In the event of primary source failure, which subsystem remains powered to maintain essential flight instrumentation?

• A. Main bus
• B. Standby bus
• C. Battery bus
• D. Avionics bus

Answer: (B)

When the primary power source fails, the Standby bus stays energized to keep essential flight instruments alive. It is tied to a separate standby power source, such as a standby battery or emergency supply, specifically to preserve critical cockpit information like attitude, altitude, airspeed, and heading. This lets the pilot maintain basic orientation and control even as the main electrical system goes down. The main and avionics buses depend on the primary source, while the standby bus is purpose-built to remain powered for safety-critical instrumentation.