On July 31, 1999, about 1630 Alaska daylight time, a wheel equipped Haering Avid Flyer, an experimental, homebuilt airplane, N511AF, sustained substantial damage during a forced landing about 10 miles north of Anchorage, Alaska. The airplane was being operated as a visual flight rules (VFR) local personal flight when the accident occurred. The solo commercial pilot was not injured. Visual meteorological conditions prevailed, and no flight plan was filed. The flight departed the Lake Hood Airstrip, Anchorage, about 1400. During a telephone conversation with the National Transportation Safety Board investigator-in-charge (IIC) on August 1, the pilot reported that after departure from the Lake Hood Airstrip, he completed a series of touch-and-go landings at two local area airstrips. He said that while returning to the Lake Hood Airstrip, the engine began to run rough, and lose power. He said that for a short time, he was able to restore partial engine power by actuating the engine fuel primer. The pilot reported that while he was attempting to land on a small, private airstrip, all engine power was lost. He said that during the approach, the airplane landed short of the airstrip, and collided with a stand of trees. The main landing gear collapsed, and the airplane sustained substantial damage to the fuselage. On August 19, the National Transportation Safety Board IIC, and a Federal Aviation Administration (FAA) airworthiness inspector, Anchorage Flight Standards District Office, examined the airplane wreckage at the pilot/owner's residence. The owner cut two 6 inch holes in each upper wing panel to gain access to the interior portion of the fuel tanks. An examination of the interior of the fiberglass fuel tanks revealed delamination of the resin material located in various locations within the tank. In addition, about three cups of a brown, plastic-like material, ranging in size from a grain of sand, up to six inches in length, was found loose within each fuel tank. Further investigation revealed that both in-line main fuel tank outlets were blocked with the same brown, plastic-like material. Examination of both wing fuel filters revealed the same material. The pilot/owner reported that he purchased the airplane in a kit form in 1988, and completed construction of the airplane in 1994. At the time of the accident the airplane had accumulated 79.5 total flight hours. The pilot/owner reported that in the past, he had operated the airplane for about 40 hours using an alcohol based fuel. AVID Aircraft service letter number 03, dated 5/91, covering all AVID Flyer models, stipulates that a placard be placed near the filler neck stating, in part: "NO ALCOHOL BLENDED FUELS. These fuels may cause deterioration of the fiberglass tanks or carburetor parts." The pilot/owner stated that since he has owned the airplane, he has not received any warnings or directives concerning the use of alcohol based fuels directly from the manufacture of the airplane kit, AVID Aircraft, Inc.