On June 24, 2020, about 1145 Pacific daylight time, a Thatcher CX4, N274DR, was substantially damaged when it was involved in an accident at Cable Airport (CCB), Upland, California. The pilot was seriously injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight.

The pilot reported that he was returning to CCB following an uneventful local flight when the propeller separated from the engine. He was about 1 mile from the airport at an altitude of about 1,100 ft above ground level (agl) at the time. The pilot said that oil covered the windscreen and canopy, restricting forward visibility; however, he continued to fly toward the airport. When the airplane crossed the threshold of the runway, it was about 50 ft agl, and “going way too fast,” as he was trying to look out to the side to keep the airplane aligned with the runway. The airplane touched down near the end of the runway and bounced. The airplane continued over a fence and collided with a dirt berm.

Postaccident examination of the airplane by the pilot revealed that both wings and fuselage were substantially damaged. The propeller assembly and a portion of the engine crankshaft were separated and not located.

The remaining portion of the crankshaft from the Hummel VW 2400CC engine was sent to the National Transportation Safety Board Materials Laboratory for further examination. Examination of the crankshaft revealed that two fractures initiated at the termination of a square keyway slot in the crankshaft. The presence of ratchet marks at the fracture origins and the presence of progressions marks along the fracture path were indicative of fatigue fracture initiation and growth.