During APU startup, what position must the battery switch be in and why?

The appropriate position for the battery switch during APU startup is ON because this ensures that the necessary electrical systems are powered and available for safe operation. With the battery switch in the ON position, critical functions such as the operation of fire handle shutdown and fire extinguisher discharge can be executed effectively if a fire situation arises during or after APU startup. This preparedness is vital for maintaining safety protocols, particularly in scenarios where there might be an emergency.

The other positions, like OFF, AUTO, or STANDBY, do not support the comprehensive operational requirements during APU startup. The OFF position would eliminate power to the essential systems, jeopardizing safety and control. The AUTO setting, typically used for different operational scenarios, wouldn't provide the same direct accessibility to emergency procedures. Finally, while STANDBY may be necessary for certain operations, it does not provide the immediate power needed during the critical startup phase of the APU when quick action is necessary. Thus, having the battery switch in the ON position ensures both operational readiness and safety during APU startup.