Explain the difference between a standby bus and a main bus in terms of loads they power.

• A. Standby bus powers most systems
• B. Standby bus powers only cabin lighting
• C. Standby bus powers critical instruments and limited loads during anomalies; main bus powers most systems
• D. They are identical

Answer: (C)

Power distribution in aircraft electrical systems uses a main bus to feed the majority of loads and a standby (essential) bus to keep critical instruments alive when power is lost. The standby bus is designed to supply only essential flight instruments and a limited set of loads during anomalies, so the crew still has basic situational awareness and control. The main bus, when power is normal, supplies the vast majority of systems and equipment, including most avionics, cabin systems, and other nonessential loads. Therefore, the standby bus preserves critical functionality during failures, while the main bus handles the bulk of electrical power needs.