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can come simply from the
movement of fuel through the lines.
Jet A weighs about 5% more per litre
than Jet B, but it gives you a longer
range, as turbines work on the
weight of the fuel they burn, not the
quantity. So, if you load the same
amount of fuel, your machine will
weigh more with Jet A, but if you fill
the tanks, you will use fewer litres
and less money.
JP4 is like Jet B but also has a
corrosion inhibitor and anti-icing
additives. It was the main military
fuel but is being superseded by JP8,
at least in the USA. JP5 has a higher
flashpoint than JP4, and was
designed for US navy ships (similar
to Jet A). JP8 is like Jet A1, but has a
full set of additives.
FCU
The Fuel Control Unit does more or
less the same job as a carburettor on
a piston engine, except it uses an
arrangement of springs and
bobweights to metre fuel according
to demand.
FADEC
The initials stand for Full Authority
Digital Electronic Control. It's just a
computer that controls the fuel
system, based on information from
various sensors, such as exhaust
temperature, engine RPM, control
movement, etc. The end result is a
more precise control of rotor or
propeller speeds under varying flight
conditions, particularly with
reference to overspeeding. Other
benefits include automatic starting,
better care of the engine (so more
time between overhauls) and
reduction of pilot workload through
automation. Being a computer, it is
software-based, and one of the
preflight checks is to ensure that the
Airframes, Engines & Systems 215
right software is loaded. Also, as it's
a computer, it can monitor many
parameters, which is why you might
see more caution lights.
It will typically consist of two main
items, the Engine Control Unit (ECU),
on the airframe, with a processor
inside (e.g. a 486 – powerful, huh?),
and the Hydro Mechanical Unit (HMU)
on the engine, which functions
rather like the old-style FCU when
the FADEC is disabled. There will
also be sensors and relays for the
transmission of information around
the system. Many signals will be
repeated to the relevant instruments.
Engine Instruments
Refer to the Instruments chapter.
Engine Handling
One of the biggest things to unlearn
when transitioning from piston to
turbine is to keep your finger on the
starter button once things start
happening (with a piston, you tend
to take your finger off straight away
when the engine starts). You take
your finger off when the engine
becomes self-sustaining. Before then, it
relies heavily on the battery or APU
to keep it turning. It follows that, if
the battery is weak to start with, the
engine won't spin as fast, the airflow
is reduced, the whole process
becomes hotter and you could melt
the back end with a hot start. You
should always check the voltage
available from the battery before
starting a turbine engine. A hung start
exists when the engine fails to
accelerate to normal idle RPM. It
just sits there, weakening the battery
and leading to a hot start. You get a
wet start when the engine doesn't
light off at all (flooded).
Pulling full power just because it's
there is not always a good idea.
Limitations may be there for other
reasons—for example, the
transmission might not be able to
take that much, which is why you
can’t go faster than 80 kts in a
Jetranger when pulling more than
85% torque (actually, in this case, the
transmission ends up in a strange
attitude). Excessive use of power will
therefore ruin your gearbox well
before the engine (and will show up
as metal particles in the oil). Many
turbine failures are the result of
pulling too many cycles from
minimum to maximum Ng, so if you
don't need 100% torque, it's best not
to use it. It's also best not to reduce
the collective lever of your
helicopter to the bottom when
descending, either, and to make
power changes gently, avoiding overand
undershoots.
Maximum Continuous Power is the
setting that may be used indefinitely,
but any between that and maximum
power (usually shown as a yellow arc
on the instrument) will only be
available for a set time limit.
While I'm not suggesting for a
moment that you should, piston
engines will accept their limits being
slightly exceeded from time to time
with no great harm being done.
Having said that, the speed at which
　
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