The carburetor air ducts consist of a fixed duct riveted to the  nose cowling and a flexible duct between the fixed duct and  the carburetor air valve housing. The carburetor air ducts  normally provide a passage for outside air to the carburetor.  Air enters the system through the ram-air intake. The intake  opening is located in the slipstream so the air is forced into the  induction system giving a ram effect to the incoming airflow.  The air passes through the air ducts to the carburetor. The  carburetor meters the fuel in proportion to the air and mixes  the air with the correct amount of fuel. The throttle plate of  the carburetor can be controlled from the cockpit to regulate  the flow of air (manifold pressure), and in this way, power  output of the engine can be controlled.  Although many newer aircraft are not so-equipped, some  engines are equipped with carburetor air temperature  indicating systems which shows the temperature of the air  at the carburetor inlet. If the bulb is located at the engine  side of the carburetor, the system measures the temperature  of the fuel/air mixture. A short discussion concerning the formation and location of  induction system ice is helpful, even though a technician is  not normally concerned with operations that occur when the  aircraft is in flight.  [Figure 3-7]  Technicians should know  something about induction system icing because of its effect  on engine performance and troubleshooting. Even when an  inspection shows that everything is in proper working order  and the engine performs perfectly on the ground, induction  system ice can cause an engine to act erratically and lose  power in the air. Many engine troubles commonly attributed  to other sources are actually caused by induction system icing.  Induction system icing is an operating hazard because it can  cut off the flow of the fuel/air charge or vary the fuel/air  ratio. Ice can form in the induction system while an aircraft  is flying in clouds, fog, rain, sleet, snow, or even clear air that  has high moisture content (high humidity). Induction system  icing is generally classified in three types: the induction system to the warm air circulating in the engine  compartment and around the exhaust manifold.  Improper or careless use of carburetor heat can be just as  dangerous as the most advanced stage of induction system  ice. Increasing the temperature of the air causes it to expand  and decrease in density. This action reduces the weight of  the charge delivered to the cylinder and causes a noticeable  loss in power because of decreased volumetric efficiency. In  addition, high intake air temperature may cause detonation  and engine failure, especially during takeoff and high power  operation. Therefore, during all phases of engine operation,  the carburetor temperature must afford the greatest protection  against icing and detonation.  When there is danger of induction system icing, the cockpit  carburetor heat control is moved to the hot position. Throttle  ice or any ice that restricts airflow or reduces manifold  pressure can best be removed by using full carburetor heat.  If the heat from the engine compartment is sufficient and the  application has not been delayed, it is only a matter of a few  minutes until the ice is cleared.  Induction system ice can be prevented or eliminated by raising  the temperature of the air that passes through the system, using  a carburetor heat system located upstream near the induction  system inlet and well ahead of the dangerous icing zones. This  air is collected by a duct surrounding the exhaust manifold.  Heat is usually obtained through a control valve that opens  Chapter 2 discusses types of icing in more detail.  3-4 •  Throttle ice •  Fuel evaporation ice •  Impact ice Induction System Icing Figure 3-7.  Location of a carburetor heat air valve.  Throttle ice Fuel evaporation ice Impact ice