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propeller gearbox. It is a free
turbine, so maintenance is easier
because the two parts of the engine
are separate. In a PT6 turboprop, the
power turbine governor reduces fuel flow
to stop the power turbine
overspeeding (exam question).
One of the most important
instruments in your cockpit is the
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.
In an aeroplane, the torquemeter
measures shaft horsepower in the
gear reduction system. The torque is
the force applied to a propeller over
a distance of rotation, and is used to
measure power output.
Turbine blades wash in. They are
hollow, so they can be cooled with
HP air (on the forward face of the
blades, it forms a labyrinth to stop
hot gases reaching the turbine wheel
– on the rear, the exhaust cone acts
as a screen). The most common
method of fitting them to the wheel
is with the fir tree:
Turbine blade creep is a permanent
change in blade length, which can be
done deliberately with bleed air to
change the temperature of the casing
(Turbine Active Clearance Control).
Pressure, density and temperature
decrease through a turbine, while
velocity remains constant.
The Nozzle
Because the nozzle restricts the air
flow, it makes the gases flow faster
to provide better thrust for a jet
aeroplane.
A thrust reverser will reverse the flow
of exhaust gases outside the engine
to help the brakes when stopping, up
to slightly less than 50% of the
engine’s rated power. Because
aircraft must stop in a specified
distance with one engine out,
meaning that reverse thrust will not
be available anyway, flight planning
calculations do not take thrust
reversal into account – in fact, the
reduction in distance with it working
is actually not that large. Their main
use is to relieve the strain on the
brakes, as performance calculations
are based on maximum braking
(which is not used routinely), aside
from allowing the brake to run
cooler in normal use, and help with
contaminated runways (as well as
backing away from the gate).
Typically, on jet aircraft without
bypass, clamshells either side of the
rear of the engine are introduced
directly into the exhaust stream to
force it back over the top and
bottom. Cascade types are used with
high bypass engines, because there is
usually little ground clearance. In this
case, the equivalent of clamshells
operate inside the engine to redirect
214 Canadian Professional Pilot Studies
cool bypass air out of the sides. Petal
Door reversers do much the same.
The controls will be piggy-backed
onto the normal thrust levers so they
are to hand when required. In
theory, reverse thrust cannot be
selected until normal thrust is at idle,
so you have to pull the throttles
back, then select reverse.
Turboprop power levers are lifted
back past an idle gate on the
quadrant. This puts the propellers
into negative pitch after initially
increasing drag by flattening it.
Engine power will increase as
negative pitch is applied. Reversers
should not be used until the
nosewheel touches down, since their
use shifts the weight on to it.
Fuel Control
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. The static
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