On June 13, 2019, about 0950 central daylight time, a Kolb Twinstar TSP-1 light sport airplane, N217JM, was substantially damaged when it was involved in an accident at Hector Municipal Airport (1D6), Hector, Minnesota. The pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 post maintenance test flight. After takeoff, the pilot climbed the airplane to 500 ft above ground level (agl) and had been in the air about 15 minutes when the airplane began to oscillate up and down. He had difficulty maintaining control, so he made a rapid descent toward the airport. During the rapid descent, the oscillation and shaking decreased, but as he reduced the descent rate, the airplane oscillated violently. The airplane landed hard on the grass runway. After the accident, the pilot stated that he thought the tail boom had fractured inflight and was the source of the oscillation. The responding Federal Aviation Administration (FAA) inspector reported that the pilot had recently completed maintenance on the airplane on June 2, 2019, and was conducting a maintenance test flight. The inspector reviewed a hand-drawn center of gravity (CG) diagram from the pilot. The CG envelope as calculated by the pilot was incorrect and the diagram contained several errors. The pilot told the FAA in an additional statement that he was adjusting his CG while airborne, and he placed a bag of bird seed in the passenger seat to assist during the inflight test. The bag was aft of the datum per the manufacturers specifications and would have moved the CG further aft. A postaccident examination of the airframe revealed that the tubular empennage structure had separated at the “H section.” The fractured section was retained for metallurgical examination to identify the mode of fracture. Both halves of the fracture surfaces on the aluminum tube were examined by the NTSB’s Materials Laboratory in Washington, DC. All fractographic features were consistent with cantilever-bending overstress-fracture due to the application of a monotonically increasing load until fracture. There were no marks on the fracture surface indicating recontact of the fracture surfaces. The inside surface of the aluminum tail boom tube, where it contacted the steel H-section via pop rivets, exhibited intergranular network cracks consistent with stress corrosion cracking. The examination determined that the location of the network cracks did not contribute to the overstress fracture of the tube section.