On August 3, 1998, at 1215 central daylight time (cdt), a Piper PA-28R-200, N4596T, piloted by a commercial pilot, was substantially damaged during an off-airport forced landing following a total loss of power while in cruise flight. Visual meteorological conditions prevailed at the time of the accident. The 14 CFR Part 91 personal flight was not operating on a flight plan. The pilot and passenger reported no injuries. The flight departed Lafayette, Indiana, at 1200 cdt. According to the pilot, the airplane was in level cruise flight at 2,800-feet above mean sea level. He said, "The plane was set up for cruise, but when the mixture control was moved to its usual cruise position, the engine slowed considerably. The mixture control was returned to full rich, and the fuel flow was observed to be 5 gallons per hour. At this power setting, it should have been 9 [to] 11 gallons per hour." The pilot said he turned the boost pump on and switched the fuel tanks. He said this did not increase the fuel flow. Changing throttle settings did not increase the power, according to the pilot. The pilot said the airplane began to lose altitude. The pilot said the airplane's automatic landing gear extension allowed the landing gear to extend while turning toward a field he was going to use for an off-airport landing. He said the airplane began to sink rapidly and he selected a field directly in front of the airplane for the landing. The airplane's left landing gear struck a powerline and "...jerked..." to the left. The pilot said right rudder did not counter the yaw attitude. The airplane landed in a yawed attitude, without its left main landing gear. The on-scene examination was conducted by a Federal Aviation Administration Principal Maintenance Inspector (PMI). During the investigation no water was found in the fuel. N4596T's fuel boost pump was powered up and fuel flowed to the fuel distribution manifold. The throttle, propeller and mixture controls were examined for continuity from the cockpit. The PMI reported, "All of the mechanical linkage appeared normal with one exception, the mixture (fuel flow) control was loose (broken) from the lug and clamp mount/bracket at the junction where the rod joins the flexible cable." The PMI continues, "...the mechanical stops for the mixture control lever on the servo fuel injector did not have wear marks at the full rich position... ." According to the Piper PA-28R-200's pilot operating handbook (POH), the best glide speed is 105 miles per hour (mph) at gross weight. N4596T is equipped with an automatic landing gear extension system. The POH states, "Gear extension is designed to occur even if the selector is in the up position at airspeeds below approximately 105 mph with power off." The POH stated the system can be over-ridden by raising a lever located between the pilot and front passenger seat. The lever must be held in the up position to keep the over-ride authority active. According to the POH, "To lock the over-ride lever in the up position.... push in the latch..." that is located on the console between the seat. Excerpts of the POH are appended to this report. The pilot said he needs to use both hands in order to accomplish the locking task. He said he was unable to do this because his attention was on flying the airplane toward the field he had chosen for the forced landing.