FAA Aircraft Dispatcher Complete Practice Exam

The correct choice for the backup source of electrical power for the Boeing 727 is the battery. In aviation, the battery serves as a crucial component in the electrical system, providing power for essential systems when other sources, like the main generators, are not available. During situations where the aircraft's engines are not running or for emergency situations, the battery supports critical functions to ensure the safety and operability of the aircraft systems.

The auxiliary power unit generally provides power while the aircraft is on the ground and can assist during engine start but is not the primary backup source during flight emergencies. The emergency generator is also a backup source, but it typically relies on engine-driven systems and is not primarily considered a standalone backup system like the battery. A capacitor bank, while useful in certain electrical configurations, does not serve as a backup power source in this context and functions differently within the overall electrical system.

Thus, the battery is specifically designed to act as an immediate and reliable source of electrical power, making it the ideal choice for backup needs in a situation where all other systems are compromised or non-operational.