FAA Powerplant Written Practice Test

When starting a piston-powered engine equipped with carburetor heat, what position should it be in?

1. Warm for optimal performance

2. Cold for starting

3. It does not matter

4. Hot to prevent icing

The correct answer is: Cold for starting

For starting a piston-powered engine equipped with carburetor heat, it is essential to have the carburetor heat in the cold position. This is because during the initial starting phase, the engine needs a optimal mixture of air and fuel to ignite properly. When the carburetor heat is set to cold, it allows the incoming air to be at ambient temperature, which is crucial for achieving the correct fuel vaporization. If the carburetor heat is on, it can introduce pre-heated air that may be less dense and can lead to a richer mixture that could hinder the combustion process, especially if the air is warm and already close to saturation with moisture, influencing the engine’s starting characteristics negatively. This practice is important not only for ensuring the engine starts efficiently but also for preventing issues related to carburetor icing, which is more relevant once the engine is running and subjected to higher throttle settings where ice formation is more likely to occur. Therefore, the cold position is ideal for the initial start of the engine.