What is the primary role of a standby bus in an aircraft electrical system?

• A. Provides essential power to all aircraft systems during normal operation
• B. Provides essential power to select instruments and systems if primary sources fail
• C. Supplies emergency hydraulic pressure
• D. Controls illumination for cockpit instruments

Answer: (B)

The stand-by bus is a backup electrical path that keeps the most critical flight information available when the primary power sources fail. Its purpose is to ensure essential instruments and systems—like key attitude and heading indicators, airspeed and altitude readouts, and other select avionics—continue to operate even if the main buses lose power. In normal operation, the larger main electrical busses handle the bulk of power distribution, while the stand-by bus remains a dedicated, resilient source for those vital displays and systems so the pilot can maintain situational awareness and control. It isn’t intended to supply hydraulic pressure or cockpit instrument illumination, which come from other systems and power paths.