Which of the following items is NOT on the battery bus?

The item that is NOT on the battery bus is the main engine start. The battery bus is designed to supply power to certain essential electrical components when the aircraft systems are operating in a battery mode, particularly during times of limited or no generator output. The main engine start system, however, typically relies on the aircraft’s electrical generator systems to start the engine.

Components like the ground communication system, aft compartment lamp, and cabin floodlights are often powered by the battery bus as they are critical for operation when there is no generator power available. These systems are essential for safety and operational integrity, especially during emergencies or engine start scenarios where battery power may be the sole source of electricity.

Understanding the specific functions and power requirements of various components is crucial for pilots and ground crews in managing the aircraft's electrical systems effectively, particularly in emergency situations where the battery bus is activated.