ANC20LA053

The pilot, who was also the owner and builder of the experimental amateur-built airplane, reported that he landed the airplane on sand dunes. Forty-five minutes later, he took off from the sand dunes, and during the takeoff roll, the airplane hit some bumps. Just before the airplane reached rotation speed, it hit a larger bump, which propelled it upward. The airplane then descended in a nose-high attitude and impacted terrain, followed immediately by the nose landing gear (NLG) collapsing. Subsequently, the airplane nosed over and came to rest inverted. Postaccident examination of the airplane revealed that the rudder and both wing struts sustained substantial damage. The nosewheel fork had separated from the nose strut in three fragments at two fracture locations, one on each side of the nose strut attachment. Examination of the fracture surfaces revealed that all the fractures exhibited features consistent with overstress fracture in upward bending of the nosewheel fork. No evidence of preexisting cracks, wear marks, or corrosion was found. The pilot reported that the accident may have been prevented if he had conducted a more “detailed” inspection of the takeoff area. Given the evidence, it is likely that, during the takeoff roll, the NLG impacted uneven terrain, which led to the airplane becoming airborne in a nose-up attitude and then impacting terrain with force that exceeded the nosewheel fork’s structural capabilities and resulted in the nosewheel fracturing.

On May 31, 2020, about 1233 Alaska daylight time, an experimental amateur-built STOL CH-701 airplane, N324WS, was substantially damaged when it was involved in an accident at the Kobuk Sand Dunes near Ambler, Alaska. The sport pilot and passenger were not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot, who was also the owner and builder of the experimental amateur-built airplane, stated that he landed the airplane on sand dunes and then shut down for 45 minutes. He then took off, and the airplane hit “some bumps, but nothing significant,” until immediately before it reached the rotation speed of about 45 mph, at which point, the airplane hit a larger bump, which propelled it upward. The airplane then impacted terrain in a “nose high” attitude, followed immediately by the nose landing gear collapsing. Subsequently, the airplane nosed over and then came to rest inverted. The pilot reported that the accident might have been prevented if he had conducted a more “detailed inspection” of the takeoff area and “preflight inspection” of the nosewheel fork. Examination of the airplane revealed that the rudder and both wing struts sustained substantial damage. The part number 7L2-4J aluminum nosewheel fork had separated from the nose strut in three fragments at two fracture locations, one on each side of the strut attachment, at or near the four attachment bolt holes. The detailed examination revealed that the outer surfaces adjacent to the fractures exhibited outward lip-like deformation and the areas near the fractures on the inside (concave) of the fork exhibited secondary cracking perpendicular to the fractures. One of the fracture’s surface exhibited a dimpled rupture and dull and rough features. All the fracture features were consistent with overstress fracture in upward bending of the nosewheel fork. No evidence of preexisting cracks, wear marks, or corrosion was found. The chemical, electrical, and hardness characteristics of the middle fork fragment were consistent with an AA 6061 aluminum alloy in a T6 temper.