What happened
The pilot in command of Air Midwest Flight 4408 reported that during the initial climb phase, the power output (Np) on the left engine suddenly decayed to 60 percent and would not recover. Following this loss of power, the crew feathered the propeller using the condition lever. However, as the aircraft turned back toward the airport for an emergency landing, the torque reading rose to 140 percent according to the pilot's account. Despite these mechanical anomalies, the aircraft landed safely without further incident.
The investigation
A post-incident examination of the aircraft revealed that an oil leak had been identified and traced to a fractured propeller control unit solenoid just one day prior to the event. This component was removed and replaced as a precautionary measure before the flight. Post-flight inspection showed that the plastic blade retaining rings inside the propeller assembly were damaged, likely due to the engine overtorque and subsequent shutdown.
A teardown of the propeller control unit indicated that the beta tube had not failed nor was it freewheeling within the unit. Additionally, spectrographic analysis of oil samples drawn from four separate sections of the engine tested satisfactorily. Consequently, investigators could find no definitive reason for the drop in engine power and the subsequent overtorque.