On November 15, 2021, about 1130 mountain standard time, an experimental amateur-built Sonex Waiex-B airplane, N535D, was substantially damaged when it was involved in an accident near Laveen Village, Arizona. The pilot was not injured. The airplane was operated by the pilot as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot reported that he had been flying for about 48 minutes when the engine began to run rough. He said the airplane did not have enough power to maintain altitude and he initiated a forced landing to an airstrip. The pilot realized he was not going to make the airstrip and landed on desert terrain, impacting rocks and a barbed wire fence which caused the airplane to nose over which resulted in substantial damage to the wings, stabilizers, and ruddervators. The pilot was able to exit the cabin area through the broken canopy. Examination of the airplane revealed that the No. 4 cylinder exhaust valve rocker arm swivel housing and swivel pad separated from its screw body. The screw body remained secured to the rocker arm. The swivel pad and swivel housing were found loose in the rocker cover. The swivel pad remained loose in the swivel housing and impact damage was visible on the swivel housing fracture surface. The rocker assembly for No. 3 and 4 cylinders and the separated swivel housing and swivel pad (see figure) were sent to the National Transportation Safety Board Materials Laboratory for forensic examination. The examination revealed a damaged fracture surface of the swivel housing and the screw body. The spherical outboard side of the swivel pad had a rough surface consistent with scoring and galling damage. The damage pattern had circular patterns on two sides with linear features in between, consistent with rotating contact within the swivel housing. The swivel housing had scoring and galling that mirrored the damage patterns observed on the swivel pad with a circular pattern on two sides and linear scratches between. The interior of the swivel housing had worn away, producing a space for the swivel pad to be recessed within the housing. This worn area was located near the facture surface of the swivel housing and screw body.

Figure - Overall view of the rocker assembly showing the inboard side of the rocker arms including the fractured No. 4 exhaust valve adjuster. Review of the airplane’s maintenance records revealed that the engine was assembled on May 9, 2018. The last inspection was completed on October 12, 2021, with about 110 hours of engine operation since the engine had been assembled. At this time, the cylinders were torqued, and the valves were adjusted. The airplane flew another 25 hours before the accident.