On September 23, 2019, about 1157 eastern daylight time, an experimental, amateur-built RV-6, N12033, sustained substantial damage when it was involved in an accident near Lakeland, Florida. The pilot received serious injuries; the passenger was uninjured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot, who was also a flight instructor, stated that the airplane owner contacted him for tailwheel instruction in his recently-purchased RV-6. The owner did not hold a pilot certificate, but the pilot agreed to fly the airplane with the owner as a passenger. The pilot performed a preflight inspection with no anomalies noted and departed on a local flight before returning to the airport. The pilot performed one go-around due to a crosswind, and during the subsequent landing approach, “the airplane banked to the left and pitch[ed] down about 8 or 10 degrees.” The pilot attempted to regain control of the airplane, but the controls were “very stiff.” He recalled heading toward the trees and tried again to move the controls; this time, the airplane pitched up slightly, but not enough to slow the descent. The airplane impacted trees and the ground, resulting in substantial damage to the fuselage, wings, and empennage. The pilot reported that he thought the passenger applied control inputs to maneuver the airplane during landing, which resulted in his inability to control the airplane. He stated that the airplane “definitely did not stall.” He also reported that the airplane had no preimpact mechanical malfunctions or failures that would have precluded normal operation. The owner of the airplane did not provide a statement regarding the accident. Examination of the airplane by a Federal Aviation Administration (FAA) inspector confirmed flight control continuity from the cockpit controls to the elevator and ailerons.