Amateur-built Experimental Cub N9WU accident near Stanley, Idaho

Probable cause

Overstress failure of the right-wing spar due to an encounter with turbulence.

NTSB analysis

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.

Text from the NTSB. Open the NTSB docket for WPR19LA170 for the full record.