HISTORY OF FLIGHT: On April 17, 1999, about 1900 hours mountain standard time, a Cessna A150L, N6085J, operated and co-owned by the pilot, collided with trees and impacted terrain while maneuvering near the shoreline of Lake Mary, approximately 13 miles southeast of Flagstaff, Arizona. The airplane was destroyed and the private pilot was seriously injured. The passenger sustained minor injuries. Visual meteorological conditions prevailed at the time of the personal flight which was performed under 14 CFR Part 91, and no flight plan was filed. The flight originated from Flagstaff, about 1815. According to helicopter crewmembers from the Arizona Department of Public Safety, who responded to the accident site vicinity, the crash occurred about 6,500 feet mean sea level, at coordinates of 35 degrees 02.682 minutes north latitude by 111 degrees 27.305 minutes west longitude. The site was about 1/8-mile from the southeast side of the lake's shoreline, and about 300 feet higher than the lake. Witnesses on and near the lake reported to Coconino County Sheriff deputies that they observed the airplane performing low altitude maneuvers over the lake. The passenger's wife reported to the National Transportation Safety Board investigator that her husband, who was not a pilot, stated to her that they had been "grazing the lake" and could not pull up when the accident occurred. The passenger reported to the Safety Board investigator that he recalled the pilot had rolled the airplane several minutes prior to the mishap. Thereafter, the pilot flew low over the lake and he (the passenger) waved to persons in a boat. The persons waved back at the airplane. Then, while the engine's tachometer was developing 2,500 rpm, the pilot flew toward rapidly rising terrain. The engine continued to operate "fine," but the airplane could not gain enough altitude to clear the trees adjacent to the shore. The pilot banked the airplane in an effort at turning away from the trees when the crash occurred. ADDITIONAL INFORMATION: The Flagstaff Pulliam Airport, elevation 7,011 feet mean sea level, is located about 13 miles west-northwest from the accident site. Based upon the 1856 weather observation at Flagstaff, the calculated density altitude for the accident site was over 7,100 feet. When the airplane departed from Flagstaff, the fuel tanks had reportedly contained a total of about 12 gallons of fuel. The passenger stated he weighs about 225 pounds. According to the Federal Aviation Administration (FAA), on the pilot's last application for an aviation medical certificate he indicated his weight was 222 pounds. The FAA estimated that the airplane's empty weight was about 1,180 pounds, and its maximum certificated gross weight is 1,600 pounds. No airplane logbooks or weight and balance papers were located. (For additional details, see the Safety Board investigator's weight computation data, attached to this report.)