FAA Powerplant Written Practice Test

What is the primary source of air used for carburetor heat?

1. Warm ducted air from around the exhaust system

2. Cold air from outside the aircraft

3. Air from the cabin

4. Cool air from the fuel tank

The correct answer is: Warm ducted air from around the exhaust system

The primary source of air used for carburetor heat is warm ducted air from around the exhaust system. This warm air is directed into the carburetor to help prevent or alleviate carburetor ice, which can form under certain conditions when temperatures drop. Carburetor icing can hinder engine performance or even lead to engine failure, so utilizing the warm air increases the temperature of the air-fuel mixture, reducing the chances of ice formation. The other sources mentioned, such as cold air from outside the aircraft and air from the cabin, would not provide the necessary warmth to combat ice. Cool air from the fuel tank is also inadequate, as it does not address the issue of carburetor icing and could actually exacerbate the problem. Thus, the use of warm air from around the exhaust system is the most effective and practical method for ensuring optimal engine operation in varying atmospheric conditions.