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engine temperatures can cause excessive engine wear
or even failure. The best way to avoid this type of situation
is to monitor the engine temperature gauges regularly
and follow the manufacturer’s guidelines for
maintaining the proper mixture.
CARBURETOR ICE
The effect of fuel vaporization and decreasing air pressure
in the venturi causes a sharp drop in temperature
in the carburetor. If the air is moist, the water vapor in
the air may condense. When the temperature in the carburetor
is at or below freezing, carburetor ice may form
on internal surfaces, including the throttle valve.
[Figure 5-10] Because of the sudden cooling that takes
place in the carburetor, icing can occur even on warm
days with temperatures as high as 38°C (100°F) and
the humidity as low as 50 percent. However, it is more
likely to occur when temperatures are below 21°C
(70°F) and the relative humidity is above 80 percent.
The likelihood of icing increases as temperature
decreases down to 0°C (32°F), and as relative humidity
increases. Below freezing, the possibility of carburetor
icing decreases with decreasing temperatures.
Although carburetor ice can occur during any phase of
flight, it is particularly dangerous when you are using
reduced power, such as during a descent. You may not
notice it during the descent until you try to add power.
Indications of carburetor icing are a decrease in engine
r.p.m. or manifold pressure, the carburetor air temperature
gauge indicating a temperature outside the safe
operating range, and engine roughness. Since changes
in r.p.m. or manifold pressure can occur for a number
of reasons, it is best to closely check the carburetor air
temperature gauge when in possible carburetor icing
conditions. Carburetor air temperature gauges are
marked with a yellow caution arc or green operating
arcs. You should refer to the FAA-Approved Rotorcraft
Flight Manual for the specific procedure as to when
and how to apply carburetor heat. However, in most
cases, you should keep the needle out of the yellow arc
or in the green arc. This is accomplished by using a carburetor
heat system, which eliminates the ice by
To Engine
Incoming Air
Ice
Ice
Venturi
Fuel/Air

Mixture
Ice
Figure 5-10. Carburetor ice reduces the size of the air passage
to the engine. This restricts the flow of the fuel/air
mixture, and reduces power.
5-8
routing air across a heat source, such as an exhaust
manifold, before it enters the carburetor. [Figure 5-11].
FUEL INJECTION
In a fuel injection system, fuel and air are metered at
the fuel control unit but are not mixed. The fuel is
injected directly into the intake port of the cylinder
where it is mixed with the air just before entering the
cylinder. This system ensures a more even fuel distribution
in the cylinders and better vaporization, which
in turn, promotes more efficient use of fuel. Also, the
fuel injection system eliminates the problem of carburetor
icing and the need for a carburetor heat system.
TURBINE ENGINES
The fuel control system on the turbine engine is fairly
complex, as it monitors and adjusts many different
parameters on the engine. These adjustments are done
automatically and no action is required of the pilot
other than starting and shutting down. No mixture
adjustment is necessary, and operation is fairly simple
as far as the pilot is concerned. New generation fuel
controls incorporate the use of a full authority digital
engine control (FADEC) computer to control the
engine’s fuel requirements. The FADEC systems
increase efficiency, reduce engine wear, and also
reduce pilot workload. The FADEC usually incorporates
back-up systems in the event of computer failure.
ELECTRICAL SYSTEMS
The electrical systems, in most helicopters, reflect the
increased use of sophisticated avionics and other electrical
accessories. More and more operations in today’s
flight environment are dependent on the aircraft’s electrical
system; however, all helicopters can be safely
flown without any electrical power in the event of an
electrical malfunction or emergency.
Helicopters have either a 14- or 28-volt, direct-current
electrical system. On small, piston powered
helicopters, electrical energy is supplied by an enginedriven
alternator. These alternators have advantages
over older style generators as they are lighter in
weight, require lower maintenance, and maintain a
uniform electrical output even at low engine r.p.m.
[Figure 5-12]
　
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