Incident Overview
A certified flight instructor (CFI) offered a familiarization flight in an experimental, homebuilt airplane to a prospective purchaser. The CFI had prior experience flying this model of airplane. The flight included stalls and landings, with the CFI demonstrating a couple of stalls to the student, who held a private pilot certificate and a tail wheel endorsement. Following the stalls, the pair practiced landings.
Detailed Account
The CFI reported monitoring the pilot's performance while the pilot handled the controls. The first landing was not performed satisfactorily; the pilot almost lost control of the conventional gear airplane during rollout. The pilot's second approach and three-point touchdown appeared satisfactory. However, after the airplane rolled several hundred feet, the private pilot suddenly and abruptly applied left and right rudder pressure while moving the flight controls erratically.
The CFI joined the pilot on the flight controls in an attempt to maintain control but could not overcome the erratic inputs, and directional control was lost. The airplane's divergent rollout course was not corrected, and the airplane exited off the left side of the runway, experiencing a ground loop. The airplane came to rest with collapsed landing gear and a bent wing.
Operational Context
The CFI had previous experience with the specific experimental homebuilt model. The source did not specify the aircraft type, registration, engine count, date, operator, location, number of occupants, injuries, damage items beyond those noted, time of day, weather characterization, or ATC involvement.