204 Canadian Professional Pilot Studies
The mixture control has a secondary
function, which is to cut fuel from
the engine on the ground when you
want to stop it (you don't just switch
the magnetos off). The Idle Cut Off
(ICO) in the carburettor is joined to
the mixture lever with a Bowden
cable. When the lever is operated at
the end of a flight, the engine is
starved of fuel, and stops.
Carburettor Icing
This is actually one aspect of
induction system icing. The other
two are fuel icing, arising from
water suspended in fuel, and
impact ice, which builds up on the
airframe around the various
intakes that serve the engine.
Even on a warm day, if it's
humid, carburettor icing is a
danger, especially with small
throttle openings where there's
less area for the ice to block off
in the first place (as when
descending, etc.). Also, the
temperature drop (between the
OAT and that in the venturi)
can be anywhere between 20-
30°C, so icing (in an R22,
anyway) can happen even when
the OAT is as high as 21°C
(70°F), or more. Tests have
produced icing at descent power
at temperatures above 30°C,
with a relative humidity below
30%, in clear air. Because it is
more volatile, and likely to
contain more water, you can
expect more fuel and carb icing
with MOGAS than AVGAS.
It usually arises from the action
of the venturi in the throat, just
before the butterfly valve, which
regulates the amount of fuel
into the engine. You will
remember the venturi's purpose
is to accelerate airflow by
restricting the size of the
passageway, which has the
effect of reducing the pressure
and pulling the fuel in.
Unfortunately, this process also
reduces the temperature, as
does the fuel vapourisation,
hence the problem (the lower
temperature means greater
relative humidity, and closeness
to the dewpoint, and the
vapourisation takes its latent
heat from the surroundings,
making the situation worse). In
fact, the vapourisation (and
cooling) can carry on most of
the way to the cylinders, causing
the problem to persist,
especially with the butterfly
semi-closed, which produces
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
Airframes, Engines & Systems 205
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
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