On July 11, 1998, at 1600 central daylight time, a Cessna 188B, N9247R, piloted by a commercial pilot, sustained substantial damage when it impacted the terrain, following a loss of engine power while maneuvering, near Atlanta, Nebraska. Visual meteorological conditions prevailed at the time of the accident. The 14 CFR Part 91 maintenance flight was not operating on a flight plan. The pilot reported no injuries. The flight departed a private airstrip near Atlanta, Nebraska, for a sprayer calibration test flight, at 1545. According to the pilot's written statement, the pilot was performing a sprayer calibration test when the engine suffered a substantial loss of power. The pilot was operating at approximately 50 feet above ground level (agl) at the time of the power loss. The pilot stated that he was at too low of an altitude to trouble shoot the power loss and maneuvered the aircraft into an open field. Upon touch down the left main gear dug into the soft mud, collapsed the left main gear, which resulted in the left wing impacting the terrain. The pilot stated that the aircraft rotated about the left wing, approximately 80-degrees, and then came to a stop. Post accident investigation revealed that the engine would not sustain power at idle or any power setting with the fuel boost pump in the "Off " position. Investigation of the fuel system revealed that the "B" nut on the fuel line from the firewall to the back of the engine was loose. After the nut had been tightened the engine preformed, without hesitation, at all power settings with the fuel pump in the "Off" position. No other aircraft or engine anomalies were found.