CARBURETOR ICE
The effect of fuel vaporization and decreasing air pressure 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 carburetor 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 temperature 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 carburetor heat system, which eliminates the ice by routing air across a heat source, such as an exhaust manifold, before it enters the carburetor. [Figure 5-11].
 

 
FUEL INJECTION
In a fuel injection system, fuel and air are metered at the fuel control unit but are not mixed. The fuel is injected directly into the intake port of the cylinder where it is mixed with the air just before entering the cylinder. This system ensures a more even fuel distribution in the cylinders and better vaporization, which in turn, promotes more efficient use of fuel. Also, the fuel injection system eliminates the problem of carburetor icing and the need for a carburetor heat system.
TURBINE ENGINES
The fuel control system on the turbine engine is fairly complex, as it monitors and adjusts many different parameters on the engine. These adjustments are done automatically and no action is required of the pilot other than starting and shutting down. No mixture adjustment is necessary, and operation is fairly simple as far as the pilot is concerned. New generation fuel controls incorporate the use of a full authority digital engine control (FADEC) computer to control the engine’s fuel requirements. The FADEC systems increase efficiency, reduce engine wear, and also reduce pilot workload. The FADEC usually incorporates back-up systems in the event of computer failure.