Lycoming O-320 76 Series Operator's Manual page 25

LYCOMING SERIES OPERATOR'S MANUAL
O-320
To avoid this, all installations are equipped with a system for preheating the incoming air supply to the
carburetor. In this way sufficient heat is added to replace the heat loss of vaporization of fuel, and the
mixing chamber temperature cannot drop to the freezing point of water (32°F). The air preheater is a tube or
jacket through which the exhaust pipe from one or more cylinders is passed, and the air flowing over these
surfaces is raised to the required temperature before entering the carburetor. Consistently high temperatures
are to be avoided because of a loss of power and a decided variation of mixture. High charge temperatures
also favor detonation and preignition, both of which are to be avoided if normal service life is to be expected
from the engine. The following outline is the proper method of utilizing the carburetor heat control.
a. Ground Operation – Use of the carburetor air heat on the ground must be held to an absolute
minimum. On some installations the air does not pass through the air filter, and dirt and foreign
substances can be taken into the engine with the resultant cylinder and piston ring wear. Only use
carburetor air heat on the ground to make certain it is functioning properly.
b. Take-Off – Set the carburetor heat in full cold position. For take-off and full throttle operation the
possibility of expansion or throttle icing at wide throttle openings is very remote.
c. Climbing – When climbing at part throttle power settings of 80% or above, set the carburetor heat
control in the full cold position; however, if it is necessary to use carburetor heat to prevent icing it is
possible for engine roughness to occur due to the over-rich fuel/air mixture produced by the additional
carburetor heat. When this happens, lean the mixture with the mixture control only enough to produce
smooth engine operation. Do not continue to use carburetor heat after flight is out of icing conditions,
and return mixture to full rich when carburetor heat is removed.
d. Flight Operation – During normal flight, leave the carburetor air heat control in the full cold position.
On damp, cloudy, foggy or hazy days, regardless of the outside air temperature, be alert for loss of
power. This will be evidenced by an unaccountable loss in manifold pressure or RPM or both,
depending on whether a constant speed or fixed pitch propeller is installed on the aircraft. If this
happens, apply full carburetor air heat and open the throttle to limiting manifold pressure and RPM.
This will result in a slight additional drop in manifold pressure, which is normal, and this drop will be
regained as the ice is melted out of the induction system. When ice has been melted from the
induction system, return the carburetor heat control to the full cold position. In those aircraft equipped
with a carburetor air temperature gauge, partial heat may be used to keep the mixture temperature
above the freezing point of water (32°F).
CAUTION MUST BE EXERCISED WHEN OPERATING WITH PARTIAL HEAT ON
AIRCRAFT THAT DO NOT HAVE A CARBURETOR AIR TEMPERATURE GAUGE. USE
EITHER FULL HEAT OR NO HEAT IN AIRCRAFT THAT ARE NOT EQUIPPED WITH A
CARBURETOR AIR TEMPERATURE GAUGE.
e. Landing Approach – In making a landing approach, the carburetor heat is generally in the "Full Cold"
position. However, if icing conditions are suspected, apply "Full Heat". In the case that full power
needs to be applied under these conditions, as for an aborted landing, return the carburetor heat to
"Full Cold" after full power application.
Revised September 2008
OPERATING INSTRUCTIONS
WARNING
SECTION 3
3-7