WPR18LA206

During a cross-country flight, the helicopter started to produce an unusual noise. The pilot initiated a precautionary landing and the noise became louder. About 5 ft off the ground before touchdown, the helicopter entered an uncontrolled climb and yaw. The pilot was unable to recover from the loss of directional control and the helicopter subsequently impacted terrain. Examination of fractured pieces from the main rotor drive shaft and the swashplate assembly exhibited features consistent with fracture from overstress; however, it is likely that this damage was sustained during the impact. Although it is possible that one of the components may have failed from overstress in-flight, this would have required an abnormally high load input to one or more components. The exam did not reveal any evidence of mechanical malfunctions or anomalies that would have precluded normal operation of the helicopter, and the source of the noise reported by the pilot could not be determined.

On July 24, 2018, about 1800 Pacific daylight time, a Bell UH-1H helicopter, N107CH, was substantially damaged during a precautionary landing near Oakdale, California. The pilot was not injured; the passenger sustained minor injuries. The helicopter was registered to Airlift Helicopters Service and operated by the pilot under the provisions of Title 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed for the cross-country flight, and no flight plan was filed. The flight originated from Madera Municipal Airport (MAE), Madera, California at 1600 and was destined for Lincoln Regional Airport/Karl Harder Field (LHM), Lincoln, California. In a post-accident telephone conversation with the National Transportation Safety Board (NTSB) Investigator in Charge, the pilot stated that he was in a straight and level flight when he heard a "strange" noise and he decided to execute a precautionary landing. He described the noise sounding like "a tarp in the back of a pickup". There were no other indications in the helicopter; all instruments were within normal operating limits, there was no vibration and the engine was producing power. About 5 ft above the ground, the helicopter entered an uncontrolled climb and yaw. The pilot was unable to recover from the loss of directional control and the helicopter impacted terrain. In the NTSB Form 6120.1 "Pilot/Operator Aircraft Accident/Incident Report", the pilot added that, during the descent, the noise "grew louder". The helicopter touched down in the upright position. In the report, the pilot did not mention uncontrolled climb and/or yaw. In a telephone call to the Federal Aviation Administration (FAA) Inspector, the passenger in the helicopter reported that, while in a level cruise flight, he heard a noise. The pilot checked the instrument panel and confirmed that all instruments were within normal operating limits. The passenger noted no vibrations or changes other than the noise. He did not disclose how long the noise was present nor what the noise sounded like. They made a normal approach with a controlled descend to land and that was the last thing he remembered surrounding the accident sequence. The NTSB Materials Laboratory examined fractured pieces from the main drive shaft and damaged remnants of the swashplate assembly. The main drive shaft and the swashplate support fracture surfaces exhibited features consistent with fracture from overstress. The overstress orientation varied from part to part, but was observed in tension, bending, shear, compression, and combinations. Postacccident exam of the helicopter did not reveal any pre-exciting malfunction that would have precluded normal operations.