Carburetor icing
Carburetor Icing, or carb icing, is an icing condition which can affect any carburetor under certain atmospheric conditions. Carburetor icing occurs when there is humid air, and the temperature drop in the venturi causes the water vapor to freeze. The ice will form on the surfaces of the carburetor throat, further restricting it. This may increase the Venturi effect initially, but eventually restricts airflow, perhaps even causing a complete blockage of the carburetor. Icing may also cause jamming of the mechanical parts of the carburetor, such as the throttle, typically a butterfly valve.
While it applies to all carburetors, it is of particular concern in association with piston-powered aircraft, especially small, single-engine, light aircraft. Aircraft powered by carbureted engines are equipped with carburetor heat systems to overcome the icing problem. In cars, carburetor icing can occasionally be a nuisance. The inlet manifold and parts of the carburetor often have warm water from the cooling system or exhaust gas circulating through them to combat this problem. Motorcycles can also suffer from carburetor icing, although some engine designs are more susceptible to it than others. Air-cooled engines may be more prone to icing, due to the absence of warm coolant circulating through the engine.
For information about when there is a chance of carburetor icing, consult a graph (compiled by the UK Civil Aviation Authority) that can be downloaded here: Carburetor icing chart.
