What is a significant indicator of APU performance issues?

Irregular fluctuations in RPM or power output are indeed a significant indicator of APU performance issues. The Auxiliary Power Unit relies on stable operational parameters to function effectively, and any inconsistencies in RPM or power output can suggest underlying problems, such as mechanical wear, fuel delivery issues, or electrical system malfunctions. These fluctuations often indicate that the APU is not operating within its designed performance envelope, which may lead to unreliable power supply and could compromise the overall functionality of the aircraft systems that rely on it. Identifying and addressing these irregularities promptly is essential to maintain operational efficiency and prevent more serious failures.

Consistent fuel consumption, stable temperature readings, and visual clarity of exhaust typically suggest that the APU is functioning normally. While consistent fuel consumption can be a positive sign, it does not provide direct evidence of performance issues, and stable temperature readings would indicate that the APU is operating within expected limits. Similarly, visual clarity of exhaust does not necessarily correlate directly to performance issues, as exhaust quality may vary without signaling a fundamental problem in operation.