Clapp Saberwing

Full Narrative

On June 10, 2024, about 2000 eastern daylight time, an experimental amateur-built Clapp Saberwing airplane, N120SW, was substantially damaged when it was involved in an accident near Valdosta, Georgia. The pilot sustained serious injuries. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 test flight.
The pilot reported that he departed Valdosta Regional Airport (VLD), Valdosta, Georgia, on a local flight to perform system checks of the airplane. During a functional check of the ignition system, while selecting between points and electronic ignition at a high engine power setting, the engine lost all power about 8 miles southwest of VLD. After multiple unsuccessful attempts to restart the engine, the pilot declared an emergency and executed a forced landing to a field.
During the landing, the airplane impacted a utility line and came to rest inverted in a field about 2 miles from the airport. The right wing, fuselage, and empennage sustained substantial damage.
Postaccident examination of the Corvair automotive conversion engine by an FAA inspector and the pilot (who also built the engine) revealed that the timing marks on the crankshaft gear and hub assembly were not aligned with the crankshaft position. Further examination revealed that the woodruff key that indexed the crankshaft gear and hub assembly to the crankshaft was sheared and the assembly was rotated about 30° from its indexed position on the crankshaft. According to the pilot, when switching from points to electronic there was a small interval for the electronics to power up, resulting in a jolt across the engine when power was restored.
The pilot reported that the accident engine was assembled as a test engine in 2007 and accrued about 20 flight hours. Since 2007, the interference fit between the crankshaft and the crankshaft gear and hub assembly increased from 0.002 inches to .004 inches, and hardened woodruff keys replaced standard woodruff keys to improve durability.