Which option best describes when the ON BAT indicator will illuminate during normal operation?

Which option best describes when the ON BAT indicator will illuminate during normal operation?

• A. When each IR section is powered by NAV
• B. On initial battery connection
• C. Only during maintenance
• D. During engine start

Answer: (B)

The ON BAT indicator shows that the aircraft’s electrical system is being powered by the battery. It lights when the battery is connected to the electrical network and the battery power is established, i.e., on initial battery connection. In normal operation, other power sources (external power, APU or engine generators) typically feed the buses, so the indicator is not a continuous lamp; it specifically marks the moment the battery becomes the power source. It isn’t tied to maintenance status, nor is it something that should be expected to illuminate only during engine start.

The ON BAT indicator shows that the aircraft’s electrical system is being powered by the battery. It lights when the battery is connected to the electrical network and the battery power is established, i.e., on initial battery connection. In normal operation, other power sources (external power, APU or engine generators) typically feed the buses, so the indicator is not a continuous lamp; it specifically marks the moment the battery becomes the power source. It isn’t tied to maintenance status, nor is it something that should be expected to illuminate only during engine start.