When might using carb heat become necessary during a flight?

Using carburetor heat becomes necessary during a flight when there is a risk of carburetor icing. Carburetor icing can occur under certain atmospheric conditions, particularly in temperatures below about 70°F (21°C) with high humidity or in temperatures colder than 20°F (-6°C) irrespective of humidity. When air passes through the carburetor venturi, it creates a pressure drop and temperature drop due to its expansion. This can cause moisture in the air to freeze, forming ice that disrupts the airflow and affects engine performance.

By applying carburetor heat, the pilot introduces warm, unfiltered air from the engine compartment into the carburetor. This raises the temperature of the air-fuel mixture and helps to melt the ice that may have formed, preventing potential engine roughness or power loss due to icing. Using carburetor heat is a crucial action to maintain proper engine operation when conditions conducive to icing are present.