On July 18, 2009, about 1100 Pacific daylight time, an experimental Leider Thorp T-18, N37GL, ground looped during the landing rollout, and came to rest inverted at Carson Airport (CXP), Carson City, Nevada. The pilot/owner operated the airplane under the provisions of 14 Code of Federal Regulations Part 91 as a personal flight. The private pilot/owner was seriously injured, and the certificated private pilot-passenger received minor injuries. The airplane sustained substantial damage to the tail section. The flight had originated from CXP at 1005 for the local area flight. The written statement submitted by the certificated private pilot-passenger stated that he was seated in the right seat. His father, the pilot/owner, was seated in the left seat, and was manipulating the flight controls at the time of the accident. The right-seated pilot stated that they were landing on runway 27. The right-seated pilot reported that after the local area flight, the left-seated pilot flew an uneventful traffic pattern and approach. As the airplane touched down it seemed “squirrelly skipping to the right.” The right-seated pilot stated that he thought his dad was going to turn off the runway at the gas pit, but the airplane veered sharply to the right into the grass and then flipped over. He also reported that there were no mechanical failures or malfunctions with the airframe or engine prior to the accident. The right-seated pilot further reported that the accident could have been prevented if he had been manipulating the flight controls along with the pilot. Both of the pilots held lapsed medical certificates. The left-seated pilot’s last biannual flight review was approximately 4 years ago; It was also reported that the second pilot had not flown the accident airplane for approximately 8 months prior to the accident. It was not known when the right-seated passenger/pilot's last biannual flight review was conducted.