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Turbine, or Exhaust Temperature gauge,
which shows the heat coming out of
the back end. It is particularly
important during starting because, if
the battery is too weak to spin the
engine properly, there will be less
airflow through it, and not as much
cooling available, leading to a hot
start and an expensive repair as the
back end melts. During flight, on hot
days, this temperature may well be
the limiting factor in the amount of
payload you can take, even if you
have lots of torque left.
Jet Fuel
Jet A, standard for commercial and
general aviation, is narrow-cut
kerosene, usually with no additives
apart from anti-icing chemicals. Jet
A1 has a different freezing point and
possibly something for dissipating
static, used for long haul flights
where the temperature gets very low.
Jet B is a wide-cut kerosene with
naphtha in, so it is lighter and has a
very low flash point (it's actually 2/3
diesel and 1/3 naphtha, but in
emergency you can swap the
naphtha for avgas to get pretty much
the same thing). It contains static
dissipators and is widely used in
Canada. Try not to mix Jet A and Jet B -
the mixture can ignite through static
in the right proportions, as Air
Canada found when they lost a DC-
8 on the ramp in the 70s. Static 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fuel but is being superseded by JP8,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, but it uses 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
Airframes, Engines & Systems 55
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 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 right software is
loaded. Also, because it's a
computer, it's able to 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).
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