FAA Ground School Practice Test

What change occurs in the fuel/air mixture when carburetor heat is applied?

1. The fuel/air mixture becomes richer

2. The fuel/air mixture becomes leaner

3. The fuel/air mixture remains unchanged

4. The fuel/air mixture evaporates quicker

The correct answer is: The fuel/air mixture becomes richer

When carburetor heat is applied, the fuel/air mixture becomes richer. This occurs because the heat from the engine raises the temperature of the air entering the carburetor. Warmer air has a lower density compared to cooler air, which means that for a given volume, there is a decrease in the amount of air molecules present when carburetor heat is applied. As a result, less air enters the carburetor relative to the amount of fuel being delivered, leading to an increase in the fuel-to-air ratio. Using carburetor heat is often a method to prevent ice formation in the carburetor, especially in conditions of high humidity and low temperatures. By enriching the mixture, it helps to ensure that the engine continues to run smoothly by maintaining optimal combustion and preventing any performance issues that can arise from a lean mixture or icing. In summary, applying carburetor heat primarily influences the mixture by increasing the fuel content relative to air, thus making it richer.