General Aircraft Technical Knowledge Practice Test

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When carb heat is selected, there is a ______ in manifold pressure because there is a ______ in ram air pressure. There is ______ restriction on the airflow and the heated air is ______ dense than previously.

Drop, Loss, Greater, Less Rise, Gain, Greater, More Drop, Gain, Greater, More Rise, Loss, Less, Less

Correct Answer:

Drop, Loss, Greater, Less

Explanation:

Carburetor heat introduces heated air into the carburetor to prevent icing. Warmer air is less dense, so for the same volume of air the engine receives less mass. With the throttle set, this reduced mass flow lowers the pressure in the intake manifold, i.e., manifold pressure drops. The path through the carb heat system adds resistance, creating a greater restriction to airflow, and the heated air is less dense than the air it replaces. The combination of lower ram air pressure behind that restricted path and the lower air density explains the drop in manifold pressure when carb heat is selected.

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