AN AC50 ON LNDG HAD THE R ENG FEATHER AND SHUT DOWN AT IDLE PWR. FOUND 4 SHEARED BOLTS ON PROP DUE TO IMPROPER O-RING SEAL INSTALLATION.

Narrative

DURING LNDG ROLLOUT; PLT RPTED R ENG FEATHERED AND SHUT DOWN WHEN PWR WAS BROUGHT TO IDLE. INITIAL INVESTIGATION APPEARED TO BE FAILED SEAL. FURTHER INVESTIGATION SHOWED 4 SHEARED BOLTS ON THE R PROP AND THE PROP O-RING INSTALLED ON THE CRANK SHAFT RATHER THAN ON THE LAND IN THE PROP HUB. IT APPEARS THE O-RING WORE AND/OR COMPRESSED AND THE BOLTS LOST THEIR TORQUE DUE TO A NON METAL-TO-METAL FIT. PROBABLE CAUSE: DURING THAT TIME WE WERE CONDUCTING PLT TRAINING AND HAD 1 FLYING ACFT. IN THE MAINT MANUAL (AIR FRAME; FOR 500'S (MODEL); AERO COMMANDERS LISTS 2 DIFFERENT PROP INSTALLATIONS VARYING BY SERIAL NUMBER THE AFFECTED ACFT FELL INTO EARLY MODEL WHEN THE MANUAL SAYS TO PUT THE O-RING ON THE CRANKSHAFT FLANGE AND IN A SEPARATE SECTION; A COUPLE PAGES LATER IN THE MANUAL IT SHOWS THE INSTALLATION FOR LATER SERIAL NUMBER 500'S AERO COMMANDERS. THE INVOLVED ACFT HAS PROPS FROM LATER MODEL ACFT INSTALLED ON IT. THE INSTALLATION WAS CORRECT FOR THE SERIAL NUMBER ACFT; HOWEVER; THE ACFT DID NOT HAVE THE ORIGINAL PROPS. THAT COMBINED WITH MOVING FROM ACFT TO ACFT OF DIFFERENT TYPES AND DOING PROP INSTALLATIONS ON BOTH; I DID NOT QUESTION THE INSTALLATION. WE REMOVED THE L PROP TO CHK ITS INSTALLATION AND FOUND IT TO BE INSTALLED THE SAME AS THE R (INCORRECTLY).