On May 1, 2018, about 0845 eastern daylight time, a weight-shift-control Cosmos Phase III, N6466, sustained substantial damage after it impacted terrain shortly after departure from Fernandina Beach Municipal Airport (FHB), Fernandina Beach, Florida. The flight instructor sustained serious injuries, and the student pilot was not injured. The aircraft was registered to and operated by the flight instructor as a Title 14 Code of Federal Regulations Part 91 instructional flight. Visual meteorological conditions prevailed, and no flight plan was filed for the flight that originated at 0830 from FHB. In a telephone interview with the National Transportation Safety Board investigator-in-charge, the flight instructor reported that the student pilot was manipulating the controls and flying the aircraft. During the initial climb, at about 700 ft above ground level, the airplane suddenly entered a "hard" left bank. The flight instructor took control of the aircraft, however, was unable to recover and the aircraft impacted terrain in a nose down attitude. The flight instructor does not recall if there was a mechanical anomaly, but reported that the student pilot was known to freeze on the controls and he could have been fighting him. The student did not provide a statement as to the circumstances of the accident. Neither the flight instructor nor the student pilot completed or submitted the NTSB Pilot/Operator Aircraft Accident/Incident Report Form 6120.1 to the investigator in charge.