Shelton, WA, USA
N166DR
BELL/GARLICK UH 1H
While the helicopter was in a hover and beginning to lift a 3,000-lb agricultural application container, the commercial pilot heard a "loud bang," followed by the low rotor warning horn. Ground crew were below the helicopter, so he pulled away from them, and the helicopter then yawed left. He was able to keep the helicopter level about 5 to 7 ft above ground level, and he then pulled the remaining collective pitch to touch down. The helicopter landed on uneven ground and subsequently rolled over onto its right side. During postaccident wreckage examination, the engine-to-transmission driveshaft adapter, which is the component between the engine output and the driveshaft, was found broken in multiple pieces. Further examination revealed thumbnail-shaped patterns and progressive crack arrest marks, consistent with fatigue, on the fracture surfaces. The bolt that attached the adapter to the mating coupling set had separated. The fracture surfaces on the bolt were generally flat with progressive crack arrest lines consistent with fatigue cracking. Given the sizes of the fatigue cracking areas on the components, it is likely that the attachment bolt separated first. Vibrations in the assembly caused by the separation of the bolt likely resulted in the numerous fatigue cracks in the driveshaft adapter. Thirty-four years before the accident, the manufacturer issued a technical bulletin, which addressed an issue with inadequate torque being applied on the attachment bolt during assembly and stated that excessive wear had been noted on the engine-to-transmission driveshaft adapter plug when engines were returned for overhaul. The bulletin noted that the wear resulted in the mating driveshaft adapter bottoming out on the antirotation tangs of the driveshaft adapter plug and causing a false torque of the bolt and wear and reduction gear damage. The bulletin identified a fix that involved reworking the driveshaft adapter spline and applying 160 to 200 inch-lbs of torque on the bolt. However, no evidence was found indicating that the recommended actions had been accomplished. The wear on the driveshaft adapter and mating washer could have been the result of an undertorqued bolt or been related to vibrations in the assembly, which resulted from the fatigue cracking of the bolt.
On February 19, 2015, about 1300 Pacific standard time, a Bell/Garlick UH 1H Helicopter, N166DR, made an emergency landing and rolled over following a loss of engine power while in a hover near Shelton, Washington. Northwest Helicopters, Inc., was operating the helicopter under the provisions of 14 Code of Federal Regulations Part 137. The commercial pilot received minor injuries, the helicopter was substantially damaged. Visual meteorological conditions prevailed, and no flight plan had been filed. The pilot reported that while in a hover and beginning to lift a 3,000-pound agricultural application container, he heard a "loud bang" followed by the low rotor warning horn sounding. The pilot pulled away from ground crew as the helicopter yawed to the left. He was able to keep the helicopter level about 5 to 7 ft above the ground when he pulled the remaining collective pitch to touch down. The ground underneath was uneven and the helicopter subsequently rolled over onto its right side. During the postaccident wreckage examination, the engine to transmission driveshaft adapter (Part Number 204-040-812-3), was found broken in multiple pieces. The driveshaft adapter is the component between the engine output and the driveshaft. The failed components were recovered and sent to the National Transportation Safety Board Materials Laboratory Division for further examination. The Materials Engineer reported that thumbnail-shaped patterns and progressive crack arrest marks consistent with fatigue were observed on the fracture surfaces. The bolt that attached the driveshaft adapter to the mating coupling set had separated. The fracture surfaces on the bolt were generally flat, with progressive crack arrest lines consistent with fatigue cracking. Bell Helicopter Alert Service Bulletin No. 204-77-6 dated September 30, 1977, Inspection of Engine Drive Shaft Adapter Bolt, P/N 204-040-813-001 for loss of torque, and Technical Bulletin No. 204-81-18 dated June 10, 1981, Engine Driveshaft Adapter P/N 204-040-812-003 rework, indicated that considerable fretting wear damage had occurred to the engine output adapter plug, P/N 1-030-360-04 when engines were returned for overhaul. Extensive wear will allow the drive shaft adapter, P/N 204-040-812-003, to seat on the anti-rotation tangs of the plug and result in a false torque of bolt, P/N 204-040-813-001. The condition will cause plug wear and reduction gear damage. The fretting wear damage of the plug will be eliminated by maintaining the required torque of 160-200 inch-pounds on bolt, P/N 204-040-813-001. Both bulletins recommend compliance times to minimize fretting wear and damage. (See public docket for more information).
The fatigue failure of the engine-to-transmission driveshaft adapter attachment bolt due to the application of inadequate torque on the bolt during assembly.
Source: NTSB Aviation Accident Database
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