What was found
DURING NORMAL GROUND RUN-UP. THE AIRCRAFT PROPELLER AUTO-FEATHER SYSTEM WAS OPERATIONALLY CHECKED. BOTH LT AND TR PROPELLER SWITCHES WERE SELECTED AND BOTH PROPELLERS FEATHERED CORRECTLY. WHEN THE SWITCHES WERE RELEASED THE LT PROPELLER DID NOT COME OUT OF THE FEATHERED POSITION BUT THE RT PROPELLER WORKED CORRECTLY. A SERVICEABLE OVERSPEED GOVERNOR WAS INSTALLED IN THE LT POSITION. OPERATIONAL CHECK WAS GOOD AND THE AIRCRAFT WAS RETURNED TO SERVICE. UNKNOWN WHAT CAUSED THE COMPONENT TO FAIL POSSIBLE INTERNAL SWITCH OR SERVO FAILURE, COMPONENT SENT FOR OVERHAUL/ REPAIR.