General Authority of Civil Aviation (GACA) Aeronautical Information Publication (AIP) Practice Exam

What triggers the APU to start automatically during flight?

1. If the APU selector is set to ON

2. When both AC transfer busses lose power

3. If the external power source fails

4. When maintenance is required

The correct answer is: When both AC transfer busses lose power

The scenario where the Auxiliary Power Unit (APU) starts automatically during flight is primarily triggered when both AC transfer buses lose power. This system design ensures that the aircraft has a reliable backup power source. If both AC buses lose power, the APU can automatically activate to provide the necessary electrical power and bleed air, which are essential for maintaining operations, ensuring safety, and minimizing the risk of power loss-related issues during flight. The other options may not accurately reflect the operational procedures related to APU activation. Setting the APU selector to ON is a manual operation and does not trigger an automatic start during flight. Similarly, while the failure of an external power source is a significant event, it would not directly cause an automatic start unless it involved a loss of both AC transfer buses. Maintenance requirements, while critical, do not factor into an automatic triggering mechanism for the APU during flight operations. Thus, the logic behind the automatic activation is fundamentally tied to maintaining power integrity in flight.