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Pilot's Handbook of Aeronautical Knowledge
Aircraft Systems
Exhaust 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




Outside air aids in cooling the engine.
Figure 6-19. Outside air aids in cooling the engine.

The outside air enters the engine compartment through an
inlet behind the propeller hub. Baffles direct it to the hottest
parts of the engine, primarily the cylinders, which have fins
that increase the area exposed to the airflow.

The air cooling system is less effective during ground
operations, takeoffs, go-arounds, and other periods of highpower,
low-airspeed operation. Conversely, high-speed
descents provide excess air and can shock cool the engine,
subjecting it to abrupt temperature fluctuations
Operating the engine at higher than its designed temperature
can cause loss of power, excessive oil consumption, and
detonation. It will also lead to serious permanent damage,
such as scoring the cylinder walls, damaging the pistons
and rings, and burning and warping the valves. Monitoring
the flight deck engine temperature instruments will aid in
avoiding high operating temperature.

Under normal operating conditions in aircraft not equipped
with cowl flaps, the engine temperature can be controlled
by changing the airspeed or the power output of the engine.
High engine temperatures can be decreased by increasing the
airspeed and/or reducing the power.

The oil temperature gauge gives an indirect and delayed
indication of rising engine temperature, but can be used for
determining engine temperature if this is the only means

Most aircraft are equipped with a cylinder-head temperature
gauge which indicates a direct and immediate cylinder
temperature change. This instrument is calibrated in degrees
Celsius or Fahrenheit, and is usually color coded with a
green arc to indicate the normal operating range. A red line
on the instrument indicates maximum allowable cylinder
head temperature.

To avoid excessive cylinder head temperatures, increase
airspeed, enrich the mixture, and/or reduce power. Any of
these procedures help to reduce the engine temperature. On
aircraft equipped with cowl flaps, use the cowl flap positions
to control the temperature. Cowl flaps are hinged covers that
fit over the opening through which the hot air is expelled. If
the engine temperature is low, the cowl flaps can be closed,
thereby restricting the flow of expelled hot air and increasing
engine temperature. If the engine temperature is high, the cowl
flaps can be opened to permit a greater flow of air through the
system, thereby decreasing the engine temperature.

Exhaust Systems

Engine exhaust systems vent the burned combustion gases
overboard, provide heat for the cabin, and defrost the
windscreen. An exhaust system has exhaust piping attached
to the cylinders, as well as a muffler and a muffler shroud.
The exhaust gases are pushed out of the cylinder through
the exhaust valve and then through the exhaust pipe system
to the atmosphere.

For cabin heat, outside air is drawn into the air inlet and is
ducted through a shroud around the muffler. The muffler is
heated by the exiting exhaust gases and, in turn, heats the
air around the muffler. This heated air is then ducted to the
cabin for heat and defrost applications. The heat and defrost
are controlled in the flight deck, and can be adjusted to the
desired level.

Exhaust gases contain large amounts of carbon monoxide,
which is odorless and colorless. Carbon monoxide is deadly,
and its presence is virtually impossible to detect. The exhaust
system must be in good condition and free of cracks.

Some exhaust systems have an EGT probe. This probe
transmits the EGT to an instrument in the flight deck. The
EGT gauge measures the temperature of the gases at the
exhaust manifold. This temperature varies with the ratio
of fuel to air entering the cylinders and can be used as a
basis for regulating the fuel/air mixture. The EGT gauge
is highly accurate in indicating the correct mixture setting.
When using the EGT to aid in leaning the fuel/air mixture,
fuel consumption can be reduced. For specific procedures,
refer to the manufacturer's recommendations for leaning
the mixture.