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Pilot's Handbook of Aeronautical Knowledge
Aircraft Systems
Powerplant

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Pilot's Handbook of Aeronautical Knowledge

Preface

Acknowledgements

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

Appendix

Glossary

Index

The formation of carburetor ice may reduce or block fuel/air flow to the engine.
Figure 6-11. The formation of carburetor ice may reduce or block
fuel/air flow to the engine.

The reduced air pressure, as well as the vaporization of fuel,
contributes to the temperature decrease in the carburetor. Ice
generally forms in the vicinity of the throttle valve and in the
venturi throat. This restricts the flow of the fuel/air mixture
and reduces power. If enough ice builds up, the engine may
cease to operate. Carburetor ice is most likely to occur when
temperatures are below 70 degrees Fahrenheit (°F) or 21
degrees Celsius (°C) and the relative humidity is above 80
percent. Due to the sudden cooling that takes place in the
carburetor, icing can occur even with temperatures as high
as 100 °F (38 °C) and humidity as low as 50 percent. This
temperature drop can be as much as 60 to 70 °F (33 to 39°C).
Therefore, at an outside air temperature of 100 °F (38°C), a
temperature drop of 70 °F (39°C) results in an air temperature
in the carburetor of 30 °F (-1 °C). [Figure 6-12]

The first indication of carburetor icing in an aircraft with
a fixed-pitch propeller is a decrease in engine rpm, which
may be followed by engine roughness. In an aircraft with a
constant-speed propeller, carburetor icing is usually indicated
by a decrease in manifold pressure, but no reduction in rpm.
Propeller pitch is automatically adjusted to compensate for
loss of power. Thus, a constant rpm is maintained. Although
carburetor ice can occur during any phase of flight, it is
particularly dangerous when using reduced power during
a descent. Under certain conditions, carburetor ice could
build unnoticed until power is added. To combat the effects
of carburetor ice, engines with float-type carburetors employ
a carburetor heat system.

Although carburetor ice is most likely to form when the temperature and humidity are in ranges indicated by this chart, carburetor ice is possible under conditions not depicted.
Figure 6-12. Although carburetor ice is most likely to form when
the temperature and humidity are in ranges indicated by this chart,
carburetor ice is possible under conditions not depicted.

Carburetor Heat
Carburetor heat is an anti-icing system that preheats the air
before it reaches the carburetor, and is intended to keep the
fuel/air mixture above the freezing temperature to prevent
the formation of carburetor ice. Carburetor heat can be used
to melt ice that has already formed in the carburetor if the
accumulation is not too great, but using carburetor heat as a
preventative measure is the better option. Additionally, the
use of carburetor heat as an alternate air source can be used
if the intake filter clogs such as in sudden or unexpected
airframe icing conditions. The carburetor heat should be
checked during the engine runup. When using carburetor
heat, follow the manufacturer's recommendations.

When conditions are conducive to carburetor icing during
flight, periodic checks should be made to detect its presence. If
detected, full carburetor heat should be applied immediately,
and it should be left in the ON position until the pilot is
certain all the ice has been removed. If ice is present, applying
partial heat or leaving heat on for an insufficient time might
aggravate the situation. In extreme cases of carburetor icing,
even after the ice has been removed, full carburetor heat
should be used to prevent further ice formation. If installed,
a carburetor temperature gauge is useful in determining when
to use carburetor heat.

 

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