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transferred to the airframe. Of
course, it also helps you get around
on the ground. The retractable variety
produces less drag in flight, at the
expense of complication. As with
transmissions, the system will have
its own oil system, in this case
hydraulics, and a backup system
should it decide not to work. This
can be operated manually, or by an
air bottle.
Otherwise, landing gear is made up
of struts, that are attached to the
fuselage, and which have the wheels
attached to them (in tailwheel
aircraft, struts are fitted slightly
ahead of the C of G – otherwise,
they will be slightly behind). For
shock absorption, an oleo strut
contains a piston and a cylinder,
moving together inside hydraulic
fluid. There are holes in the piston to
allow the fluid through and damp
down the shocks. There may be
nitrogen or dry air instead of fluid.
Another function of a strut is to
force the tyre on to the ground.
With wheels, there will be brakes,
used for stopping, and sometimes
steering (not recommended in a
multi-engined aeroplane – use
differential engine power instead).
There will be a master cylinder for
each brake pedal, a reservoir, the
brakes themselves and connecting
pipes. The toe pedal activates the
master cylinder directly. A piston
inside forces fluid along the lines to
activate the brakes on the wheels.
Small aircraft use single disk brakes,
and large ones use multi disks. The
former can be checked visually for
wear, and the latter use wear
indicator pins when they are applied.
On large aeroplanes, a brake
temperature monitor will indicate
whether the brakes are overheating
or not. If they do, they will suffer
from brake fade, and not work so well
222 Canadian Professional Pilot Studies
(brake fade happens when the disks
glaze from heat – drum brakes suffer
most from this). In fact, one of the
jobs of the NFP (Non Flying Pilot) is
to note the time maximum brakes
are applied during an aborted takeoff
and have the brake cooling chart
handy, which will state when the
brakes can be used again (sometimes
you can’t even taxi off the runway
for a while).
Antiskid systems monitor deceleration
and release the brakes at intervals to
prevent the wheels from locking. A
control unit compares the rotation
of each wheel with what it should be
on a dry runway. If it is less than
85%, valves are opened to release
the brakes. Each wheel has a speed
measurement device.
To stop a jet after a slightly high
approach speed, set it down firmly
(i.e. drive it onto the ground), initiate
reverse thrust, put the nosewheel
down then apply full reverse thrust
and brake (exam question).
Transmissions
This is how you get the power from
the engines to the propellers or rotor
blades. Because some engines work
at higher speeds than other
components, there will also be an
element of reduction involved.
Transmission systems will also have
their own, self-contained oil supply.
In a helicopter, a clutch may be fitted
between the engine and main rotor
gearbox. A centrifugal clutch (as found
on the Bell 47) is automatic and will
have more effect as speed is
increased so, if the engine stops, the
blades are free to rotate. A friction
or belt drive clutch (as in the
Enstrom) is manually operated, so
the engine can be warmed up
without the blades rotating (safer for
passengers, too). Some turbine
helicopters have clutches, too, like
the Gazelle. Other turbines have a
freewheel unit to let the blades spin
without engine power.
The engine and rotor RPM gauges
are normally combined, with the
needles superimposed on each other:
They are split in the above diagram.
Pneumatics
We have seen elsewhere that flight
instruments are operated by air, as
are deicing boots. Compressed air is
used for many purposes on older
aircraft, such as operating the
landing gear, doors, flaps, etc., all
things that are generally now done
with hydraulics, but air is still useful
in other ways.
Although it is more compressible
than hydraulic fluid, a good reason
for using it instead is that you
already fly through it, so you don't
need to carry it in storage tanks, or
need return pipes and systems, as it
can be exhausted to the atmosphere.
This saves a great deal of weight.
APU
The Auxiliary Power Unit is usually a
turbine engine that delivers power
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Canadian Professional Pilot Studies2(13)
