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Airframes, Engines & Systems 223
or compressed air, or both, but not
intended for direct propulsion of an
aircraft. In other words, it's a spare
engine that runs the air and electrical
systems when the aircraft is on the
ground, or in emergency when in
flight, through a gearbox or
something. It saves you relying on
ground power units.
It will be found in an unpressurised
area, typically in the tail section,
behind a firewall. Starting will be
done from the ship's batteries, or
one specifically for the APU. It is
ready for loading 1 minute after the
blue (AVAIL) light comes on. It can
be shut down 2 minutes after
working without load.
Being a turbine, it qualifies as one of
the most powerful vacuum cleaners
around, so loose articles, etc. should
not be around. Also, they are noisy,
so ear defenders should be worn.
Engine Starting
Modern turbines are spun over
until they are self-sustaining
with compressed air, commonly
bled from the APU, to start the
No 1, or Master, engine, which
is then used to start any others
through a crossfeed system of
pipes and valves.
Compressed Air
This can be created by a compressor
powered by an engine, or come from
storage bottles on the aircraft.
A compressor consists of a piston
inside a cylinder, much like one in a
reciprocating engine. There are even
cooling fins, as air heats up when it
is subject to pressure (compressors
will also be positioned to get the
maximum benefit from air cooling).
When the piston descends, air is
sucked in through an inlet valve at the
top of the cylinder. There is a delivery
valve about halfway down the
chamber, through which air from the
previous upstroke is forced as the
piston descends – it will have got
there through a transfer valve in the
top of the piston, which allows air
through when it gets compressed as
the piston goes up. Continuous
delivery happens because the space
underneath the piston is small, and
the excess goes out directly through
the delivery valve. All valves are
spring-loaded.
A high pressure relief valve is fitted near
the compressor to keep it warm, so
it doesn't freeze up. Its purpose is to
protect the system by allowing
excess air to escape if a component
fails and blocks the system.
Although it is adjustable, it is not the
way to control the pressure! That's
done with the pressure regulating valve.
Also near the compressor is an antifreezer
full of methanol, which stops
ice forming from the moisture in the
air being compressed. An alternative
solution is a dehydrator with silica
crystals in, that absorb the moisture.
An air bottle acts as a temporary
supply of pressure when the system
is overloaded or switched off – it
gets charged up during normal
operations (the system is filled
before flight through a ground charging
valve). The pipes going into the bottle
actually extend inside by about 4
inches (forming stack pipes), so that
oil or water that escapes the relevant
traps doesn't get out.
Where such a system powers wheel
brakes, a typical gauge in the cockpit
would show the pressure in the main
224 Canadian Professional Pilot Studies
system along the top, and the
pressure in the wheel circuit on each
side underneath.
Cabin Pressurisation
Contrary to popular belief, a
pressurised cabin is not an airtight
container – rather, it makes use of
outflow valves, or variable discharge valves,
that allow pressurised air to escape
in an orderly fashion, depending on
whether the First Officer remembers
to set the controls or not. Think of it
as a large paper bag with a hole in
the end – the inside pressure is
maintained as long as you keep
blowing a constant mass flow of air
into it. In our case, however, engine
bleed air is used. The reason the
cabin is not airtight is that plugging
all the holes caused by routing
cables, etc. through the cabin would
be very inconvenient and expensive.
The desired cabin altitude is usually
set before takeoff, and it will "climb"
automatically to it afterwards at an
assigned rate. Typically, there will be
two gauges in the cockpit, one to
show the cabin’s rate of climb (looks
like a VSI), and the other with two
needles, one to show the cabin
altitude and a smaller one to show
the ratio of the cabin pressure to the
outside (the max diff (see below) will
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