On November 28, 1997, about 1520 central standard time, a Cessna 172G, N3908L, registered to a private individual, operating as a 14 CFR 91 personal flight, crashed in a wooded area in the vicinity of Stephens Airfield, a private grass airstrip near Hazel Green, Alabama. Visual meteorological conditions prevailed, and no flight plan was filed. The aircraft received substantial damage, and the private-rated pilot received serious injuries. The flight originated from that field 1 hour before the accident. An eyewitness to the accident reported that the propeller was stopped just before aircraft impact with trees to the north of the airport. The aircraft came to rest 300 to 400 feet short of runway 18, outside airport boundary. The pilot states that he has no recollection of the events that led up to the accident. Before he bought the aircraft on September 8, 1997, he had the seller correct a leaking fuel selector valve. The replacement valve was subsequently found to allow some tank-to-tank fuel migration, so reportedly, the plan was to overhaul the removed fuel valve for reinstallation. The recently reworked valve, in a plastic bag, some fuel component lubricant, hand tools, access panels, various fasteners and a spiral notebook with times and fuel calculations were found in the aircraft at the accident site. Reportedly, arrangements had been made with an A & P mechanic to effect the fuel valve change after landing. Wreckage examination by the FAA and manufacturer's representatives revealed no evidence of precrash mechanical failure or malfunction of the airframe, flight controls, engine/accessories, or their components. Propeller damage was limited to one blade, bent aft 90 degrees, parallel to the fuselage longitudinal axis, and neither blade nor spinner showed rotational type damage. The two fuel tanks appeared not compromised by crash impact, and were empty. The FAA and police authorities reported no odor of fuel at the crash site. The wreckage was positioned upright and level and the fuel system was tested negatively for water and completely drained through the fuel strainer drain. The amount drained, according to the FAA report was, "less than two gallons". Type certificate data states that Cessna 172G weight calculations must consider 18 pounds ( 3 gallons) as unusable fuel.