What is the primary purpose of the service bus before cockpit power is turned on?

• A. Powers items needed to pre-flight and service the airplane
• B. Powers in-flight entertainment systems
• C. Powers cargo doors
• D. Powers meteorological sensors

Answer: (A)

The service bus is there to power items that ground crew need for preflight and servicing while the cockpit power is still off. It provides just enough electrical power for non-essential systems like cabin lighting, galley and lavatory services, and other ground-support tasks so crews can inspect, service, and prepare the airplane without turning on the main cockpit avionics. This keeps critical flight systems off until the aircraft is ready to be powered up for flight.

That’s why the primary purpose is to power items needed to pre-flight and service the airplane. The other options describe systems that aren’t required for preflight tasks or aren’t intended to be energized by the service bus before cockpit power is on.