Cessna 180B N180GK accident near Piscataquis County, Maine

Probable cause

The fatigue fracture of the No. 3 exhaust valve, which initiated at the outer surface of the valve stem and resulted in the subsequent total loss of engine power over terrain unsuitable for landing.

NTSB analysis

The pilot reported that, while maneuvering the float airplane about 2,500 ft above ground level, he heard a "slight squeal" coming from the engine before it started to sputter. After the airplane had descended to about 1,500 ft, the engine and propeller "completely stopped." The pilot determined that the airplane would not reach a nearby lake and chose an unpaved road in the woods for the forced landing. The right wing struck a tree about 60 ft above the ground, and the airplane then impacted the road, which resulted in substantial damage to the wings, fuselage, and empennage.

Examination of the engine revealed that the crankshaft could be rotated freely by hand, and removal of the oil sump revealed piston material and the No. 3 exhaust valve head, which was fractured at the stem below the head. Disassembly of the engine revealed normal wear and appearance on all of the cylinders except for the No. 3 cylinder, piston, and exhaust valve. Metallurgical examination of the No. 3 exhaust valve revealed that the fracture had initiated at the outer surface of the valve stem and that the valve had fractured due to fatigue at the transition of the stem to the valve.

Text from the NTSB. Open the NTSB docket for ERA14LA446 for the full record.