Celsius degrees) (Remember there are 180 Fahrenheit degrees  from freezing to boiling versus 100 degrees for the Celsius  scale.) Therefore, an outside air temperature of 100 F (38 C),  a temperature drop of an absolute 70 F degrees (38.89 Celsius  degrees) results in an air temperature in the carburetor of 30  F (-1 C).  [Figure 7-12]  The first indication of carburetor icing in an aircraft with  a fixed-pitch propeller is a decrease in engine rpm, which  may be followed by engine roughness. In an aircraft with a  constant-speed propeller, carburetor icing is usually indicated  by a decrease in manifold pressure, but no reduction in rpm.  Propeller pitch is automatically adjusted to compensate for  loss of power. Thus, a constant rpm is maintained. Although  carburetor ice can occur during any phase of flight, it is  particularly dangerous when using reduced power during a  descent. Under certain conditions, carburetor ice could build  unnoticed until power is added. To combat the effects of  carburetor ice, engines with float-type carburetors employ a  carburetor heat system.  Carburetor heat is an anti-icing system that preheats the air  before it reaches the carburetor and is intended to keep the  fuel-air mixture above freezing to prevent the formation of  carburetor ice. Carburetor heat can be used to melt ice that has  already formed in the carburetor if the accumulation is not too  great, but using carburetor heat as a preventative measure is  the better option. Additionally, carburetor heat may be used  as an alternate air source if the intake filter clogs, such as in  sudden or unexpected airframe icing conditions. The carburetor  heat should be checked during the engine runup. When using  carburetor heat, follow the manufacturer’s recommendations.  When conditions are conducive to carburetor icing during  flight, periodic checks should be made to detect its  It is imperative for a pilot to recognize carburetor ice when it  forms during flight to prevent a loss in power, altitude, and/or  airspeed. These symptoms may sometimes be accompanied  by vibration or engine roughness. Once a power loss is  noticed, immediate action should be taken to eliminate ice  already formed in the carburetor and to prevent further ice  formation. This is accomplished by applying full carburetor  heat, which will further reduce power and may cause engine  roughness as melted ice goes through the engine. These  symptoms may last from 30 seconds to several minutes,  depending on the severity of the icing. During this period, the  pilot must resist the temptation to decrease the carburetor heat  usage. Carburetor heat must remain in the full-hot position  until normal power returns.  The use of carburetor heat causes a decrease in engine  power, sometimes up to 15 percent, because the heated  air is less dense than the outside air that had been entering  the engine. This enriches the mixture. When ice is present  in an aircraft with a fixed-pitch propeller and carburetor  heat is being used, there is a decrease in rpm, followed by  a gradual increase in rpm as the ice melts. The engine also  should run more smoothly after the ice has been removed.  If ice is not present, the rpm decreases and then remains  constant. When carburetor heat is used on an aircraft with a  constant-speed propeller and ice is present, a decrease in the  manifold pressure is noticed, followed by a gradual increase.  If carburetor icing is not present, the gradual increase in  manifold pressure is not apparent until the carburetor heat  is turned off.  Whenever the throttle is closed during flight, the engine cools  rapidly and vaporization of the fuel is less complete than if  the engine is warm. Also, in this condition, the engine is more  susceptible to carburetor icing. If carburetor icing conditions  are suspected and closed-throttle operation anticipated, adjust  the carburetor heat to the full ON position before closing the  throttle and leave it on during the closed-throttle operation.  The heat aids in vaporizing the fuel and helps prevent the  formation of carburetor ice. Periodically, open the throttle  smoothly for a few seconds to keep the engine warm;  otherwise, the carburetor heater may not provide enough  heat to prevent icing.  presence. If detected, full carburetor heat should be applied  immediately, and it should be left in the ON position until  the pilot is certain that all the ice has been removed. If ice  is present, applying partial heat or leaving heat on for an  insufficient time might aggravate the situation. In extreme  cases of carburetor icing, even after the ice has been removed,  full carburetor heat should be used to prevent further ice  formation. If installed, a carburetor temperature gauge is  useful in determining when to use carburetor heat.  Carburetor Heat  Figure 7-12.  Although carburetor ice is most likely to form when  the temperature and humidity are in ranges indicated by this chart,  carburetor icing is possible under conditions not depicted.  7-10  Relative humidity  Outside air temperature  100%  50%  80%  60%  70%  90%  20°F/-7°C 32°F/0°C  70°F/21°C  100°F/38°C  High carburetor icing potential Carburetor icing possible