THE HELICOPTER WAS COMPLETING AN AERIAL OBSERVATION FLIGHT (FISH SPOTTING) OVER THE PACIFIC OCEAN NEAR THE ISLAND OF CANTON AND WAS LANDING ON THE SHIP'S HELIPAD. WHEN THE SKIDS TOUCHED DOWN, THE ROTOR MAST ASSEMBLY SEPARATED FROM THE ROTOR MAST BASE CAUSING THE HELICOPTER TO TURN TO THE LEFT 90 DEGREES. THE ROTOR MAST ASSEMBLY AND ALL ITS COMPONENTS FELL INTO THE OCEAN AND WERE NOT RECOVERED. THE HELICOPTER SUSTAINED SUBSTANTIAL DAMAGE. THE PILOT REPORTED THAT THE HELICOPTER DID NOT EXPERIENCE ANY ABNORMAL CYCLIC MOVEMENTS OR ANY UNUSUAL VIBRATIONS BEFORE THE OCCURRENCE. THE HELICOPTER HAD ACCRUED 328.5 HOURS SINCE THE LAST ANNUAL INSPECTION. A 300-HOUR AND A 100-HOUR INSPECTION HAD RECENTLY BEEN COMPLETED. METALLURGICAL EXAMINATION OF THE ROTOR MAST BASE FRACTURE SURFACES REVEALED OVERLOAD CHARACTERISTICS WITH SOME POST FRACTURE MECHANICAL SMEARING AND DAMAGE. NO EVIDENCE OF ANY CORROSION OR FATIGUE WAS FOUND. THE HARDNESS MEASUREMENTS COMPLIED WITH THE MATERIAL SPECIFICATIONS.