The weather was CAVOK with 5 kt of wind along Runway 21, which has a grass surface. The pilot had 134 hours flying time, all on the accident type, of which 33 hours were in command. The pilot’s report stated that he crossed the threshold at 60 KIAS, and that at the flare the aircraft struck the runway hard and the nose dug in, causing it to pitch over onto its back. He exited the aircraft unaided and uninjured, and there was no fire. The pilot commented that he later noticed a deep rut in the runway, but did not know whether it had been made during the accident or was there previously. He was unsure of the cause of the crash.