On August 18, 1994, at 1445 mountain daylight time (mdt), a Hughes 369HS, N74AM, registered to X Data Corporation of Rapid City, South Dakota, and piloted by a commercially certificated pilot, was substantially damaged during landing following an auto-rotation demonstration. Visual meteorological conditions prevailed at the time of the accident. The 14 CFR Part 91 flight was not operating on a flight plan. The pilot and passenger reported no injuries. The flight departed Custer, South Dakota, at 1443 mdt. The pilot's written statement on NTSB Form 6120.1/2 said, "...practicing touch down auto rotation landing procedure. Upon touch down, right landing skid connected with rock not visible due to ground cover in area. At time of impact with rock, helicopter was jolted over approximately 4 to 6 inches sideways causing ground resonance." The pilot stated the rotor blades began "...gyrating out of normal path..." and struck components on the helicopter. According to the pilot, the terrain used for the landing had been used in the past without any incidents. He said the terrain was a "... rolling, grassy field with no visible obstructions." An interview with the pilot revealed that he did one autorotation before the one that resulted in the accident. The pilot said he observed rocks in front of the helicopter, but not below during the autorotation. According to the pilot, the rock his helicopter struck was obscured by tall grass.