FAA Powerplant Written Practice Test

What happens when the main air duct air filter becomes blocked in an airplane with an alternate air door?

1. Engine loses all power

2. Engine draws warm, unfiltered air

3. Engine automatically shuts down

4. Air is filtered from the cabin

The correct answer is: Engine draws warm, unfiltered air

When the main air duct air filter becomes blocked in an airplane equipped with an alternate air door, the aircraft's engine will draw warm, unfiltered air. This scenario arises because the alternate air door serves as a backup air supply to ensure that the engine continues to receive airflow even when the primary filter is obstructed. The alternate air intake sources air from a location that is not filtered, which may be warm due to the influence of the engine or other components. This shift to warm, unfiltered air allows the engine to maintain operation, albeit potentially at reduced performance and efficiency since the air is not conditioned or filtered like normal operational air. This mechanism is critical in preventing total engine failure and allowing pilots to manage the engine's operation under less than ideal conditions. The other options do not accurately reflect the function of the alternate air door in this scenario. The engine does not lose all power or automatically shut down solely because of a filter blockage, as the alternate air door is designed to prevent such events. Air being filtered from the cabin is unrelated to the operation of the engine and does not play a role in this specific situation.