-mix systems on military
aeroplanes. They feed oxygen
from a chemical generator into
the mask mixed with cabin air
to give the wearer the right
partial pressure of oxygen to
stay alive long enough to survive
until the aeroplane has made an
emergency descent, if there has
been a loss of cabin pressure,
down to 10,000ft or lower. The
oxygen tastes unpleasant from
the chemicals, and a few may
lose consciousness for a little
while, but everybody should live
which is what really matters.
Cockpit masks in airliners are
usually from a separate system,
similar to a military system with
mixed and 100% oxygen modes
taken from a separate bottled
oxygen system. Clearly 100%
oxygen whilst only nice-to-have
for passengers is essential for
pilots who may have fumes in
the cockpit and need to land the
beast (they'll also usually have
goggles as well to ensure they
can see through the fumes).
And so, at most altitudes we
ground-living ape descendants
can survive, and function,
and keep managing our flying
machines both normally and
during emergency conditions.
Go high enough, and we
start to need space suits, but
that's for another month. ·
Typical times of useful consciousness.
Under Pressure