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normal once the engine warms up.
Mineral oil has no additives and is
used in new engines. Detergent oil has
chemicals added to help with
cleaning, etc., including keeping
particles suspended. Do not mix the
two. Synthetic oils have come from
turbine oil development, but they
have one drawback, in that the
sludge tends to centrifuge out inside
the dome of a constant speed
propeller and make cycling a bit
difficult. They also hold
contaminants longer.
When flying, the oil temperature and
pressure gauges work with each
other (they are measured after the
pump and before the engine). If the
pressure is low, you can either
expect the temperature to rise
because it is working harder (PA 31),
or reduce because there is less going
over the temperature detector (Bell
407). Check your flight manual, but
it also depends on whether oil is
leaking, or whether the detectors are
near each other.
With Hiller 12 and Bell 47
helicopters, the main rotor gearbox
is lubricated with engine oil, because
it is bolted directly to the top of the
engine. An engine oil pressure
problem with these machines (or any
similar) is therefore quite serious.
After starting a cold engine,
particularly in winter, you can allow
the oil pressure not to rise for 30
seconds, because it may be too thick
to get through the passages until it
gets warm. Otherwise you should
shut the engine down immediately.
If the temperature rises even with
the cowl flaps open, the oil flow is
blocked through the core of the
cooler, which is probably why it is
known as coring.
Airframes, Engines & Systems 59
Fuel Supply
The simplest system is gravity feed,
which needs the fuel cells to be
above the engine to work properly
(as with the Bell 47). Modern design
requirements, however, mean that
the fuel cells are in all manner of
strange places, and come in many
different shapes and sizes (together
with a C of G system all of their
own). Because of this, various
methods are used to get the fuel
from them to the engine, all
involving fuel pumps and filters. Each
engine will have its own pump, but
there will also be an inline backup,
just before the carburettor or
actually at the fuel tank. Note that
boost pumps are lubricated by the
fuel they work on, so don’t run them
dry or you will burn them out.
A fuel primer is a small hand pump
designed to put neat fuel directly
into either the induction manifold
(near the combustion chamber) or
the inlet valve port before you start
in the cold to promote the presence
of fuel vapour that will ignite to start
the engine (very rarely do you need
to prime a warm engine). They are
not there with fuel injection systems.
Fuel tanks will be vented to
atmosphere, to prevent a vacuum
forming inside the tanks as the fuel
level is reduced. The vents might be
in the fuel cap, or be an overflow pipe
in the tank.
Although many fuel gauges are
accurate, they should never be relied
upon as the final guide to what you
have. Reading the book Free Fall,
about the Gimli Glider is very
instructive about this - a 757 had to
make a dead stick landing at Gimli
after running out of fuel in the
cruise, due to a combination of
circumstances, including misleading
fuel gauges. Actually, the episode is
also instructive with regard to CRM
procedures.
Hydraulics
Liquids have minimal compressibility,
meaning that, when pressure is
applied, it will be pretty much taken
up throughout the whole system.
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
think of an accumulator as a shock
absorber, since a valve opening in a
highly pressurised system causes
quite a shock to the lines. It is a
cylinder in which a piston separates
hydraulic pressure from air, which is
　
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