Amateur-built PANZL S331E N331ST accident near Apple Valley, California

Probable cause

A fatigue failure of the left aileron control rod end during aerobatic flight, which resulted in the pilot's inability to maintain control of the airplane.

NTSB analysis

The private pilot was practicing her aerobatics routine. During the sixth figure of a nine-figure sequence, the pilot performed a snap roll to the right. Following the snap roll, the pilot performed a left roll as planned; however, rather than entering the next maneuver, the airplane continued to roll left. In-cockpit video indicated that, during the left roll, the right aileron exhibited a trailing-edge-down deflection consistent with a left turn. The pilot then moved her body toward the right side of the cockpit, consistent with a right control stick input. However, the airplane's rate of left roll increased, and the airplane continued to roll to the left as the pilot continued to lean toward the right. At this time, the right aileron indicated a trailing-edge-up deflection consistent with the pilot's right control stick input; the deflection of the left aileron could not be determined. The airplane continued to roll to the left until it impacted the ground.

Postaccident examination of the engine revealed no anomalies that would have precluded normal operation. Examination of the airframe revealed a break in the left aileron control assembly and one in the elevator control assembly. Both fracture surfaces of the left aileron control rod end exhibited relatively flat areas emanating from the inner diameter of the outer ring, which transitioned to angular planes adjacent to the outer diameter. One of fracture surfaces exhibited feathery features in the flat area consistent with fatigue cracking; the other fracture surface had smearing damage that obscured most of the finer features. The elevator control fracture exhibited features consistent with shear overstress.

The smearing damage indicated that the fatigue crack had been forming for some time. It is likely that the high stresses exerted on the airplane during aerobatic flight resulted in the final fracture of the left aileron control rod end due to fatigue.

Text from the NTSB. Open the NTSB docket for WPR17FA161 for the full record.