On August 19, 2008, about 1020 mountain standard time, a Cessna TU206F, N62DB, collided with a fence during a forced landing near Glendale, Arizona. Airwest Helicopters LLC was operating the airplane under the provisions of 14 Code of Federal Regulations (CFR) Part 135. The certificated airline transport pilot and two passengers were not injured. The airplane sustained substantial damage to the fuselage. The cross-country business flight departed Glendale about 1012, with a planned destination of Grand Canyon Valle Airport, Williams, Arizona. Visual meteorological conditions prevailed, and no flight plan had been filed. The pilot reported that the departure was uneventful until he leveled the airplane at 3,000 feet mean sea level (msl). He noticed that all of the exhaust gas temperature readings were much higher than normal. He adjusted the manifold pressure to 27 inches of mercury, and reduced the revolutions per minute (rpm) to 2,500. The engine began to run rough, so the pilot turned back toward the airport. He tried to get the engine to run more smoothly, but was unsuccessful. The airplane could not maintain altitude, and could not make it back to the airport. He elected to land in a short, open field. After colliding with the fence and coming to a stop, the pilot evacuated his passengers and himself without injuries. Investigators examined the engine. They removed the top spark plugs. All spark plugs were clean with no mechanical deformation. The spark plug electrodes were circular and gray, which corresponded to normal operation according to the Champion Aviation Check-A-Plug AV-27 Chart. A borescope inspection of the cylinders revealed no mechanical deformation on the valves, cylinder walls, or internal cylinder head. Recovery personnel strapped the airframe to supports and the floor. They plumbed a fuel can into the left wing fuel supply. When they opened the fuel valve allowing gravity flow, fuel leaked from the fuel line fitting that was connected to the fuel flow transducer inlet. There were blue stains on the crankcase immediately below the fitting. The fitting took about 180 degrees of turn to close finger tight, and stop the leak. Investigators tightened the fitting. Investigators started the engine with no difficulties encountered. Investigators left the engine at a low rpm setting until the engine oil temperature was indicating within the green operating arc. They advanced the throttle, and increased the rpm to 1,800, and the engine ran smoothly. The fuel flow was about 14 gallons per hour (gph). A magneto check revealed a smooth rpm drop on each magneto. An investigator manually loosened the fitting with the engine running at 1,800 rpm. The fuel pressure decreased to 6 gpm, which was below the bottom of the green operating arc; the engine began to run rough. The investigator tightened the fitting, and smooth engine operation resumed. The last maintenance recorded that would have involved removal of this fitting was during an engine replacement on July 7, 2007, which was 127 hours prior to the accident. Maintenance personnel completed an annual inspection on May 21, 2008.