Incident Overview
During initial climb, the pilot of Air Midwest Flight 4408 reported that the power (NP) on the left engine decayed to 60% and would not recover. The engine was feathered using the condition lever. While returning for landing, the torque rose to 140%. The aircraft landed safely.
Investigation Findings
A post-incident investigation revealed that an oil leak had been traced, the day prior to the incident, to a fractured propeller control unit solenoid. That part was removed and replaced as a precautionary measure.
The plastic blade retaining rings inside the propeller assembly were damaged due to the engine overtorque and subsequent shutdown. A teardown of the propeller control unit (PCU) showed that the beta tube had not failed nor was it freewheeling inside the PCU. Spectrographic analysis of oil samples drawn from four separate sections of the engine tested satisfactorily.
As such, no reason could be found for the drop in engine power (NP) and subsequent overtorque.