UH 1H N166DR accident near Shelton, Washington

Probable cause

The fatigue failure of the engine-to-transmission driveshaft adapter attachment bolt due to the application of inadequate torque on the bolt during assembly.

NTSB analysis

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.

Text from the NTSB. Open the NTSB docket for WPR15LA109 for the full record.