FAA Powerplant Written Practice Test

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

1. It will shut down the engine completely

2. Cool air will be drawn into the engine

3. Warm, unfiltered air is allowed in through a spring-loaded alternate air valve

4. The air filter will automatically clean itself

The correct answer is: Warm, unfiltered air is allowed in through a spring-loaded alternate air valve

When the main air duct air filter becomes blocked, the aircraft's engine management system is designed to maintain engine performance and safety through the use of an alternate air door. In this scenario, the alternate air door opens, allowing warm, unfiltered air to enter the engine instead of restricted airflow through the blocked filter. This is particularly important because, despite being unfiltered, the alternate air ensures that the engine continues to receive the necessary air to function effectively, preventing a complete shutdown due to lack of air intake. This mechanism helps to safeguard against potential engine failure in an emergency situation where the main filter is compromised. The warm air typically comes from within the engine compartment, which can help to prevent ice accumulation that might occur if only cooler ambient air is allowed into the system. By diverting airflow through the alternate air valve, the engine can maintain operation even under less than ideal conditions.