Papillon Helicopters Sikorsky/Orlando S-55 accident

**This report was modified on 2/23/2016. Please refer to the public docket for this accident for more information.**

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 511 SW. 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. When the next-to-last and last engine oil samples taken prior to the accident were compared, iron increased 412.5 percent, and magnesium increased 571.6 percent. Just before the accident flight, the engine chip detector warning light illuminated and maintenance personnel removed, cleaned, and reinstalled the chip light sensor.

Verbatim NTSB analysis and probable cause from the NTSB dataset