N59AT

On June 1, 2015, about 1905 central daylight time, a Sea & Sky INC Cygnet weight-shift-control aircraft, N59AT, was substantially damaged when it impacted the water near Panama City, Florida. The commercial pilot was seriously injured. The aircraft was registered to and operated by the pilot under the provisions of 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed and no flight plan had been filed for the personal flight, which departed Panama City Beach, Florida at 1905.According to the pilot, this was his first solo flight in his new aircraft after a sign off by his instructor. He stated that after takeoff, he climbed the to approximately 300 feet and leveled off. He began a turn to the right and noted an "unstable" feeling in the flight controls. He attempted to roll the aircraft to straight and level, it continued to the left and rolled into an uncommanded "steep" left banking turn. The pilot was unable to maintain control of the aircraft; subsequently it entered an aerodynamic stall and impacted the water.

According to the pilot's son he watched as the aircraft departed St. Andrews Bay. He said that as the aircraft began a slight right turn, it began to oscillate from left to right while descending. At about 50 feet above the bay the aircraft turned to the right in a 90° bank before "crashing" into bay. A review of a video recording revealed that the pilot was in stable flight prior to the accident. In a statement made to the NTSB; the pilot's son assisted with the postaccident recovery of the aircraft and noted the wing assembly was separated from its fuselage attachment point, and was being held on by cables before the aircraft was recovered.

A Federal Aviation Administration inspector examined the airplane after the accident. According to the inspector, the wing and fuselage (trike) were buckled, and the aluminum hang block attachment and three attachment bolts had fractured. The hang block assembly was forwarded to the NTSB Materials Laboratory for further examination.

The hang block assembly consisted of a strap and saddle. The saddle was attached to the strap by three flush-head bolts on each side of the strap. The strap for the saddle was fractured on both sides through the three saddle attachment holes on the right side and the forward saddle attachment hole on the left side. Bolts for attaching the right side of the saddle to the strap were sheared. The lower fracture surfaces through the strap at the left and right had an overall twisting deformation, and both fracture surfaces had a uniform rough matte gray appearance consistent with ductile overstress fracture. The saddle attachment bolts on the right side of the saddle were fractured. The fracture features and associated deformation and contact damage were consistent with shear fracture.

The United Kingdom's Air Accidents Investigations Branch (AAIB) commissioned a safety study of the tumble mode, a peculiarity of weight-shift-control aircraft. This safety study described the inherent spiral instability of the aircraft type. According to the report, "Many weightshift microlight aircraft are spirally unstable (particularly at higher power settings); thus, an initial small bank angle is likely to increase without (unless horizon reference is available) the pilot's ability to control it. The aircraft would roll, potentially past 90° of bank to a condition where the pendulum stability which keeps the trike below the wing ceases to act – inevitably causing some loss of control."