What are common APU abnormal conditions and their typical symptoms?

• A. High EGT with normal RPM.
• B. Low oil pressure or high oil temperature (lubrication problem).
• C. Overspeed (excess RPM).
• D. Abnormal vibration indicating mechanical fault.

Answer: (B)

Lubrication problems are a common APU abnormal condition, typically showing low oil pressure or high oil temperature. Low oil pressure means bearings and gears aren’t getting enough lubrication, which raises friction, speeds wear, and can trigger protective shutdowns. High oil temperature suggests the oil isn’t circulating effectively or is being overheated by excess friction, signaling potential pump or flow issues and risking component damage. These lubrication-related signs are the most frequent early indicators of APU trouble. In contrast, an overspeed points to a control or mechanical speed fault, abnormal vibration indicates a mechanical imbalance or mounting issue, and a high exhaust gas temperature with normal RPM usually points to combustion or fuel-control concerns rather than lubrication problems.