NTSB Factual Report

On August 3, 2015, about 1000 mountain daylight time, a Cessna TU206G, N235HM, made an off airport forced landing following a loss of engine power near Saint Ignatius, Montana. The pilot/owner was operating the airplane under the provisions of 14 Code of Federal Regulations (CFR) Part 91. The private pilot, the sole occupant, was not injured. The airplane sustained substantial damage to the wings, fuselage, and empennage. The cross-country personal flight departed Bozeman, Montana, at 0812, with a planned destination of Polson, Montana. Visual meteorological conditions (VMC) prevailed, and no flight plan had been filed.

The pilot reported that he was going to pick up a friend at Polson. About 45 minutes into the flight at a cruise altitude of 8,500 feet, the exhaust gas temperature (EGT) increased from 1,420 to 1,460 degrees F. The pilot stated he increased the fuel flow, which brought the temperature back down; but it soon started to climb again. Another increase in mixture brought the temperature back down.

Investigating the problem, the pilot switched fuel tanks with no change. The fuel flow gauge indication had decreased to 17.2 gallons per hour (gph). Pulling the throttle back to reduce the temperature and observing the fuel flow, the pilot turned on the electric fuel pump at 15 gph. The fuel flow went back to 18.2 gph, and remained there for 10 minutes. He noted the alternator light and its needle started to move around, which stopped upon turning the electric fuel pump off. The fuel flow dropped to 14 gph, so the pilot adjusted the throttle, and called his friend, who was an airplane mechanic. The pilot tried several recommendations including varying engine rpm, switching tanks, and adjusting the mixture control in and out. But there were no changes to the engine performance; the fuel flow continued to drop.

The pilot informed his friend that he was 13 miles from Polson, and was going to turn the electric fuel pump back on for the remainder of the flight, and they could check the situation out when he arrived there. Upon activation of the electric fuel pump, fuel flow went to 17.5 gph. The engine ran normally for 2 minutes, and then quit; the pilot noted that the fuel flow was 3 gph.

The pilot slowed the airplane to 90 knots, and checked his GPS for the nearest airport location, which was the St. Ignatius airport at 8 nautical miles (nm). The airplane was descending at 800 feet per minute, and once it was 3 nm from the St. Ignatius, the pilot determined that he was going to have to land off airport. He headed for a paved road that had two crossing power lines and one parallel power line. He stayed high enough to cross both lines, and was over a ditch on the left side of the road so that the airplane's right wing was under the parallel line. Once the wing was below the parallel line, the pilot moved the airplane over, and landed on the road at 70 knots; the brakes seemed ineffective. After rolling about 900 feet, the airplane approached a four-way junction; the pavement continued to the right, while a gravel road was straight ahead. The pilot attempted to stay on the gravel road; however, a fence line and bushes began to encroach on the road, and the left wing hit a choke cherry bush. The airplane rotated left into a fence, and came to stop facing the opposite direction.

The pilot reported that the airplane had just undergone an annual inspection. He called the mechanic who did the inspection, and informed him of the symptoms. The mechanic stated that it sounded like a fuel line was loose. When the pilot's mechanic friend arrived on scene, they removed the top cowling, and observed heavy blue stains on top of the cylinders. They observed that the main fuel line to the fuel flow metering unit was loose.

A Federal Aviation Administration (FAA) inspector examined the airplane, and observed blue fuel stains on top of the engine. The fuel line attached to the fuel flow transducer was finger tight, and leaking. He examined maintenance records, and determined that this was the first flight after maintenance performed several months earlier. Three cylinders had been removed and reinstalled, including cylinder number six, the front left cylinder. The fuel line attached to the fuel flow transducer was just above this cylinder, and had been removed during the maintenance. The maintenance facility did a ground run with no anomalies noted. The airplane was returned to the owner's hangar, and was not operated until the accident flight.

NTSB Probable Cause

Maintenance personnel’s failure to tighten a fuel line, which resulted in a loss of fuel pressure and a subsequent loss of engine power during cruise flight and an off-airport landing and collision with obstacles.