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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




The manifold pressure gauge is color coded to indicate the
engine's operating range. The face of the manifold pressure
gauge contains a green arc to show the normal operating
range, and a red radial line to indicate the upper limit of
manifold pressure.

For any given rpm, there is a manifold pressure that should not
be exceeded. If manifold pressure is excessive for a given rpm,
the pressure within the cylinders could be exceeded, placing
undue stress on the cylinders. If repeated too frequently, this
stress can weaken the cylinder components and eventually
cause engine failure. As a general rule, manifold pressure
(inches) should be less than the rpm.

A pilot can avoid conditions that overstress the cylinders by
being constantly aware of the rpm, especially when increasing
the manifold pressure. Conform to the manufacturer's
recommendations for power settings of a particular engine to
maintain the proper relationship between manifold pressure
and rpm.

When both manifold pressure and rpm need to be changed,
avoid engine overstress by making power adjustments in
the proper order:
• When power settings are being decreased, reduce
manifold pressure before reducing rpm. If rpm is
reduced before manifold pressure, manifold pressure
will automatically increase, possibly exceeding the
manufacturer's tolerances.
• When power settings are being increased, reverse the
order—increase rpm first, then manifold pressure.
• To prevent damage to radial engines, minimize
operating time at maximum rpm and manifold
pressure, and avoid operation at maximum rpm and
low manifold pressure.

The engine and/or airframe manufacturer's recommendations
should be followed to prevent severe wear, fatigue, and
damage to high-performance reciprocating engines.

Induction Systems
The induction system brings in air from the outside, mixes
it with fuel, and delivers the fuel/air mixture to the cylinder
where combustion occurs. Outside air enters the induction
system through an intake port on the front of the engine
cowling. This port normally contains an air filter that inhibits
the entry of dust and other foreign objects. Since the filter
may occasionally become clogged, an alternate source of
air must be available. Usually, the alternate air comes from
inside the engine cowling, where it bypasses a clogged air
filter Some alternate air sources function automatically,
while others operate manually.

Two types of induction systems are commonly used in small
aircraft engines:
1. The carburetor system, which mixes the fuel and air
in the carburetor before this mixture enters the intake
2. The fuel injection system, which mixes the fuel and air
immediately before entry into each cylinder or injects
fuel directly into each cylinder

Carburetor Systems
Carburetors are classified as either float type or pressure type.
The float type of carburetor, complete with idling, accelerating,
mixture control, idle cutoff, and power enrichment systems is
probably the most common of all carburetor types. Pressure
carburetors are usually not found on small aircraft. The basic
difference between a float-type and a pressure-type carburetor
is the delivery of fuel. The pressure-type carburetor delivers
fuel under pressure by a fuel pump.

In the operation of the float-type carburetor system, the
outside air first flows through an air filter, usually located
at an air intake in the front part of the engine cowling. This
filtered air flows into the carburetor and through a venturi, a
narrow throat in the carburetor. When the air flows through
the venturi, a low-pressure area is created, which forces the
fuel to flow through a main fuel jet located at the throat. The
fuel then flows into the airstream where it is mixed with the
flowing air. [Figure 6-10]

The fuel/air mixture is then drawn through the intake manifold
and into the combustion chambers where it is ignited. The
float-type carburetor acquires its name from a float, which
rests on fuel within the float chamber. A needle attached to
the float opens and closes an opening at the bottom of the
carburetor bowl. This meters the correct amount of fuel into
the carburetor, depending upon the position of the float,
which is controlled by the level of fuel in the float chamber.
When the level of the fuel forces the float to rise, the needle
valve closes the fuel opening and shuts off the fuel flow to
the carburetor. The needle valve opens again when the engine
requires additional fuel. The flow of the fuel/air mixture to
the combustion chambers is regulated by the throttle valve,
which is controlled by the throttle in the flight deck.

The float-type carburetor has several distinct disadvantages.
In the first place, imagine the effect that abrupt maneuvers
have on the float action. In the second place, the fact that its
fuel must be discharged at low pressure leads to incomplete
vaporization and difficulty in discharging fuel into some
types of supercharged systems. The chief disadvantage of
the float carburetor, however, is its icing tendency. Since
the float carburetor must discharge fuel at a point of low
pressure, the discharge nozzle must be located at the venturi
throat, and the throttle valve must be on the engine side of the
discharge nozzle. This means the drop in temperature due to
fuel vaporization takes place within the venturi. As a result,
ice readily forms in the venturi and on the throttle valve.