On December 23, 2002, approximately 1430 Pacific standard time, a Grumman American AA-1, N5985L, impacted the surface of a small pond about one-quarter mile south or runway 33 at Chehalis-Centralia Airport, Chehalis, Washington. The private pilot received serious injuries, his passenger received minor injuries, and the aircraft, which was owned and operated by the pilot, sustained substantial damage. The pilot remained in a coma for a considerable period after the accident, and has no recollection of the associated events. The 14 CFR Part 91 personal pleasure flight, which departed the same airport about 15 minutes earlier, was operating in visual meteorological conditions. No flight plan had been filed. On the day of the accident, the pilot landed at the Chehalis-Centralia Airport and then had lunch with his son. After lunch, the pilot and his son decided to take a short flight to look at some nearby property. Prior to takeoff, the pilot mentioned that when they got back to Chehalis, he wanted to add fuel to the aircraft's tanks before heading back to his home airport. After takeoff, the pilot climbed to a little over 2,000 feet, and flew toward the location of the property. As the aircraft neared the subject area, they discovered that the property was covered by dense low-level fog. They therefore headed back to the airport, and because of fog to the north and west of the field, the pilot modified his pattern so as to make a 45 degree entry to an extended final approach to runway 33. During the last part of the descending approach, the pilot advanced the power lever, but there was no response from the engine. The pilot attempted to stretch the glide to the end of the runway, but he was unable to do so, and because there was no other suitable terrain, he ultimately ended up ditching in a long narrow pond that was closely aligned with his approach heading. After recovering the aircraft from the pond, it was determined that the right fuel tank was a little over one-half full of fuel, but the left tank contained only about one gallon of fuel. The fuel selector was found to be on the left tank, and the electric fuel pump switch was in the off position. The bottom of the electric fuel pump was removed and the pump screen was found to have no contamination. Less than a teaspoon of fuel was drained from the pump body and the lines from it to the carburetor. The carburetor bowl and passages contained no fuel. The throttle and mixture controls worked normally, but the carburetor heat flapper valve was locked in a mid-range position by impact damage deformation. Although the engine and its accessories where flooded with water, an inspection of the engine components did not reveal any anomaly that would have kept the engine from running normally. After the water was drained from the engine and its accessories, the crankcase was filled with new oil and the engine was test run using an external fuel source. During the test run, the engine was operated at a number of power settings for varied periods of time, and there was no indication of any anomalies or malfunctions. During the investigation, it was also noted that at the time of the accident, the temperature and dew point were the same throughout a large area around Chehalis. At Toledo-Winlock Airport, which is located approximately 15 miles southeast of Chehalis, the temperature and dew point had been equal to each other throughout most of the day, and remained so for at least four hours after the accident. At the time of the accident, the temperature/dew point at Toledo was 36 degrees and 36 degrees Fahrenheit. At Olympia Airport, which is located about 20 miles north of Chehalis, the equal temperature and dew point situation was present for many hours both before and after the accident. At the time of the accident it was 37 degrees and 37 degrees. Throughout most of the day, the Interstate Highway Five corridor, between Olympia, Washington and Kelso, Washington, was covered by dense low-level fog. Because of the fog and mist in the valley, the visibility at both the aforementioned airports varied between one-eight and three-quarters mile during all of the daylight hours. Witnesses reported that at the time of the accident, much of the area around Chehalis was covered with fog, including the northern end of the runway. According to the attached FAA/DOT Carburetor Icing Probability Chart, this aircraft was operating in an environment where icing could be expected to accumulate in the carburetor throat at cruise power and below. Due to these conditions, the IIC asked the passenger if he remembered his dad engaging the carburetor heat as he descended back into the airport, but he was unable to positively recall if he had seen him do so. Maintenance personnel who had worked on the aircraft reported that the pilot had said that in the past that the aircraft experienced carburetor icing in conditions where he had not expected it to occur. At the termination of the investigation, it could not be positively determined if the aircraft's engine was starved of fuel due to the low quantity in the selected left tank, or whether the pilot had forgotten to apply full carburetor heat resulting in a loss of power due to an accumulation of ice in the carburetor.