FAA Powerplant Written Practice Test

What effect does applying carburetor heat have during engine operation?

1. Increases air pressure to the carburetor

2. Decreases air density to the carburetor

3. Increases fuel efficiency

4. Improves power output

The correct answer is: Decreases air density to the carburetor

Applying carburetor heat has the effect of decreasing air density to the carburetor. This occurs because the carburetor heat system draws warm air, often heated by exhaust gases, into the carburetor. Warm air is less dense than cold air, which means that when carburetor heat is applied, the mixture going into the engine is less dense. The primary purpose of using carburetor heat is to prevent or eliminate carburetor icing, which can happen when moisture in the air freezes on the carburetor surface, leading to a blockage of the fuel-air mixture. By using heated air, the likelihood of ice formation is minimized, allowing for more reliable engine operation. While some might think that warming the air could improve fuel efficiency or power output under certain conditions, the immediate and most relevant effect during operation is indeed the reduction in air density as a result of the heated air entering the carburetor. This understanding is crucial for pilots and mechanics when managing engine performance and ensuring safe operation, particularly in cold and humid conditions.