Smyth SIDEWINDER

Full Narrative

On September 14, 2019, about 1125 mountain daylight time, an experimental, amateur-built Sidewinder airplane, N492AC, was substantially damaged when it was involved in an accident near Tucumcari, New Mexico. The commercial pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight.

The pilot reported that, during cruise flight at 11,500 ft mean sea level (msl), the engine momentarily started to run rough. He then heard a loud bang, followed by a portion of propeller blade separating from the fixed-pitch propeller assembly. The pilot shut the engine down by moving the mixture handle to the idle cut-off position and turning off the magneto switch; the engine continued to windmill during the descent and was “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.

Review of photos of the airplane revealed that one of the propeller blades was separated about 12 inches from the hub.

Examination of the propeller fracture surface revealed that the fracture origin area exhibited a small pit. Further examination with a scanning electron microscope revealed fracture features consistent with progressive crack growth from a flat fracture surface in the area that was identified as the origin. The fracture features were consistent with a fatigue fracture originating from the pit. Energy dispersive spectroscopy of the origin area revealed chlorine-containing salts, which were consistent with a corrosion-initiated pit.

No propeller logbooks were located during the investigation. It could not be determined when the propeller was last inspected or overhauled, or its total time in service.