Cold Weather Operation and Carb Heat Usage
By Dave Domeier
The Lycoming Flyer has some tips on using carburetor heat....what follows is a clip on the subject, compliments of Lycoming.
Every pilot who flies an aircraft powered by a carbureted engine should be thoroughly educated about carburetor ice. They should know that under moist conditions (a rela-tive humidity of 50% to 60% is moist enough), carburetor ice can form with any outside
air temperature from 20° to 90°F. It is most likely in the 30° to 60°F range. Temperatures in the carburetor can drop 60° to 70°F (refrigerator effect) as a result of fuel vaporization and the carburetor venturi effect. It also happens that carburetor ice forms more readily when the engine is operated in the lower power range. It will form while taxiing and this makes it very important to check engine power before takeoff and to remove the ice if necessary. Care should be taken to avoid dusty or dirty conditions when utilizing carburetor heat on the ground. Next, it is imperative that the pilot recognize carburetor ice when it forms during flight. The loss of power that occurs will cause a reduction of RPM when flying with a fixed pitch propeller, and a loss of manifold pres-sure when a controllable pitch propeller is used. In either case, a loss of altitude or airspeed will occur. These symptoms may sometimes be accompanied by vibration or engine roughness. 

In any case, it is a good idea to consider carburetor ice as the cause of any unex-plained power loss during cruise flight. Once a power loss is noticed by the pilot, immediate action should be taken to eliminate ice which has already formed in the carbu-retor,
and to prevent further ice formation. This is accomplished by applying full carburetor heat which will initially cause a further loss of power (perhaps as much as 15%) and, possibly, engine roughness. The additional power loss is caused by the heated air that is being directed into the induction system. Heated air makes the mixture richer and also melts the ice which then goes through the engine as water. The throttle may be advanced and the mixture may be leaned to help get some of the lost power back, but immediately after the application of carburetor heat the pilot must be patient and keep the airplane flying until the ice has completely melted and normal power returns. How long this will take depends on the severity of the icing, but the pilot should expect a delay of 30 seconds to several minutes. Under the circumstances, this period of time will be stressful and always seems longer than it really is, but the knowledgeable pilot will not retreat from use of carburetor heat. Carburetor heat should remain in the hot position until power returns. 

In conditions where carburetor ice is likely to form, the pilot may use heat during cruise to prevent the formation of ice in the carburetor. It is also appropriate to use full carbu-retor heat, if needed, to prevent icing when operating at low power for instrument approaches, or for flight in the traffic pattern. Unless the aircraft is equipped with a carburetor air temperature (CAT) gage, and very few general aviation aircraft are, use of full carburetor heat is recommended.

An unknown amount of partial heat can actually cause induction ice in the float type carburetor. This may occur when moisture in crystal form in the incoming air that would ordinarily pass through the induction system without any problem is melted by the partial heat. This moisture then freezes when it comes in contact with the cold metal of the throttle plate. Whenever carburetor heat is used in the landing configuration, and a go-around or touch-and-go takes place, there are some important steps for the pilot to
remember. The throttle must be advanced and the carburetor heat lever placed in the cold position. The order in which these steps are accomplished is not too important, but both must be done. Leaving the carburetor heat on during a go-around will result in a
loss of power that could be critical at low altitude and low airspeed. Do not use carburetor heat for takeoff or climb with a Lycoming engine as it is not necessary, and it may bring on detonation and possible engine damage. An exception to this rule might be justified in extremely cold weather conditions such as those found in the Arctic, and these conditions require a special knowledge to accommodate operation under such extreme conditions. A review of the material discussed in this article should help pilots to cope with reduc-tion of engine power when it is caused by loss of intake air for combustion. A thorough understanding of the air intake system and the knowledge to competently deal with induction icing are essential to safe flight in general aviation aircraft. Pilots are encouraged to enhance the safety of their flying by knowing what to expect and what steps to
take when the air flow to the engine is cut off for any reason.

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