Accident Overview
On June 13, 2019, about 0950 central daylight time, a Kolb Twinstar TSP-1 light sport airplane, registration N217JM, was substantially damaged during an accident at Hector Municipal Airport (1D6), Hector, Minnesota. The pilot, who was the sole occupant, was not injured. The flight was conducted as a Title 14 Code of Federal Regulations Part 91 post-maintenance test flight.
Flight Details
After takeoff, the pilot climbed the airplane to approximately 500 ft above ground level. About 15 minutes into the flight, the airplane began to oscillate up and down. The pilot experienced difficulty maintaining control and initiated a rapid descent toward the airport. During the rapid descent, the oscillation and shaking decreased; however, when the pilot reduced the descent rate, the oscillation became violent. The airplane subsequently landed hard on the grass runway. Following the accident, the pilot stated that he believed the tail boom had fractured in flight and was the source of the oscillation.
The responding Federal Aviation Administration (FAA) inspector noted 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 provided by the pilot, which was incorrect and contained several errors. The pilot informed the FAA that during the flight, he was adjusting the CG and had placed a bag of bird seed in the passenger seat to assist with the in-flight test. The bag was located aft of the datum per the manufacturer's specifications, which would have moved the CG further aft.
Postaccident Examination
A postaccident examination of the airframe revealed that the tubular empennage structure had separated at the “H section.” The fractured section was sent to the NTSB’s Materials Laboratory in Washington, DC, for metallurgical examination. Both halves of the fracture surfaces on the aluminum tube were examined. All fractographic features were consistent with cantilever-bending overstress-fracture resulting from the application of a monotonically increasing load until fracture. No marks indicating recontact of the fracture surfaces were observed. 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 these network cracks did not contribute to the overstress fracture of the tube section.