What could happen if the APU experiences a fault while operating under lean fuel conditions?

• A. The RPM would increase beyond expected without fault.
• B. A fault could cause flameout or abnormal EGT and RPM.
• C. The system would automatically re-stabilize without effects.
• D. The APU would overcool the oil.

Answer: (C)

Lean fuel operation keeps the APU's fuel control loop close to its stability margins, with RPM and EGT tightly monitored by the system. When a fault occurs in this mode, the built‑in control logic and protection circuits continuously compare actual values to the commanded setpoints. If the fault is detected, the system can automatically adjust fuel flow and other parameters to bring RPM and EGT back to the target operating point. That automatic re‑stabilization means the APU returns to stable operation without requiring manual intervention, and there aren’t external effects seen at the surface.

While a fault can sometimes lead to flameout or abnormal EGT and RPM in other scenarios, the described behavior focuses on the APU’s automatic stabilization capability under lean fuel conditions, which is why this answer best fits. Overcooling the oil isn’t a typical immediate consequence of a lean-fault scenario.