RECIPROCATING ENGINES
Fuel is delivered to the cylinders by either a carburetor or fuel injection system.
CARBURETOR
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 position. 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 control. 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 helicopters in-flight.
Most mixture adjustments are required during changes of altitude or during operations at airports with field elevations well above sea level. A mixture that is too rich can result in engine roughness and reduced power. The roughness normally is due to spark plug fouling from excessive 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 situation is to monitor the engine temperature gauges regularly and follow the manufacturer’s guidelines for maintaining the proper mixture.