Carburetor Heat when applied causes mixture to become rich, since the air density of the hot air decreases and the flow of the fuel is constant.
Avoid excessive ground usage of the Carburetor heat since the hot air entering the carburetor is not filtered.
Usually the fuel mixture ratio maintained for the normal conditions are 15 parts of air mixed with 1 part of the fuel.
Conditions for Carburetor icing
Requirements-
- Visible moisture
- High humidity
- Temperature above 0° degrees Celsius to +20° degrees Celsius.
The icing can take place at temperature in the range of 0 to 20 degrees Celsius.
The icing occurring at such a high temperature of +20 degrees occurs because when the fuel is converted to a vapor in the carburetor, the adiabatic cooling takes place.
Adiabatic cooling is the process of a fuel changing into vapor that requires the heat to be absorbed from the surrounding air.
In the carburetor when the fuel is in the process of turning into a vapor it absorbs heat from the surrounding construction, that is of aluminum alloy, that has melting temperature of approx 600 degrees, (approximately half the melting value of Cast Iron) due to which it cools down faster, causing the metal to cool down quickly.
The butterfly valve controls the flow of air passing through the carburetor,
The chances of Carburetor icing increases significantly when throttle is operated in a partially open position, because of the venturi effect created by the butterfly valve partially open, causing air to further cool down.
There is no instrument to indicate or to confirm the accumulation of icing in the Carburetor.
On applying the Carburetor Heat the a/c experiences a sudden drop in the Rpm or if fitted with Manifold press gauges a decrease in the Manifold Pressure.
The presence of the Carburetor icing is confirmed by a drop of Manifold Pressure initially due to mixture becoming rich, due to air density of the hot air being low followed by a further drop in Manifold Pressure accompanied by sudden engine roughness, since the melted ice passes through the carburetor and finally an increase in the Manifold Pressure confirms that the Ice has melted.
The Carburetor ice should always be applied to full; it should never be applied partially as it can cause the passage to cause a venturi effect causing temperature of the applied air to decrease.
The chances of Carburetor icing reduces at temperature of 0 degrees Celsius and below since the air can not hold water in the vapor form below it, and causes water to crystallize in the form of ice, which passes through the carburetor harmlessly.