Accident Overview
On May 31, 2020, about 1233 Alaska daylight time, an experimental amateur-built STOL CH-701, registered N324WS, was substantially damaged during a takeoff accident at the Kobuk Sand Dunes near Ambler, Alaska. The sport pilot and passenger were not injured. The flight was conducted as a personal flight under Title 14 Code of Federal Regulations Part 91.
Pilot Account
The pilot, who was also the owner and builder of the airplane, stated that he landed on sand dunes and shut down for 45 minutes. He then initiated a takeoff. The airplane encountered "some bumps, but nothing significant" until just before reaching rotation speed of about 45 mph, when a larger bump propelled the aircraft upward. The airplane impacted terrain in a nose-high attitude, causing the nose landing gear to collapse. The aircraft subsequently nosed over and came to rest inverted.
Post-Accident Examination
Examination of the airplane revealed substantial damage to the rudder and both wing struts. The nosewheel fork, part number 7L2-4J made of aluminum, had separated from the nose strut in three fragments at two fracture locations, one on each side of the strut attachment near the four attachment bolt holes. The outer surfaces adjacent to the fractures showed outward lip-like deformation, while the inside concave areas exhibited secondary cracking perpendicular to the fractures. One fracture surface displayed a dimpled rupture with dull and rough features. All fracture features were consistent with an overstress fracture from upward bending of the nosewheel fork. No evidence of preexisting cracks, wear marks, or corrosion was found. Chemical, electrical, and hardness tests on the middle fork fragment confirmed the material was AA 6061 aluminum alloy in a T6 temper.
Pilot's Remarks
The pilot reported that the accident might have been prevented if he had conducted a more detailed inspection of the takeoff area and a preflight inspection of the nosewheel fork.