On April 2, 2000, at 1600 central daylight time, a Piper PA-28-140, N54454, piloted by a commercial pilot, sustained substantial damage on impact with a fence and vegetation during a forced landing in a field near Pine City, Minnesota following an in-flight loss of engine power. The aerial observation flight was operating under 14 CFR Part 91. Visual meteorological conditions prevailed at the time of the accident. The flight was not operating on a flight plan. The pilot sustained minor injury and the passenger was uninjured. The local flight originated from Mora Municipal Airport, near Mora, Minnesota at 1430. The pilot stated, "We flew for about 1 1/2 hours with no problems. Suddenly our engine loss power as I rolled out of a 360[degree] turn, at a check point about 5 mi east of Pine City. I immediately gave full power, mixture rich and carb heat on. Our power loss was so dramatic that I knew it wasn't carb ice. Our power dropped from 2400 rpm to 1500. We were at 2500' MSL and losing altitude. Between us and the airport was a lake which I felt wasn't safe to try and cross. I tried to fly north around the lake when the plane started to lose more power and it was evident we were going to have to ditch." He said, "With the lake to our west, a bunch of trees to our north, a house to our south behind us, when the engine quit. I chose to turn 90[degrees] to the east fly under some high tension wires, which I knew I could do safely, to a field on the other side. The field was not very wide, but I felt it was safer for landing without any houses close by. Being 200 to 300 feet above the ground I didn't have the altitude to turn 180[degrees] and land in the field below. I could not get stopped in the narrow field, so consequently we went through the barb wire fence on the east side and hit a clump of bushes with our left wing, which brought us to a stop." The accident airplane's engine was salvaged and was disassembled. Its number four cylinder's exhaust valve spring and valve were found failed. The spring's uncompressed length from end to end as recovered was 2.25 inches. The spring was fractured approximately 0.75 inches from an end. The springs fracture surface was approximately 45 degrees to the longitudinal axis of the coil at that point. The valve head was found with approximately a third of its face fractured off. A semicircular section of the valve head, approximately 0.75 inches in length, was found within the cylinder. The valve head's face and valve head section were found with indentations. See appended valve photographs.