What was found
THE FLIGHT CREW REPORTED THAT AT APPROXIMATELY 800-1000' AGL, THE LT PROPELLER INADVERTENTLY FEATHERED. THE FLIGHT CREW RAN THE "INADVERTENT PROPELLER FEATHER" QRH AND WAS ABLE TO UNFEATHER THE PROPELLER, PERFORMED A PRECAUTIONARY AIR RETURN WITH BOTH ENGINES OPERATING. UPON LANDING, THE LT EEC ALSO FAILED WHEN SELECTING REVERSE. MAINTENANCE FOUND THE LEFT ENGINE HYDRO MECHANICAL UNIT (HMU) BEHAVING ERRATICALLY CAUSING THE LEFT ENGINE TORQUE TO FLUCTUATE. THE FLUCTUATING TOQUE WAS THE CAUSE OF THE LEFT PROP FEATHERING AS THE AUTO FEATHER SYSTEM SENSED LOW TORQUE ON THE LEFT ENGINE. REPLACED LEFT ENGINE HMU WITH A SERVICEABLE UNIT. GROUND RUNS PERFORMED AND FOUND SATISFACTORY. AN OPERATIONAL CHECK FLIGHT OF THE AIRCRAFT WAS PERFORMED AND WAS SUCCESSFUL.