On June 4, 1997, about 1609 eastern daylight time, a Piper PA-18-150, N4310Z, was substantially damaged as it collided with trees during a forced landing while en route to the Shenandoah Valley Regional Airport (SHD), Harrisonburg, Virginia. The certificated commercial pilot received minor injuries and the passenger/observer was not injured. Visual meteorological conditions prevailed and no flight plan was filed for the local flight conducted under 14 CFR Part 91. The airplane, owned and operated by the State of Virginia Commission of Game and Inland Fisheries, departed SHD about 1340. The pilot and observer were performing an official survey flight for the Virginia Game and Inland Fisheries. The observer reported that they had completed their survey and were returning to the airport when the engine began to "spit and sputter." He stated that the airplane began to lose altitude, but the pilot was able to get the engine to run again for about 5 seconds, before it lost power again. The observer recalled that the pilot called out that they were forced to land. The pilot reported that he spotted a grass strip and attempted to land the airplane. He stated that he had made a downwind approach and was flaring the airplane for landing when he noticed a mobile home at the end of the grass strip. The pilot reported that he decided to stop the airplane by running the right wing into a tree. Inspection of the wreckage by a State Trooper revealed that the fuel selector was set to the right fuel tank. The State Trooper reported that there was no fuel in the right fuel tank, while the left tank was full. The pilot stated that he got preoccupied during the flight and did not remember to switch tanks.