What happened
The FAA’s narrative, in sentence case. The FAA writes it in capitals and removes private details.
(-23) During part 141 training maneuver flight instructor had student shut down left hand engine and feather left hand propeller. During engine restart attempt the left hand aircraft propeller failed to unfeather, preventing an air start of engine. An in-flight engine start was attempted using the electric starter which also failed to restart engine. After numerous engine restart attempts flight instructor opted to land aircraft at Stockton airport with left hand engine off and feathered. Maintenance personnel later performed an inspection of the propeller feathering system and observed that the propeller accumulator head failed to hold a required 75 PSI air pressure. The measured accumulator head pressure was observed at 20 PSI. This observed low pressure was attributed to a leaking accumulator service port valve core.
As the FAA filed it
(-23) DURING PART 141 TRAINING MANEUVER FLIGHT INSTRUCTOR HAD STUDENT SHUT DOWN LEFT HAND ENGINE AND FEATHER LEFT HAND PROPELLER. DURING ENGINE RESTART ATTEMPT THE LEFT HAND AIRCRAFT PROPELLER FAILED TO UNFEATHER, PREVENTING AN AIR START OF ENGINE. AN IN-FLIGHT ENGINE START WAS ATTEMPTED USING THE ELECTRIC STARTER WHICH ALSO FAILED TO RESTART ENGINE. AFTER NUMEROUS ENGINE RESTART ATTEMPTS FLIGHT INSTRUCTOR OPTED TO LAND AIRCRAFT AT STOCKTON AIRPORT WITH LEFT HAND ENGINE OFF AND FEATHERED. MAINTENANCE PERSONNEL LATER PERFORMED AN INSPECTION OF THE PROPELLER FEATHERING SYSTEM AND OBSERVED THAT THE PROPELLER ACCUMULATOR HEAD FAILED TO HOLD A REQUIRED 75 PSI AIR PRESSURE. THE MEASURED ACCUMULATOR HEAD PRESSURE WAS OBSERVED AT 20 PSI. THIS OBSERVED LOW PRESSURE WAS ATTRIBUTED TO A LEAKING ACCUMULATOR SERVICE PORT VALVE CORE.