When must the APU ALARM switch be turned to ON?

The APU ALARM switch should be turned to ON prior to APU start to ensure that any potential abnormalities or malfunctions during the startup phase can be adequately monitored. This step is crucial as it activates the audible and visual alarm systems, which will alert personnel to any issues that may arise during the APU's operation. By engaging the alarm system beforehand, you are taking proactive measures to maintain safety and ensure that any alerts are readily available during an important operational phase, thereby promoting effective response and troubleshooting should any issues arise.

Other situations, like during APU operation or immediately after shutdown, do not address the necessary preparation phase that occurs before the APU is started, and activating the alarm system only during maintenance doesn't have the same urgency or preventive nature as doing so before the APU becomes operational.