Why is regular monitoring of RPM and temperature necessary during APU startup?

Regular monitoring of RPM and temperature during APU startup is crucial because it allows for the early identification of potential stability issues. As the APU starts, any irregularities in RPM or temperature can indicate problems such as improper fuel flow, ignition system malfunctions, or mechanical issues within the APU itself. By observing these parameters closely, operators can take prompt corrective actions to prevent further complications or failure of the APU, ensuring safe and reliable operation.

In contrast, other factors mentioned, while important for various operational scenarios, do not directly relate to the critical nature of monitoring RPM and temperature specific to startup conditions. Reducing noise can be a concern in certain operational environments, but it is not a primary reason for monitoring RPM and temperature. Likewise, ensuring a clean fuel filter is essential for overall performance but primarily relates to consistent operation rather than the immediate startup phase. Finally, while a quicker engine startup may be desired, it is not the primary purpose of RPM and temperature monitoring, which is focused on stability and safety during the initial phase of operation.