CEN22LA318

While the pilot was conducting an aerial application flight, he felt a small jolt, the main rotor mast failed, and the helicopter collided with terrain. Four days before the accident, a mechanic performed a dye penetrant inspection on the rotor mast; the helicopter had only flown 9 hours since the inspection. Attempts to retrieve the rotor mast parts from the operator were unsuccessful and when contact was reestablished with the owner, he had already disposed of the parts. Photos of the broken rotor hub were reviewed by the National Transportation Safety Board (NTSB) Materials Laboratory, which determined that several preexisting, progressive cracks were present.

On July 6, 2022, about 1015 central daylight time, a Hiller UH-12E helicopter, N64272, was substantially damaged when it was involved in an accident near Becker, Minnesota. The pilot was seriously injured. The helicopter was operated under the provisions of Title 14 Code of Federal Regulations Part 137 as an aerial application flight. According to information collected by a Federal Aviation Administration (FAA) inspector, the pilot was applying product to a potato field when he felt a small jolt and the helicopter collided with terrain. The operator transported the helicopter back to the their facility about a week before they notified the NTSB or FAA of the accident, so neither agency was able to perform on-scene documentation. The operator reported that the main rotor mast had failed; the yellow blade was found about 500 ft west of the main wreckage and the blue blade was found about 300 ft east of the main wreckage. The mast, with a part of the transmission, was found about 150 ft northeast of the blue blade. An FAA review of maintenance records found that the main rotor hub (p/n 51437-23) was installed on the helicopter on December 14, 2018. Four days before the accident, a mechanic performed a 100-hour inspection, which included compliance with Airworthiness Directive (AD) 73-20-03. This AD requires dye penetrant inspection of the main rotor hub at 50-hour intervals to detect cracks. The helicopter had accumulated 9 flight hours since the inspection. Attempts to retrieve the parts from the operator were unsuccessful; when the NTSB reestablished contact with the owner, he had already disposed of the parts. The NTSB Materials Laboratory reviewed photos of the broken rotor hub. The photos showed evidence of several preexisting, progressive fatigue failures.