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Pilot's Handbook of Aeronautical Knowledge
Aircraft Systems

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Pilot's Handbook of Aeronautical Knowledge



Table of Contents

Chapter 1, Introduction To Flying
Chapter 2, Aircraft Structure
Chapter 3, Principles of Flight
Chapter 4, Aerodynamics of Flight
Chapter 5, Flight Controls
Chapter 6, Aircraft Systems
Chapter 7, Flight Instruments
Chapter 8, Flight Manuals and Other Documents
Chapter 9, Weight and Balance
Chapter 10, Aircraft Performance
Chapter 11, Weather Theory
Chapter 12, Aviation Weather Services
Chapter 13, Airport Operation
Chapter 14, Airspace
Chapter 15, Navigation
Chapter 16, Aeromedical Factors
Chapter 17, Aeronautical Decision Making




Whenever the throttle is closed during flight, the engine cools
rapidly and vaporization of the fuel is less complete than if
the engine is warm. Also, in this condition, the engine is more
susceptible to carburetor icing. If carburetor icing conditions
are suspected and closed-throttle operation anticipated, adjust
the carburetor heat to the full ON position before closing the
throttle and leave it on during the closed-throttle operation.
The heat will aid in vaporizing the fuel and help prevent the
formation of carburetor ice. Periodically, open the throttle
smoothly for a few seconds to keep the engine warm;
otherwise, the carburetor heater may not provide enough
heat to prevent icing.

The use of carburetor heat causes a decrease in engine
power, sometimes up to 15 percent, because the heated air
is less dense than the outside air that had been entering the
engine. This enriches the mixture. When ice is present in an
aircraft with a fixed-pitch propeller and carburetor heat is
being used, there is a decrease in rpm, followed by a gradual
increase in rpm as the ice melts. The engine also should run
more smoothly after the ice has been removed. If ice is not
present, the rpm will decrease and then remain constant.
When carburetor heat is used on an aircraft with a constantspeed
propeller and ice is present, a decrease in the manifold
pressure will be noticed, followed by a gradual increase.
If carburetor icing is not present, the gradual increase in
manifold pressure will not be apparent until the carburetor
heat is turned off.

It is imperative for a pilot to recognize carburetor ice when
it forms during flight because a loss of power, altitude,
and/or airspeed will occur. These symptoms may sometimes
be accompanied by vibration or engine roughness. Once a
power loss is noticed, immediate action should be taken to
eliminate ice already formed in the carburetor, and to prevent
further ice formation. This is accomplished by applying full
carburetor heat, which will cause a further reduction in power,
and possibly engine roughness as melted ice goes through the
engine. These symptoms may last from 30 seconds to several
minutes, depending on the severity of the icing. During this
period, the pilot must resist the temptation to decrease the
carburetor heat usage. Carburetor heat must remain in the
full-hot position until normal power returns.

Since the use of carburetor heat tends to reduce the output
of the engine and to increase the operating temperature,
carburetor heat should not be used when full power is required
(as during takeoff) or during normal engine operation, except
to check for the presence or to remove carburetor ice.

Carburetor Air Temperature Gauge
Some aircraft are equipped with a carburetor air temperature
gauge, which is useful in detecting potential icing conditions.
Usually, the face of the gauge is calibrated in degrees Celsius,
with a yellow arc indicating the carburetor air temperatures
where icing may occur. This yellow arc typically ranges
between -15 °C and +5 °C (5 °F and 41 °F). If the air
temperature and moisture content of the air are such that
carburetor icing is improbable, the engine can be operated
with the indicator in the yellow range with no adverse effects.
If the atmospheric conditions are conducive to carburetor
icing, the indicator must be kept outside the yellow arc by
application of carburetor heat.

Certain carburetor air temperature gauges have a red radial,
which indicates the maximum permissible carburetor inlet
air temperature recommended by the engine manufacturer. If
present, a green arc indicates the normal operating range.

Outside Air Temperature Gauge
Most aircraft are also equipped with an outside air
temperature (OAT) gauge calibrated in both degrees Celsius
and Fahrenheit. It provides the outside or ambient air
temperature for calculating true airspeed, and also is useful
in detecting potential icing conditions.

Fuel Injection Systems
In a fuel injection system, the fuel is injected directly into
the cylinders, or just ahead of the intake valve. The air
intake for the fuel injection system is similar to that used
in a carburetor system, with an alternate air source located
within the engine cowling. This source is used if the external
air source is obstructed. The alternate air source is usually
operated automatically, with a backup manual system that
can be used if the automatic feature malfunctions.

A fuel injection system usually incorporates six basic
components: an engine-driven fuel pump, a fuel/air control
unit, fuel manifold (fuel distributor), discharge nozzles,
an auxiliary fuel pump, and fuel pressure/flow indicators.
[Figure 6-13]

The auxiliary fuel pump provides fuel under pressure to the
fuel/air control unit for engine starting and/or emergency
use. After starting, the engine-driven fuel pump provides
fuel under pressure from the fuel tank to the fuel/air control

This control unit, which essentially replaces the carburetor,
meters fuel based on the mixture control setting, and sends it
to the fuel manifold valve at a rate controlled by the throttle.
After reaching the fuel manifold valve, the fuel is distributed
to the individual fuel discharge nozzles. The discharge
nozzles, which are located in each cylinder head, inject the
fuel/air mixture directly into each cylinder intake port.