Accident Overview
On September 14, 2019, at approximately 1125 mountain daylight time, an experimental, amateur-built Sidewinder airplane, registered N492AC, was substantially damaged in an accident near Tucumcari, New Mexico. The commercial pilot was not injured. The flight was conducted as a personal flight under Title 14 Code of Federal Regulations Part 91.
Pilot Report
The pilot reported that while cruising at 11,500 feet mean sea level (msl), the engine momentarily began to run roughly. Shortly after, a loud bang was heard, followed by a portion of a propeller blade separating from the fixed-pitch propeller assembly. The pilot shut down the engine by moving the mixture handle to the idle cut-off position and turning off the magneto switch. Despite this, the engine continued to windmill during the descent and was described as "still very wobbly." The pilot performed a forced landing to open desert terrain, during which the airplane impacted vegetation, resulting in substantial damage to the left wing.
Examination Findings
Review of photographs taken after the accident showed that one propeller blade had separated approximately 12 inches from the hub. Examination of the propeller fracture surface revealed a small pit at the origin area. Further analysis using a scanning electron microscope showed fracture features consistent with progressive crack growth from a flat fracture surface at the identified origin. These features were indicative of a fatigue fracture originating from the pit. Energy dispersive spectroscopy of the origin area detected chlorine-containing salts, consistent with a corrosion-initiated pit.
Propeller Records
No propeller logbooks were located during the investigation. Consequently, it could not be determined when the propeller was last inspected or overhauled, nor its total time in service.