What must the UNDERFLOOR Switch be set to before starting the APU?

The correct setting for the UNDERFLOOR Switch before starting the Auxiliary Power Unit (APU) is to set it to the FAN position. When the switch is in the FAN mode, it enables the fans to circulate air through the system, ensuring that any potential buildup of heat is mitigated before the APU is started. This is crucial for maintaining the components within their operational temperature range and preventing overheating during APU initiation.

Furthermore, if the switch were set to AUTO, OFF, or START, the necessary airflow to cool the system effectively would not be provided, which could lead to overheating and possibly damage the system during start-up and operation. Setting the switch to FAN allows for effective ventilation, which is a vital step in ensuring the reliability and longevity of the APU operation.