RECIPROCATING ENGINES CARBURETOR CARBURETOR ICE Fuel is delivered to the cylinders by either a carburetor or fuel injection system. In a carburetor system, air is mixed with vaporized fuel as it passes through a venturi in the carburetor. The metered fuel/air mixture is then delivered to the cylinder intake. Carburetors are calibrated at sea level, and the correct fuel-to-air mixture ratio is established at that altitude with the mixture control set in the FULL RICH posi- tion. However, as altitude increases, the density of air entering the carburetor decreases while the density of the fuel remains the same. This means that at higher altitudes, the mixture becomes progressively richer. To maintain the correct fuel/air mixture, you must be able to adjust the amount of fuel that is mixed with the incoming air. This is the function of the mixture con- trol. This adjustment, often referred to as “leaning the mixture,” varies from one aircraft to another. Refer to the FAA-Approved Rotocraft Flight Manual (RFM) to determine specific procedures for your helicopter. Note that most manufacturers do not recommend leaning hel- icopters in-flight. Most mixture adjustments are required during changes of altitude or during operations at airports with field eleva- tions well above sea level. A mixture that is too rich can result in engine roughness and reduced power. The rough- ness normally is due to spark plug fouling from exces- sive carbon buildup on the plugs. This occurs because the excessively rich mixture lowers the temperature inside the cylinder, inhibiting complete combustion of the fuel. This condition may occur during the pretakeoff runup at high elevation airports and during climbs or cruise flight at high altitudes. Usually, you can correct the problem by leaning the mixture according to RFM instructions. If you fail to enrich the mixture during a descent from high altitude, it normally becomes too lean. High engine temperatures can cause excessive engine wear or even failure. The best way to avoid this type of situ- ation is to monitor the engine temperature gauges regu- larly and follow the manufacturer’s guidelines for maintaining the proper mixture. The effect of fuel vaporization and decreasing air pres- sure in the venturi causes a sharp drop in temperature in the carburetor. If the air is moist, the water vapor in the air may condense. When the temperature in the car- buretor is at or below freezing, carburetor ice may form on internal surfaces, including the throttle valve. [Figure 5-10] Because of the sudden cooling that takes place in the carburetor, icing can occur even on warm days with temperatures as high as 38°C (100°F) and the humidity as low as 50 percent. However, it is more likely to occur when temperatures are below 21°C (70°F) and the relative humidity is above 80 percent. The likelihood of icing increases as temperature decreases down to 0°C (32°F), and as relative humidity increases. Below freezing, the possibility of carburetor icing decreases with decreasing temperatures. Although carburetor ice can occur during any phase of flight, it is particularly dangerous when you are using reduced power, such as during a descent. You may not notice it during the descent until you try to add power. Indications of carburetor icing are a decrease in engine r.p.m. or manifold pressure, the carburetor air tempera- ture gauge indicating a temperature outside the safe operating range, and engine roughness. Since changes in r.p.m. or manifold pressure can occur for a number of reasons, it is best to closely check the carburetor air temperature gauge when in possible carburetor icing conditions. Carburetor air temperature gauges are marked with a yellow caution arc or green operating arcs. You should refer to the FAA-Approved Rotorcraft Flight Manual for the specific procedure as to when and how to apply carburetor heat. However, in most cases, you should keep the needle out of the yellow arc or in the green arc. This is accomplished by using a car- buretor heat system, which eliminates the ice by  5-7 Figure 5-10. Carburetor ice reduces the size of the air pas- sage to the engine. This restricts the flow of the fuel/air mixture, and reduces power. To Engine Incoming Air Ice Ice Venturi Fuel/AirGLYPH<.notdef> Mixture Ice