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that the alternate air source (carburetor heat on) be used anytime the power setting of the aircraft is below
a certain point even though high atmospheric moisture content is not present. For fixed pitch propeller
configurations, most aircraft should use carburetor heat below 2000 - 2100 RPM. The location of ice in
the carbuerator is shown in the diagram at right.
The vaporization of the fuel in the venturi additionally cools the throttle area. Even a small amount of ice
in the carburetor or the induction system will reduce power. Usually this condition is detected by a
gradual drop in RPM (or Manifold Pressure). Application of carburetor heat will usually cause an
additional temporary decrease in power, but as the ice melts, the power should be restored. If icing is
persistent, it may be necessary to operate with some carburetor heat on continuously.
Fuel Injection
In these systems, the air intake system is similar to carbuerated systems. However, the fuel is not
vaporized in a venturi, but rather is injected directly into the engine cylinder just prior to the spark plugs
firing. A specific amount of fuel is injected and an appropriate amount of air is vented though the air
induction system to provide for proper combustion.
There are several advantages to fuel injected systems.
· Less susceptibility to icing
· Fuel flow is better controlled
· Faster throttle response since the fuel is directly injected into the cylinder.
· Better distribution of fuel to each cylinder.
· Easier starting in cold weather.
Some disadvantages are:
· Starting a hot engine can sometimes be difficult.
· Vapor lock during ground operations on hot days.
· Difficult engine re-start if engine quit due to fuel starvation.
INDUCTION SYSTEM
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INDUCTION SYSTEM
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Electrical System
Electrical System
Most small aircraft are equipped with a 28 volt direct current electrical system. The system is powered by
an Alternator which drives the electrical devices and stores energy in the battery.
A simplified diagram is shown at below. Not shown are the starter, and starter switch, and various other
electrical regulators and devices.
Electrical System
The Master Switch (labled MS) causes the electrical system to connect the electrical buses and devices to
the battery. The battery provides the power to crank the starter. Once the engine is running, power is
supplied by the alternator and the battery is recharged.
Numerous circuit breakers feed off the Primary Electrical Bus, and provide individual circuits to power
the electrical devices. Although the arrangement will vary from one make and model aircraft to another,
the basic principles are the same. By providing numerous circuit breakers and dividing the electrical load
into several different circuits, a malfunction in one system can be turned off without adversely affecting
the other circuits. The breakers will be labeled as to their general use, and the amperage will be marked
on the face of the breaker push button. On older model aircraft, fuses are used instead of circuit breakers.
Usually an alternator light is located on the instrument panel to provide a means for the pilot to determine
alternator is providing power to the system. In addition, an ammeter on the instrument panel can
determine the general health of the electrical system. After the battery is used for starting, a considerable
“charge” should be shown, indicating that the alternator is replenishing the power drained from the
battery during engine cranking. If the indicator shows zero while electronic equipment is ON, failure of
the alternator to charge the battery is indicated.
A second bus is provided to power the electronic and avionics equipment. This bus is connected to the
Primary Bus via the Avionics Switch. This switch should not be turned on until the engine is started to
prevent the possibility of high voltage transient currents resulting from engine starting from feeding into
sensitive electronic equipment. The pilot should also turn this switch OFF prior to engine shut-down for
the same reason.
Prior to start-up the pilot should check the status of all circuit breakers as a part of the pre-flight check. A
“tripped” breaker will project out farther from the control panel than does a properly functioning breaker.
Pushing the breaker in will reset it to it’s normal operating position. If it pops out again, there is a
malfunction in the circuit which it feeds, and repair should be made prior to flight.
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