What are common signs of impending APU failure?

The presence of unusual vibrations and fluctuating RPM is a strong indicator of impending APU failure. In an Auxiliary Power Unit, a steady performance is essential for its reliability, and the APU's ability to maintain stable RPM is crucial for generating the necessary power and pneumatic pressure. When vibrations are unusual, it suggests that there may be internal malfunctioning or issues with components such as bearings or the drivetrain. Fluctuating RPM can indicate problems with fuel supply, air intake, or mechanical failure, all of which can compromise the APU's performance and lead to a complete failure if not addressed promptly.

Monitoring these indicators allows for early detection of potential issues, leading to timely maintenance or replacement before a total failure occurs, which could result in operational delays or safety hazards.