曝光台 注意防骗
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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
Airframes, Engines & Systems 219
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
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
above the engine to work properly.
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 are 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
220 Canadian Professional Pilot Studies
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
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