The BFIs provide backup indications in the event of failure of the electronic flight instrument system or the aircraft electrical systems. The BFIs are normally powered by the ______.

• A. Battery bus
• B. Auxiliary battery
• C. Generator bus
• D. Main AC buses

Answer: (A)

Backup flight indications must stay powered even when the main instruments and electrical system fail. To ensure this, they are fed from the battery bus, which remains energized by the aircraft’s main battery and is dedicated to essential/standby equipment. This setup guarantees the BFIs continue to display critical information when generators are offline or other power sources are unavailable. The other options depend on power that isn’t reliable during a failure—generator power, main AC power, or limited-time emergency power from an auxiliary battery—so they wouldn’t provide the guaranteed standby indication power the battery bus does.