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(although wet sumps systems have
pumps, too). The scavenge pump
(which pulls oil from the engine) has
a greater capacity than the pressure
pump, to make sure the tank gets
filled properly. The filter will be
between the engine and the scavenge
pump. The oil cooler is between the
scavenge pump and the reservoir.
Pumps are usually mesh gear types:
Oils come in various thicknesses, or
viscosities, which measure resistance to
flow. The lower the viscosity
number, the thinner it is, so you
would use 120 oil in Summer, 100 in
Fall or Spring, 80 in Winter and 65
in the Arctic. To keep the oil thin, in
the cold, one trick is to pour a few
litres of petrol into the oil system
just after closing down at night, so it
is very thin in the morning and you
can start the engine. By the time the
oil has warmed up, the petrol has
evaporated and you can carry on
(but check your flight manual to see
if this Oil Dilution is acceptable for
your machine).
58 JAR Private Pilot Studies
Oil is cooled by pumping it through
an oil cooler, which is just like a
radiator. An oil filter is used to trap
any impurities, and the pressure relief
valve is there to make sure it doesn't
get too high (if the pressure
increases, due to a blockage, maybe,
the valve opens and dumps the oil
back to the tank).
Chip Detectors are small magnets that
attract slivers of metal suspended in
the oil. Sometimes, they are
connected to a warning panel in the
cockpit in which a light glows if the
sliver makes a circuit across the
detector. It's always a good idea to
be prepared to land straight away if
you see a chip light come on, and
some flight manuals say do so
immediately. If the light is in a
transmission system, keep it loaded,
as unloading a disintegrating one has
been known to make it worse (in a
helicopter, make sure you land next
to a pub; the engineers like it better).
An engine that is not used enough
develops corrosion very quickly on
the inside, and rust flakes, which are
very abrasive, will circulate when the
engine is started, which is why you
have to change the oil even when
you don’t fly a lot. Another reason is
an increased water content, which
will have an acidic effect once it
mixes with the byproducts of
combustion, which is why you
should just pull the propeller
through several rotations if you
cannot fly. The most wear takes
place in the first seconds of a cold
start, after the oil has been allowed
to settle. Priming will wash whatever
oil is left off the cylinder walls, so
don't do too much, and maintain
minimum RPM to let the oil
circulate. The pressure will be high
just after starting, but will reduce to
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
Engines & Systems 59
blocked through the core of the
cooler, which is probably why it is
known as coring.
Fuel Supply
The simplest system is gravity feed,
which needs the fuel cells to be
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