During normal operations, when should the ON BAT light come on?

During normal operations, when should the ON BAT light come on?

• A. When each IR section is first powered by selecting NAV
• B. When the aircraft is on the ground with batteries off
• C. Only after engine start
• D. When the APU is running

Answer: (A)

The ON BAT light is an indicator tied to powering the Inertial Reference System (IRS). In normal ops, you energize each IRS by selecting NAV. When this happens, the IRS sections receive power from the aircraft’s battery, and the ON BAT light comes on to show that battery power is being used for the initial energizing and alignment of the IRS. This tells you the IRS is not yet on the main electrical bus and is in the battery-powered startup phase for that unit. Once the electrical system is fully up and the IRS runs off the normal power source, the light will no longer indicate battery power. The other scenarios—being on the ground with batteries off, starting engines, or the APU running—do not define the normal first-energizing event for the IRS, so the light is not expected to be described by those situations.

The ON BAT light is an indicator tied to powering the Inertial Reference System (IRS). In normal ops, you energize each IRS by selecting NAV. When this happens, the IRS sections receive power from the aircraft’s battery, and the ON BAT light comes on to show that battery power is being used for the initial energizing and alignment of the IRS. This tells you the IRS is not yet on the main electrical bus and is in the battery-powered startup phase for that unit. Once the electrical system is fully up and the IRS runs off the normal power source, the light will no longer indicate battery power. The other scenarios—being on the ground with batteries off, starting engines, or the APU running—do not define the normal first-energizing event for the IRS, so the light is not expected to be described by those situations.