Figure 3-4: Carburettor Icing Graph
Carburettor icing may be controlled, or avoided, by adopting the following practices:
1. Start the engine with the carburettor heat control in the “cold” position, to avoid damage to the carburettor heat system.
2. When relative humidity is high and the summer ambient temperature is below 27 degrees Celsius, use carburettor heat immediately before takeoff. In general, carburettor heat should not be used while hovering because of increased power requirement.
3. Avoid using carburettor heat during takeoff since it may cause detonation and possible engine damage. An exception to this might be in very low temperature areas, which call for special procedures.
4. Remain alert after takeoff for indications of carburettor icing, especially when visible moisture is present. Remember that the relative humidity approaches 100% close to the base of clouds.
5. When carburettor ice is suspected, immediately apply full heat. Watch for a power loss to indicate the presence of carburettor heat, then an increase in power as ice melts.
6. Carburettor icing can occur with the ambient temperature as high as +38 degrees Celsius and humidity as low as 50 per cent. Remain especially alert with a combination of ambient temperature below +28 degrees Celsius and high relative humidity. The possibility of carburettor ice decreases:
a) in the range below 0 degrees Celsius, because of lessened humidity as the temperature decreases,
b) at around -10 degrees Celsius because of ice crystals which pass through the indication system harmlessly. It should be remembered that if the intake air does contain ice crystals, carburettor heat might actually cause carburettor icing by melting the crystals, and raising the moisture-laden air to the icing temperature.
7. During low power descents, when carburettor icing is present or suspected, apply full carburettor heat and adjust collective, so that enough engine heat is produced to prevent or disperse ice. This is a general rule for many helicopters. Consult the flight manual for the procedures in a specific helicopter type.
There is a misconception that it does not matter, to the efficiency of the engine, whether the carburettor heat is on or off. If this were true, engine manufacturers would design their engines so that heated air was constantly directed through the carburettor air intake system to completely eradicate the problem of carburettor icing. But they don’t because the application of carburettor heat in standard atmospheric conditions will:
1. Reduce the maximum power output of the engine, and
2. Increase fuel consumption.
As the ambient temperature decreases, the effect of carburettor heat on the efficiency of the engine also decreases. Light helicopter engines, operated at extremely low (winter) ambient temperatures, may require the warming influence of carburettor heat to ensure adequate response to throttle application.
MIXTURE CONTROL
Most mixture adjustments are required during changes of altitude or during operations at high altitude. As an aircraft gains altitude, the surrounding air becomes less dense. At altitude, the engine draws a lesser weight of air into its cylinders than it does on the ground. If the weight of the fuel drawn into the cylinders remained the same, regardless of altitude, the mixture would become too rich at altitude.
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 Approved Rotorcraft Flight Manual (RFM) to determine specific procedures for your helicopter. Note that most manufactures do not recommend leaning helicopters in flight. Generally, the accepted procedure for leaning the mixture is to move the mixture control slowly toward the “lean” position until maximum RPM is obtained with a fixed power setting. Then, move the mixture control toward “rich” until a decrease in RPM is just perceptible. This produces optimum power for the throttle setting, with a slightly rich mixture to prevent overheating, since sustained operations with the mixture too lean can damage the engine: 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.
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