Rolling Fork, MS, USA
N93PH
BELL 206
The pilot was conducting an aerial application flight. He reported that he was almost finished spraying a river when he suddenly felt the helicopter yaw right then left. Subsequently, he raised the helicopter’s nose up and right to clear a line of trees that ran parallel to the river. When the helicopter reached the height of the treetops, the pilot realized that the airspeed was slow, so he held the collective down to start an autorotation. He then pulled full collective just as the skid impacted the ground hard. The helicopter then came to rest on its right side. Examination of the helicopter revealed that flight control continuity was established. The tailboom was found fracture separated, consistent with main rotor blade contact. Both main rotor blades exhibited impact damage and warping due to slight spanwise deflection opposite direction of rotation but with low inertia. The 1st-stage compressor inlet and blades exhibited damage consistent with foreign object ingestion. Foreign object damage was also noted in the compressor, and the case was pierced in several areas. Black, sludge-like material that contained aluminum debris was found throughout the gas path from the compressor to the combustor and turbine inlet, and it was mostly along the inner surface of the outer combustion case, consistent with splatter during N1 rotation. The ingestion of material throughout the engine and foreign object damage to the compressor is consistent with engine operation during the impact sequence. The reason for the uncommanded yaw could not be determined based on the available information because postaccident examination of the airframe, engine and fuel system revealed no evidence of any mechanical malfunctions or failures that would have precluded normal operation.
November 7, 2017, about 0930 central standard time, a Bell 206B helicopter, N93PH, was substantially damaged when it was involved in an accident near Rolling Fork, Mississippi. The pilot sustained minor injuries. The helicopter was operated as a 14 Code of Federal Regulations Part 137 aerial application flight. The pilot reported that he was almost finished spraying a river when he suddenly felt the helicopter yaw right then left. Subsequently, he raised the helicopter’s nose up and right to clear a line of trees that ran parallel to the river. When the helicopter reached the height of the treetops, he realized that its speed was slow, so he held the collective down to start an autorotation. He pulled full collective just as the skid impacted the ground hard. The helicopter then came to rest on its right side. The tailboom was found fracture separated, consistent with main rotor blade contact. Both main rotor blades exhibited impact damage and warping due to slight spanwise deflection opposite the direction of rotation with low inertia. One blade was found cracked chordwise in the center section. Several breaks were found throughout the drive train; however, flight continuity was established. The tail rotor hub and blades were still attached to the tail rotor gearbox. One blade was intact, but the other was broken chordwise at the doubler. The tail rotor hub rotated freely by hand with no binding. During engine examination, the transmission rotated freely by hand with no binding, and no abnormal sounds were noted. The chip detectors were clear of debris. The main transmission input shaft was fracture separated, and circumferential scraping was noted adjacent to the firewall opening. Metallurgical examination of the compressor revealed no evidence of fatigue on the compressor blade fracture surfaces. The 1st-stage compressor inlet and compressor blades exhibited damage consistent with foreign object ingestion, and the case halves were pierced in several areas. N1 and N2 rotated freely. The 4th- and 1st- stage turbine wheels exhibited no damage. Black sludge-like debris was found throughout the gas path from the compressor to the combustor and turbine inlet, and it was mostly along the inner surface of the outer combustion case, consistent with splatter during N1 rotation. The black sludge-like debris was an aluminum alloy not consistent with materials used in the manufacture of any of the engine components. Fuel was found in the fuel lines and engine-mounted fuel filters. No contaminants were found in the fuel control inlet strainer. The bleed valve was found in the “open” position; the inner surface of the valve plunger was coated with black sludge-like debris. The fuel control unit and power turbine governor were functionally tested, and both units tested slightly outside of specified limits per the manufacturer. Postaccident examination of the airframe, engine, and fuel system revealed no evidence of any preaccident mechanical malfunctions or failures that would have precluded normal operation.
An uncommanded yaw and subsequent loss of main rotor rpm for reasons that could not be determined.
Source: NTSB Aviation Accident Database
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