As air moves through a carburettor its temperature drops and, if conditions are right, water vapour in the air can condense and form ice. Most carburetted aircraft are equipped with a control that routes heated air to the carburettor to melt the ice and keep it from re-forming.

Applying carb heat enriches the mixture and increases fuel consumption for a given power setting because the heated air is less dense than ambient air. Pilots should therefore lean while operating with carb heat and enrich when it’s no longer needed.

Accident Report: A private pilot was en-route from Boston to an airport in northern Virginia. Shortly after passing Dulles Airport, the engine failed due to fuel exhaustion. The airplane was destroyed and the occupants were severely injured in the off-airport landing. The pilot stated that he had made the trip many times before with enough fuel to reach his destination but, on this flight, carburettor heat was applied shortly after take-off and remained on until the landing. The richer mixture resulted in fuel exhaustion ten miles from his destination.

Tony Birth