CHI02LA285

The helicopter was substantially damaged during a forced landing following a complete loss of engine power during cruise flight at night. The pilot stated that during the flight, the engine failed, the emergency annunciator lights illuminated, and he felt a yaw due to the loss of full torque. He executed an autorotation to a bean field. The pilot stated that it was very dark and that depth perception was difficult and he flared too soon and "ran out of lift at about 10-12 feet above the ground." The helicopter subsequently impacted the ground. The main rotor blades sheared off the front right upper portion of the cabin and the tailboom. Examination of the engine at the accident site revealed that the N1 rotor system would not rotate. Subsequent examination revealed that the first stage turbine wheel had all of the blades broken off 0.2 to 0.4 inch above the rim at the leading edge and 0.1 to 0.15 inch above the rim at the trailing edge. Scanning electron microscope examination of the fractures revealed no fatigue progression. The fracture surface was oxidized and the macroscopic features were interdendritic, indicating that separation occurred in overload. A metallographic section of one of the blades was made. This section showed a loss of metal as a result of hot corrosion. Distinct spherical particles were observed ahead of the corrosion front, consistent with sulfadation. The chemistry of the spherical particles was found to be rich in sulfur. Examination of the second stage turbine wheel also showed features consistent with hot corrosion damage. Further examination of the submitted items showed that the microstructure, hardness and chemistry conformed to the engineering drawing requirements.

On September 16, 2002, at 2135 central daylight time, a Hughes 369D, N500EK, piloted by a commercial pilot, sustained substantial damage during a forced autorotation into a bean field, near O'Neill, Nebraska, after an engine failure. The 14 CFR Part 91 personal flight was operating in night visual meteorological conditions without a flight plan. The pilot was not injured and his passenger received minor injuries. The flight originated from the Fort Dodge Regional Airport (FOD), Fort Dodge, Iowa, about 1900 and was en route to the Ainsworth Municipal Airport, Ainsworth, Nebraska.

In a written report, the pilot stated that he departed from his home in Harvard, Illinois, at 1530. The pilot stated that he flew 2.7 hours to FOD where the helicopter was refueled. He stated that they departed from FOD about 1900 and landed in Creighton, Nebraska, for a rest stop. He stated that during this rest stop, the engine was not shut down. The pilot said that after continuing the flight, the engine failed about 5 nautical miles west of O'Neill. He stated that the emergency annunciator lights illuminated, he felt the helicopter yaw due to the loss of full torque and he executed an autorotation to a bean field. The pilot stated that it was very dark and that depth perception was difficult and he flared too soon and "ran out of lift at about 10-12 feet above the ground." The helicopter subsequently impacted the ground. The main rotor blades sheared off the front right upper portion of the cabin and the tailboom.

The helicopter was a Hughes model 369D powered by a 420 shaft horsepower Allison model 250-C20B engine. The helicopter had accumulated 2,323.0 hours total time is service, and the engine had accumulated 2,320.0 hours total time in service at the time of the accident. The helicopter and engine had undergone an annual inspection on September 9, 2002. The airframe time was 2,310.5 hours at the time of the inspection.

Examination of the engine at the accident site revealed that the N1 rotor system would not rotate. The helicopter engine was shipped to the manufacturers facility where it underwent a teardown examination under the supervision of a National Transportation Safety Board Investigator. The examination revealed that the first stage gas producer (GP-1) wheel, had evidence of heat distress with approximately 90% of the blade material missing. The second stage gas producer (GP-2) wheel showed heat distress with approximately 10-20% of the blade material missing. The GP-1 wheel, GP-2 wheel, GP-1 nozzle, GP-2 nozzle, and GP-3 nozzle assemblies were submitted to the Rolls-Royce failure analysis department for further investigations.

The NTSB Materials Laboratory reviewed the metallurgical examination performed by Rolls Royce. The examination showed that the first stage turbine wheel had all of the blades broken off 0.2 to 0.4 inch above the rim at the leading edge and 0.1 to 0.15 inch above the rim at the trailing edge. The separated blades were not recovered. Scanning electron microscope examination of the fractures revealed no fatigue progression. The fracture surface was oxidized and the macroscopic features were interdendritic, indicating that separation occurred in overload. A metallographic section of one of the blades was made. This section showed a loss of metal as a result of hot corrosion. Distinct spherical particles were observed ahead of the corrosion front, consistent with sulfadation. The chemistry of the spherical particles was found to be rich in sulfur. Examination of the second stage turbine wheel also showed features consistent with hot corrosion damage. Further examination of the submitted items showed that the microstructure, hardness and chemistry conformed to the engineering drawing requirements.