What is the purpose of the standby/emergency bus?

• A. To power cabin lighting during normal operations
• B. To power nonessential systems to improve comfort
• C. To power critical instruments and basic flight functionality if the main or essential buses lose power
• D. To replace the main bus during routine maintenance

Answer: (C)

The standby/emergency bus exists to keep the airplane's most essential flight information alive when the main electrical system loses power. In an electrical failure, you still need attitude, heading, airspeed, and altitude to fly safely. The standby bus provides power to those critical instruments (such as the attitude indicator, heading/gyros, airspeed indicator, and altimeter) and other basic flight functions so the crew can maintain control and complete an emergency procedure or safe landing.

This bus is usually powered by its own independent source, like a dedicated generator or a battery, so it can continue operating even if the main generators or buses fail. It’s not intended to run cabin lighting or comfort systems, nor to replace the main bus during routine maintenance.