When is the use of carburetor heat necessary?

The use of carburetor heat is necessary when carburetor icing is suspected because colder air entering the carburetor can lead to the formation of ice on the internal components. This is especially true when conditions are conducive to icing, such as when flying in humid air at temperatures between approximately 20°F and 70°F, where the temperature drop caused by fuel vaporization can combine with moisture to form ice.

Applying carburetor heat warms the air entering the carburetor, which helps prevent or mitigate the formation of ice, ensuring that the engine receives the appropriate fuel-air mixture for optimal performance. This is crucial during critical phases of flight, such as takeoff and climb, where engine performance must be reliable.

While it's true that flying in heavy rain or climbing might introduce other concerns (such as the need for additional engine power or concerns over water ingestion), these conditions do not specifically necessitate the use of carburetor heat the way suspected icing does. Also, carburetor heat is not typically required when the temperature is above freezing, as icing risk is significantly lower under those conditions. This makes the scenario of suspected carburetor icing the most appropriate context for the use of carburetor heat.