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another restriction and more of
the same. Any water vapour
under those conditions will
sublimate, or turn directly to ice.
Note also that warm air will
produce more ice because it can
hold more moisture.
With smaller engines, use full
settings for every application—
that is, carb heat either on or
off, with no in-betweens - the
greatest risk is at reduced
power. Out of Ground Effect
hover performance charts for
helicopters usually assume the
carb air is cold (the R22 requires
carb heat below 18" MP). In
fact, when heat is applied, an
engine will typically lose around
9% of its rated power.
Rough running may increase as
melted ice goes through the
engine. Also, be careful you
don't get an overboost or too
much RPM when you reselect
cold. Of course, aeroplanes
have some advantage if the
engine stops from carb icing, as
the propeller keeps the engine
turning, giving you a chance to
do something about it. In a
helicopter, due to the freewheel
that allows autorotation, the
practice of only selecting hot air
when you actually get carb ice
may not be such a good idea –
usually, a gauge is used with a
yellow arc on it, showing the
danger range:
Use carb heat as necessary to
keep out if it. The other
peculiarity with regard to
helicopters is that they tend to
use power as required on
takeoff, whereas aeroplanes use
full throttle. This makes them
more vulnerable, as the butterfly
opening is smaller, and is
particularly apparent on the first
takeoff of the day, when the
engine and induction system are
still cold. If it is filtered, your
carb heat may be used to
preheat the induction system
during the engine warm-up.
With a fixed pitch propeller, the
first indications will be a slight
loss in RPM, followed by rough
running, then more loss of RPM
until the engine stops. With a
constant speed propeller (and in
a helicopter) keep an eye on the
manifold pressure gauge and the
EGT gauge, if you have one,
which should show a decrease.
Fuel Injection
Most of the above problems with
the carburettor are avoided with fuel
injection, where the fuel is metred
directly to the cylinders according to
power requirements, automatically
taking air density into account. Ice is
not formed because there is no
venturi to cause temperature drops
48 JAR Private Pilot Studies
(there's no carburettor in the first
place, as it is replaced with a fuel
control unit). In fact, the only control
you have (apart from the throttle) is
Idle Cut Off (ICO). The biggest
problem with fuel injected engines is
blocked jets, from dirt in the fuel.
A lesser one, but still significant, is
difficulty in starting, particularly on a
hot day, where the feed pipes lie
across the top of the engine and
consequently get warm (like the PA
31), with the fuel inside them
evaporating nicely and creating a
vapour lock, so you need a short burst
of fuel pressure from the pumps to
prime the lines.
If the primary airway is blocked
(maybe from ice, or a bird), there
will be an alternate air switch in the
cabin to change the source.
An electric fuel injection system will
be backed up by a mechanical one.
The benefits include being able to
use it without the engine running,
making it useful for priming.
Superchargers
A supercharger is a compressor run
directly by the engine, situated
between the carburettor and the inlet
manifold. Its function is to extend the
service ceiling of the aircraft, by
compressing the fuel/air mixture to
maintain sea level power at altitude,
or to increase normal power lower
down. This forced induction only really
works in a particular temperature
range. The essential point to
remember is that the extra air is
sucked through the carburettor and
blown into the cylinder.
Turbochargers
A turbocharger, on the other hand,
is powered by exhaust gases which
are deflected from their normal
course outwards by a wastegate. When
the wastegate is closed, the engine is
being turbocharged. The gases drive
an impeller that also compresses air,
but at the intake (before the carb, so
the air is being blown into it). Also, the
engine doesn't lose power by driving
it. Since the exhaust system is
involved, preflight checks should
include security of the pipes, so
carbon monoxide doesn't get into
　
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