S-55 used for cherry drying in WA  

The helicopter was being operated as a sightseeing flight to the Grand Canyon. The pilot reported that during cruise flight the rotor and engine noise suddenly decreased and the helicopter began a descent. During the ensuing forced landing, the helicopter landed hard. The helicopter was fabricated from an ex-military remanufactured and modified Sikorsky S-55 (UH-19D). The original radial reciprocating engine had been replaced with a Garrett Gas Turbine TPE 331 modified to a TSE 331 IOUA 511SW. It had a 6,000-hour time between overhaul. According to maintenance records dating back to July 3, 1985, the accident engine P-54083 operated in more than one Fairchild SA226 airplane in Australia, and several rotary wing S-55 aircraft during their certification in the United States. Following the accident the engine was shipped to the modifier for functional testing in a cell. The engine started and idled normally. Increasing the rpm to the certified operating specification of 101.5 percent revealed no abnormal conditions. The test propeller was moved from the start locks and increasing torque load was applied. As the torque was increased the engine responded normally until approximately 60 percent torque when the indicated engine rpm rapidly decreased and an emergency shutdown was initiated. The process was repeated through 12 cycles, during which the indicated rpm began a sudden rapid decrease when torque was increased. Test cell troubleshooting failed to identify or correct this condition. During manual rotation of the propeller following the final cycle some "slipping" was noted between the output shaft and the main rotating group. Subsequent examination of the gearbox revealed disengagement of the sun and bull gear assembly. Examination revealed fretting damage to the retaining nut and mating surfaces related to loss of specified torque. Visual examination of the sun and bull gears revealed severe damage to the mating splines of both gears. About 9 months before the accident the original manufacturer issued a service bulletin providing for the replacement of the affected components; the engine modifier recommended replacement of the components prior to further operation. Examination of past oil analysis samplings revealed increases of iron and magnesium over a period of time, with analysis report comments of "Alert" and "Iron Appears High."

Probable Cause: The failure of maintenance personnel to accomplish a service bulletin addressing the potential failure of engine drive gears, and the ignored oil analysis testing results that indicated an impending internal engine failure that subsequently resulted in the loss of engine drive to the main rotor during flight.