WPR19LA170

While overflying mountainous terrain, the airplane encountered turbulence. The pilot heard a “pop” from the right wing and shortly thereafter, the wing began to shake violently. The pilot was maneuvering the airplane to a forced landing site when the airplane encountered turbulence again and subsequently entered a spin that continued until impact with terrain. The accident site was adjacent to a mountain range where strong thermals likely existed as well as an unstable low-level pressure system. This is likely the cause of the localized turbulence that was encountered by the flight. Examination of the wing spar revealed in-plane fractures consistent with tensile overstress failure. The fracture sequence started with the fracture of the wing spar, which led to subsequent spar fractures, and eventual buckling and fracture of the connected supporting members and struts. It is likely that the turbulence encounter resulted in excessive loads on the wing and subsequent overstress failure.

On June 13, 2019, about 1320 mountain daylight time, an experimental, amateur-built Experimental Cub airplane, N9WU, was substantially damaged when it was involved in an accident near Stanley, Idaho. The pilot and passenger were seriously injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot reported that, while in the vicinity of the Sawtooth Mountain Range, the airplane encountered turbulence. As the flight continued, they encountered severe turbulence and he heard a “pop” sound. A few seconds later, the right wing began to shake "violently." He slowed the airplane and began to look for a place to land. He placed the airplane in a slight turn and the airplane encountered turbulence again. The airplane subsequently entered a spin and impacted the ground. The accident site elevation was 7,460 ft msl and adjacent to a mountain range. There were no pilot reports of turbulence in the area. Review of the surface analysis chart for 1200 on the day of the accident showed a low-pressure system over Idaho just south of the accident site, which was associated with a thermal low and a trough of low pressure that extended southward. Southeast of the accident site was a high-pressure system located over western Wyoming with a ridge of high pressure that extended westward into southeastern Idaho. The closest weather reporting station was the Stanley Ranger Station, Stanley, Idaho, about 2 miles northwest of the accident site at an elevation of 6,496 ft mean sea level (msl) and a calculated density altitude of 8,844 ft at the time of the accident. The station did not report precipitation, visibility, visibility restrictions, or sky conditions. At 1348, the wind was variable at 5 knots, temperature 23°C, dew point 6°C, altimeter 30.10 inches of mercury. A High-Resolution Rapid Refresh model sounding for the area over the accident site indicated a high probability of strong thermal activity from the surface through 7,000 ft agl. Portions of the right-wing strut and front and rear spar were sent to the National Transportation Safety Board Materials Laboratory for examination. The forward wing spar displayed an in-plane fracture surface, consistent with a tensile overstress. The other fracture surfaces were identified as overstress and were consistent with out-of-plane plastic deformation. These fracture surfaces occur after the initial fracture or failure event and are often associated with ground impact.