AOPA Turbine Pilot Magazine - December 2018 - T-8

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is applied to it. For these reasons, a GPUpowered start will tend to be much faster
and cooler than a battery start.
Voltage is only one of three parameters that can assist or stress a start. The
other two are environmental: the density of the ambient air where the start
is being attempted, and the wind conditions through the engine. The lower the
density altitude present at start, the more
air there is in any volume the engine pulls
in and compresses, so the cooler the hot
section will be. So a start on a summer
afternoon in Aspen, Colorado (elevation
7,820 feet), will be more challenging to
the engine than one at sea level at zero
Celsius. Similarly, a headwind during start will assist airflow through the
engine, while a tailwind creates wrongway flow and, if strong enough, will
rotate the low pressure section of the
engine backwards-creating more work
for the starter to overcome.
I tell pilots to think of these three
variables as points on a triangle-you can
start an engine as long as you have two of
three points in your favor. No GPU available and need to do a battery start with a
questionable battery? Only consider this
if you are in a low density altitude condition and have the airplane pointed directly
into the wind. Starting at high altitude on
a hot day? Make sure you use a GPU and
don't try a tailwind start. Tailwind right
at the start limit on a crowded ramp and
the FBO is saying it can't turn you into the
wind? Grab a GPU, but don't try this if the
air is thin.
Once the pilot has optimized the initial conditions for the engine start, the
actual process in FADEC-controlled airplanes is trivial. In some light jets there is
only one step to take-turn a knob and the
FADEC does the rest. In others two steps
are required: a button-push to engage the
starter, and a movement of the thrust lever
out of the cutoff position to allow for fuel
introduction. The pilot's real work is in
monitoring the start.
Confirming that the high pressure
(N 2) section begins rotating immediately with starter activation is step one.
Concurrent with rotation should be activation of the ignition. It's essential for the
spark to be there before fuel is introduced,
so unburned fuel doesn't pool and ignite
T-8 | AOPA PILOT December 2018

in a spectacular manner rather than in a
controlled burn.
With proper rotation and ignition the
conditions are set for the introduction of
fuel. In some airplanes the pilot triggers
this step with the thrust lever at a specified rotational speed, while in others it's
completely automatic. In either case, the
pilot needs to confirm that fuel is actually
flowing, as seen by activation of the electric boost pump and by confirming that
the fuel flow value is as expected, typically
between 100 and 200 pounds per hour.
Right around the time fuel is introduced,
the core section of the engine should be rotating fast enough that the mechanically linked
oil pump begins producing measurable pressure. This is another key step to verify. Even
at idle the engine is spinning at thousands of
rpm, and inadequate lubrication is a quick
way to destroy the engine. As the core section spins faster, it pushes enough air through
the turbines to spin the fan (N1) section. With
no mechanical linkage between the two sections, seeing N1 accelerate is an indication of
proper movement of gas through the engine,
as well as an indication that the fan section
hasn't seized because of damage or any icing
between the fan and the inlet.
Finally, the pilot should see the fruits
of the starting labor-a rising inter-turbine temperature (ITT) readout that shows
combustion is occurring. This should bootstrap the rest of the start. The gas produced
by combustion further accelerates the core,
which compresses more air, which can be
burned with more fuel to allow for even
more acceleration, and so on until a stable
idle rpm is achieved.
As acceleration is occurring the pilot
must carefully monitor the ITT gauge for
the two most common start problems: a
hot start, or ITT rapidly rising in a way
that could exceed the ITT limits-or a
hung start, where the core engine speed
stops climbing and plateaus before reaching the proper idle rpm. Most light jets'
FADECs will automatically shut off fuel
to the engine (almost immediately causing a decrease in temperature) if either is
sensed. But the pilot should still be carefully watching ITT and N2 rpm, in case the
automatic protections fail.
AOPA
NEIL SINGER is a Master CFI with more than

9,500 hours in 15 years of flying.

http://www.seaerospace.com

AOPA Turbine Pilot Magazine - December 2018

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