A 17 INCH PORTION OF ONE OF THE PROP BLADES FRACTURED AND SEPARATED FROM THE 76 INCH DIA PROP WHILE THE ACFT WAS AT CRUISE. DURING THE FORCED LANDING THE ACFT STRUCK A FENCE POST. EXAMINATION OF THE PROP REVEALED THE PRESENCE OF FILIFORM CORROSION ACROSS THE BACKS OF BOTH BLADES. METALLURGICAL ANALYSIS OF THE FRACTURE SURFACE REVEALED THAT THE FRACTURE ORIGINATED AT A CORROSION PIT ON THE BLADE BACK. TOTAL TIME OF THE PROP WAS 2225 HRS, THE PROP WAS RECONDITIONED BY THE MFR 1241 HRS PRIOR TO THE ACCIDENT. THE MFR RECOMMENDS THAT THIS MODEL PROP BE RECONDITIONED EVERY 1000 HRS. THE ORIGINAL RECONDITIONING PROCEDURE SPECIFIED ANODIZING THE PROP AND PAINTING WITH LAQUER AFTER REPAIR. EXAMINATION OF THE PROP INDICATES THAT THIS WAS DONE. THE MFR HAS SINCE REVISED THE RECONDITIONING PROCEDURE TO USE A CHEMICAL CONVERSION COATING INSTEAD. THE MFR SPECIFIES THAT THE PROP BE FREQUENTLY WIPED WITH AN OILY RAG BETWEEN RECONDITIONINGS TO ASSIST IN CORROSION PREVENTION.

The National Transportation Safety Board determines the probable cause(s) of this accident to be:

The National Transportation Safety Board determines the probable cause(s) of this accident to be:

Contributing Factors

PROPELLER SYSTEM/ACCESSORIES,BLADE..CORRODED

Contributing Factors

MAINTENANCE,OVERHAUL..INADEQUATE..MANUFACTURER

Contributing Factors

PROCEDURE INADEQUATE..MANUFACTURER

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