What happened
On August 3, 2015, a Cessna TU206G, registration N235HM, was performing a cross-country flight from Bozeman, Montana, to Polson, Montana. Approximately 45 minutes into the flight at 8,500 feet, the pilot noticed the exhaust gas temperature (EGT) rising from 1,420 to 1,460 degrees F. The pilot attempted to manage the temperature by increasing fuel flow and adjusting the mixture, but the temperature continued to fluctuate.
During troubleshooting, the pilot noted a decrease in fuel flow to 17.2 gallons per hour (gph). After turning on the electric fuel pump, the fuel flow increased to 18.2 gph for 10 minutes. However, the fuel flow eventually dropped to 14 gph. The pilot attempted several recovery actions, including varying engine RPM, switching fuel tanks, and adjusting the mixture control, but engine performance did not improve. After the pilot reactivated the electric fuel pump, the engine ran normally for two minutes before the fuel flow dropped to 3 gph and the engine quit.
The pilot initiated an off-airport forced landing near Saint Ignatius, Montana. He targeted a paved road, maneuvering the aircraft to avoid power lines. The aircraft landed on the road at 70 knots, but the pilot reported the brakes were ineffective. During the landing roll, the aircraft encountered a fence line and bushes; the left wing struck a choke cherry bush, and the aircraft rotated into a fence before coming to a stop.
The pilot was not injured, but the aircraft sustained substantial damage to the wings, fuselage, and empennage.
The investigation
An examination of the engine revealed heavy blue stains on top of the cylinders. Investigators found that the main fuel line attached to the fuel flow transducer was finger tight and leaking.
Maintenance records showed that the aircraft had recently undergone an annual inspection, and this was the first flight since maintenance performed several months prior. During that maintenance, three cylinders—including cylinder number six—had been removed and reinstalled. The fuel line in question was located just above the No. 6 cylinder and had been removed to access the cylinder during the maintenance process. A ground run following that maintenance had shown no anomalies.