On July 3, 2020, a Sopwith 9400, N5539, was substantially damaged when it was involved in an accident near Van Alstyne, Texas. The pilot received minor injuries and the passenger was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 test flight.

The airplane was a reproduction of a Sopwith 1 ½ Strutter built from original factory drawings. According to the pilot, who was also the builder of the airplane, this was the 23rd test flight of the airplane. The flight was scheduled to be a short flight to confirm flying and landing wire deflections, flight control synchronization while under air loading, and to perform an aerial reconnoiter for potential obstacles at the approach end of the runway. A second crewmember was also on board, responsible for monitoring cable deflections and vibrations and control surface positions during flight and was communicating with the ground crew via text message.

The airplane performed normally through the first overflight of the airfield and a series of turns. However, when the pilot moved the control stick to the right to bring the wings level from a left turn, he noticed no resistance and the stick continued until it had reached its maximum travel. The pilot recognized this as a failure in the lower aileron circuit. Not knowing the exact location or cause of the failure, the pilot kept the control stick at its far right limit and waited for the effects of dihedral to slowly bring the wings back to level, while maintaining marginal control with back pressure on the elevator and right rudder. The airplane eventually settled wings level with a slight nose down attitude about 350 ft above ground level (agl). The pilot elected to perform an emergency landing in the cornfield in front of the airplane, during which the airplane impacted trees. The wooden-framed wing structures and fuselage were substantially damaged.

The pilot reported that a rocking tube that connected the front cockpit control stick to the rear control stick had moved rearward about 1 inch. The aileron cables connected to a horn affixed to this tube. The rearward movement of the rocking tube was sufficient for the front part of the tube to come loose from its socket, which would have affected the control authority of the ailerons. The pilot explained that there was nothing in the original design that would prevent the rocking tube from sliding aft.