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be marked with a line).
A system’s efficiency is measured by
its max diff, or maximum differential for
short, which is the maximum ratio of
cabin pressure to outside air pressure
that the system can maintain. The
pressurisation cycle concerns the rise
and fall of differential pressure. The
VSI will show a descent as cabin
pressure is increased.
Just in case a modern system fails,
you will need to know how to revert
to doing it manually, so a few
calculations are in order. To find out
the cabin rate of climb, find the
change in altitude, then the time
taken to get to the one you want.
Then divide the change by the climb
time. So, if you start at 2,000 feet
airfield elevation and want a cabin
altitude of 8,000 feet, you divide the
resulting 6,000-foot difference by the
time taken to get to the real altitude
of, say, FL 250.
When descending, remember that
cabin altitude must decrease
proportionally with the real altitude
so that both reach sea level (or
airfield elevation) at the same time,
so apply whatever you calculate as
the time for descent to the cabin
altitude as well (this is important,
because a pressurised airframe can
be stressed during a landing,
although some aircraft are always
slightly pressurised. Not only that, it
can be hard to open the doors).
Modern systems need to know the
destination elevation at TOD so it
can be worked out automatically.
Positive pressure relief valves dump
excess pressure overboard when max
diff is exceeded (they work at 1 PSI).
Negative pressure relief valves stop the
outside air pressure getting above
that of the cabin ( ½ PSI) – they
keep cabin pressure to a minimum
level for when the cabin can’t catch
up, as with a fast descent. They may
sometimes be called pressurisation
safety valves, or inward relief valves,
respectively. Dump valves allow you to
manually dump the pressure in an
emergency, which could be from a
system failure or smoke in the cabin.
Squat switches on the landing gear
Airframes, Engines & Systems 225
make sure the cabin is depressurised
before takeoff and after landing (it
activates the dump valve).
A ditching valve closes all valves to
stop water getting in.
Ground tests include pumping up the
pressure until the max diff is reached
and checking that the discharge
valves operate. When the discharge
valves are isolated, increase pressure
slowly to check the safety valves.
Air Conditioning
This is a required item in many
aircraft, especially if they use engine
bleed air to operate the systems. This
air is compressed, and therefore hot,
and the cabin would very quickly get
uncomfortable if there were no way
of cooling it. This is not to forget
solar radiation, skin friction, and
heat coming from assorted bodies
and radio equipment in the cabin.
On the other hand, you feel cooler
because moisture is evaporating
from your skin at an advanced rate
due to the dry air - humidity needs
to be 60% at 18°C, but, as this
would mean carrying large amounts
of water, somewhere between 30-
70% is acceptable.
In addition, the ISA temperature at
36,000 feet is –56.5°C, which is a bit
colder than the range of 18-24° that
the human body needs to operate
comfortably in. Thus, the air
conditioning system is needed to
regulate the temperature, refresh the
air in the cabin (at a rate of ½ lb per
person per minute, or .4 lb in
emergency), and maintain humidity.
There are several parts to the
average air conditioning system,
including the air supply, flow
control, cooling and heating systems,
temperature control and distribution.
They all work in the same way, by
initially compressing a liquid, and
taking some of the heat created by
that process away, by running cool
air past it. When that air is expanded
again, it becomes cooler than it was
in the first place.
Air Supply
Air is drawn into the cabin and
mixed with a supply of warm air for
the right temperature. How air is
sucked in depends on the system
being used:
· Ram Air systems are found on
small, unpressurised machines
not needing much throughput,
with heating either coming from
the engine exhaust (through a
heat exchanger, which is actually
a muff round the piping) or a
unit that is self-contained - fuel
burnt in it provides the heat,
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Canadian Professional Pilot Studies2(15)
