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after running out of fuel in the
cruise, due to a combination of
circumstances, including misleading
fuel gauges. Actually, the episode is
also very instructive with regard to
CRM procedures.
Large aircraft will have all sorts of
connections between fuel tanks, for
rerouting purposes in an emergency,
and even small twins, like the PA 23,
have a crossfeed capacity, to feed the
right engine from the left tank, and
vice versa.
A collector bay is a small tank that
collects the input from several wing
tanks to absorb minor interruptions
in fuel supply – in other words, it
acts like a relay tank. Surge tanks near
wingtips also control fuel movement.
Systems
There are various ways of operating
the rest of the aircraft, described
below. Their motive power mostly
comes from the engine (with n
independent backup), and you will
find them working singly, or in
combination, frequently with one
type controlling another (electric
switches control the undercarriage,
for example). It is important to
know which way any system goes
when it fails – that is, does it fail open
or fail closed? Busbars are discussed in
the Electricity & Radio chapter.
Hydraulics
Liquids have minimal compressibility,
meaning that, when pressure is
applied, it will be pretty much taken
up throughout the whole system (in
fact, Pascal's Principle states that static
pressure exerted by a fluid is the
same on all surfaces touched by it).
This makes it a useful way of
transferring movement round
corners and into strange places, as
the forces produced by a hydraulic
system can be very powerful indeed,
which is why they are used in
helicopters to reduce the forces that
would otherwise be required to
move the flying controls.
Keeping to the helicopter theme,
some (such as the AStar) may have
an accumulator instead of a second
hydraulic system, to save weight.
Aside from smoothing out
fluctuations in hydraulic pressure,
the accumulator's job is to store
pressure that can be used for a short
time if the main system fails. That is,
it can be used in emergency. You can
also think of an accumulator as a
shock absorber, since a valve
opening in a highly pressurised
system makes quite an impact on the
lines. It is a cylinder in which a
piston separates hydraulic pressure
from air, which is pressurised on the
ground. if the charge is low, you can
expect rapid fluctuations in pressure
when you try to use the system.
Speaking of shocks to the system,
hydraulic fluids are specially made to
withstand high pressures and
temperatures without vapourising, so
make sure you use the proper stuff.
Speaking of shocks to the system,
hydraulic fluids are specially made to
withstand high pressures and
temperatures without vapourising, so
make sure you use the proper stuff.
The three main types are vegetable,
mineral or synthetic, which will be used
in high performance aircraft. They
Airframes, Engines & Systems 221
are coloured straw, red or
green/purple, respectively. Synthetic
fluid becomes acidic if overheated.
The basic system will have a jack,
with a control to direct the fluid into
whichever end of the jack is desired
to move, a pump and a reservoir. The
engine drives the pump, which
moves the fluid out of the reservoir
and applies pressure to it. When the
jack gets as far as it can go, the
increased pressure forces a relief valve
open so the fluid can be dumped
back to the reservoir. To keep
everything clean there will also be a
filter somewhere.
Hammering is due to fluctuating
pressure, and should be investigated
immediately.
Some helicopters have a rotor brake
that is hydraulically operated, but is
nothing to do with the main system.
It consists of a disk around the main
drive shaft that is gripped with brake
pads operated by the lever in the
cockpit. There will be a range of
rotor RPM that the brake must be
operated within.
Undercarriages
These can be skids, skis, wheels or
floats. Helicopters can also have
inflatable floats that are used when
over water – they are packed tightly
inside a covering and are inflated
when the pilot operates an air bottle
inside the cabin. Wheeled helicopters
typically taxi like aeroplanes, and
rarely hover for long periods.
Landing gear is there to take the
shock of landing, so it isn't
　
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