What happened
On August 19, 2008, at approximately 10:20 mountain standard time, a Cessna TU2 enough 206F, registration N62DB, was performing a cross-country business flight from Glendale, Arizona, to Grand Canyon Valle Airport. The flight was being operated by Airwest Helicopters LLC under 14 CFR Part 135.
After departing at 10:12, the pilot leveled the aircraft at 3,000 feet mean sea level. The pilot observed that all exhaust gas temperature readings were significantly higher than normal. In an attempt to manage the engine, the pilot adjusted the manifold pressure to 27 inches of mercury and reduced the engine speed to 2,500 rpm. Shortly thereafter, the engine began running rough.
Unable to maintain altitude or return to the airport, the pilot performed a forced landing in a short, open field. During the landing roll, the aircraft collided with a fence. There were no injuries to the pilot or the two passengers, though the aircraft sustained substantial damage to the fuselage.
The investigation
Investigators performed a borescope inspection of the cylinders, which showed no mechanical deformation of the valves, cylinder walls, or internal cylinder heads. The top spark plugs were inspected and found to be clean and in a condition consistent with normal operation.
During the examination of the engine, investigators found that fuel leaked from a fitting connected to the fuel flow transducer inlet when the fuel valve was opened. Blue stains were observed on the crankcase directly below this fitting.
Testing confirmed that the fitting was the source of the issue. When the fitting was manually loosened while the engine was running at 1,800 rpm, the fuel pressure dropped below the green operating arc and the engine began to run rough. Tightening the fitting restored smooth engine operation. Records indicated that the last maintenance involving the removal of this fitting occurred during an engine replacement 127 hours prior to the accident.