On August 21, 1999, at 1830 hours Pacific daylight time, a Piper PA-18-18, N3356B, collided with mountainous terrain after encountering what the pilot described as a downdraft near Eagleville, California. The aircraft was operated by Jim & I Aviators of Redding, California, and rented by the pilot for a personal flight under the provisions of 14 CFR Part 91. Visual meteorological conditions prevailed and no flight plan was filed. The aircraft incurred substantial damage. Neither the private pilot nor his passenger was injured. The flight originated at Redding on the morning of the accident and made stops at Lakeview, Oregon, and Cedarville, California, and was en route to Eagleville when the accident occurred. In a telephone interview and his written statement, the pilot said he departed Redding early in the morning and stopped in Lakeview to take care of some family business. Then he flew to Cedarville, arriving about 1200, and spent several hours on the ground. About 1800, he left Cedarville for Eagleville and arrived in the town's valley just before 1830. After completing some sightseeing on the southwest side of the mountain valley, he headed north toward the town, where he intended to land and spend the night. About 3 miles south of the town, the aircraft flew over a small ridge, which resulted in the aircraft being about 500 feet agl. His passenger remarked on some corrals on the ground and the pilot performed a 360-degree turn to view the area. During the turn, the pilot noted the airspeed had increased slightly, and, as the aircraft rolled out on a northerly heading, the aircraft suddenly felt like it stalled and entered a rapid descent toward the ground. The pilot lowered the nose and flared as the aircraft approached the ground. The aircraft landed flat, breaking off the landing gear. The pilot said he turned off the fuel and exited the aircraft. Suddenly, as a large cloud moved overhead it began to lightening and rain heavily, with very gusty surface winds. The pilot stated that there were no mechanical malfunctions or failures prior to ground contact. Based on the pressure altitude and the temperature at a weather reporting station 15 miles east of the site, the density altitude was calculated to be about 7,500 feet.