On October 2, 2009, at 1716 Pacific daylight time, N31KT, a Cessna 421B, rolled backwards into a parked car at the Hearst Castle Airstrip, San Simeon, California. The empennage sustained substantial damage. Western Air Charter was operating the airplane under the provisions of 14 Code of Federal Regulations Part 135. The airline transport pilot and five passengers were not injured. Visual meteorological conditions prevailed and the pilot had requested and received flight following. The pilot departed from Santa Monica Municipal Airport, Santa Monica, California, at 1600 and was at his destination when the accident occurred. The pilot reported that he had shutdown the engines and secured the airplane. The pilot indicated that although the checklist called for setting the parking brake, he elected to not use it since there were several chocks available on the ramp and no movement of the airplane was observed. He walked back to open the passenger door when the airplane began rolling backwards and impacted the windshield of a parked car. Although the parking area appeared level, the pilot indicated that later analysis showed there was a gradual slope.