During which state must a qualified operator monitor ATM operations?

Monitoring ATM operations during start-up and operation is crucial because this phase involves critical systems coming online and functioning under various loads. The APU provides essential power and pneumatic support to start the main engines, making it necessary for a qualified operator to be vigilant about the operational parameters and potential anomalies during this time. Any irregularities could compromise aircraft systems or procedures, and having an operator monitor ensures prompt action can be taken if necessary.

In contrast, while fueling, during taxiing, and during maintenance checks, the APU's operation does not have the same immediate impact on the aircraft's ability to function safely and effectively. Although monitoring is still important in those scenarios, the start-up and operational phase uniquely demands continuous oversight due to the potential for immediate consequences related to engine starts and system checks.