On June 20, 2008, approximately 1400 central daylight time, an Ayres S2R-G6, N61374, registered to and operated by Swan Lake Flying Service, Inc., and piloted by a commercial pilot, was substantially damaged when it struck trees and impacted terrain during a forced landing following a loss of engine power near Pine Bluff, Arkansas. Visual meteorological conditions prevailed at the time of the accident. The aerial application flight was being conducted under the provisions of Title 14 Code of Federal Regulations (CFR) Part 137 without a flight plan. The pilot, the sole occupant on board, was not injured. The local flight originated approximately 1330 at Altheimer, Arkansas. The pilot said he was spraying a rice field with fertilizer when the "airplane began to lose altitude." He attempted to make a forced landing in an open bean field, but the airplane's right wing clipped a tree, spinning the airplane around to the right. The airplane impacted terrain sideways, shearing off the landing gear. The wings were bent, and the fuselage was buckled. FAA inspectors from the Little Rock Flight Standards District Office (FSDO) examined the airplane and found the fuel remaining in the engine fuel filter housing, which is integral to the engine fuel pump and is located in-line between the centrifugal boost pump and the positive displacement high pressure pump, was clean and clear. The threads on the fuel pressure line fitting on the fuel filter housing were found to be stripped, allowing the fitting to wobble about 5 to 10 degrees. When the fitting was moved, the O-ring would not seat. Examination of the adjacent aircraft cowling structure did not reveal fuel residue, indicative that the fuel pressure fitting damage was not pre-existing but rather as the result of impact with terrain. The engine was removed from the airframe and shipped to Honeywell in Phoenix, Arizona. Under Federal Aviation Administration (FAA) auspices, the engine was disassembled and examined on October 8 and 9, 2008. According to its report, Honeywell said that "the type and degree of damage to the engine was indicative of an engine not operating at the time of impact with the terrain. No pre-existing condition was found with the engine that would have caused a loss of power or an in-flight shutdown." FAA inspectors later learned that about six flights before the accident flight, the airplane sustained an uncommanded engine shutdown while being refueled and reloaded. Ayres, in their design process, did not install engine speed switches to automatically and electronically control the fuel system. Instead, they installed pilot-activated switches for manual control of the fuel system. Examination of the wiring system schematic revealed that if there was a short in the wires or if the switch failed due to contamination or corrosion, voltage could be sent to pin "B" on the Fuel Shutoff (Solenoid) Valve, causing the fuel shutoff valve to close. In the case of N51374, there was evidence of wiring and fuel shutoff switch contamination. As a result of this investigation, a safety recommendation proposal was submitted.