Carburetor Icing

Carburetor icing occurs when moisture in the induction air freezes inside the carburetor, restricting or, in severe cases, completely blocking the flow of the fuel-air mixture to the engine. It is caused by the sharp temperature drop produced as fuel vaporizes and the venturi effect accelerates airflow through the carburetor throat — a drop that can exceed 60-70°F (about 21°C) below the outside air temperature.

Because of this, carburetor icing can form even in outside air temperatures well above freezing, and even on clear days — it is not limited to cold, visibly icy conditions, which is one of the most commonly tested traps in this area. Relative humidity above roughly 50-60% is enough to supply the necessary moisture.

Applying carburetor heat routes unfiltered, heated air into the induction system to melt the ice. Immediately after applying it, engine RPM typically drops further before recovering, as the warmed, less dense air momentarily reduces power — a pilot who mistakes this initial drop for a sign carb heat is not working, and turns it back off, allows the icing to continue building.