Lycoming IVO-540 Series, Operator'S Manual

Page: 24 / 84

SECTION 3 LYCOMING OPERATOR’S MANUAL
OPERATING INSTRUCTIONS VO, IVO & TIVO-540 SERIES
a.
Use of Carburetor Heat Control –
Under certain moist atmospheric conditions, it is possible for ice to
form in the induction system even in summer weather. This is due to the high air velocity through the
carburetor venturi and the absorption of heat from this air by evaporation of the fuel. The temperature
in the mixture chamber may drop 20°F to 70°F below the temperature of the incoming air. If this air
contains a large amount of moisture, the cooling process will cause precipitation in the form of ice.
These ice formations generally begin in the vicinity of the butterfly throttle and will often build up to
such an extent that engine operation is noticeably affected. This ice will obstruct the carburetor
passage resulting in a decreased flow of mixture and consequently a drop in manifold pressure and
RPM. If not detected this condition will continue to such an extent that the reduced power will cause
complete engine stoppage.
To avoid this condition, 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 to vaporization of
fuel, and the mixture chamber temperatures cannot drop to the freezing point of water. This pre-heater
is essentially 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 the danger of detonation,
especially when operating at high power output. The application of excessive heat will produce
expansion of the charge with a resultant loss of density. Since power output depends upon the mass of
charge induced into the cylinders, heating the mixture will involve a loss of power and a decided
variation of the mixture. High charge temperatures favor both detonation and pre-ignition, 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:
The carburetor air heat control should be left in the cold position during normal flight operations.
On damp, cloudy, foggy or hazy days, regardless of the outside air temperatures, keep a sharp lookout
for loss of power. This loss of power will be shown by an unaccountable loss of manifold pressure and
RPM. When this situation arises, apply full carburetor air heat. 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 the ice has been melted from the induction system, the carburetor heat
control should be returned to the 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
(32°F).
WARNING
CAUTION MUST BE EXERCISED WHEN OPERATING WITH PARTIAL HEAT ON
AIRCRAFT THAT DO NOT HAVE A CARBURETOR AIR TEMPERATURE GAUGE.
MOISTURE IN CRYSTAL FORM THAT WOULD ORDINARILY PASS THROUGH THE
INDUCTION SYSTEM, CAN BE RAISED IN TEMPERATURE BY USE OF PARTIAL HEAT
TO THE POINT WHERE THE CRYSTALS ARE MELTED INTO LIQUID FORM. THIS
MOISTURE IN TURN CAN FORM CARBURETOR ICE DUE TO THE TEMPERATURE
DROP AS IT PASSES THROUGH THE VENTURI OF THE CARBURETOR. IT IS
ADVISABLE, THEREFORE, TO USE EITHER FULL HEAT OR NO HEAT IN AIRCRAFT
THAT ARE NOT EQUIPPED WITH A CARBURETOR AIR TEMPERATURE GAUGE.
b. See airframe manufacturer’s instru ctions for correct manifold pressure for various power settings.
3-4