Aircraft: Cessna U206G. Injuries: 2 Fatal, 2 Serious. Location: Leverett, Mass. Aircraft damage: Destroyed.
What reportedly happened: The airplane was in cruise flight at 3,000 feet AGL when the engine developed a vibration, then lost power. The pilot attempted a forced landing in an open field. The airplane cleared trees adjacent to the field but the landing gear collided with electrical wires on the other side of the trees. The airplane flipped, hitting the ground upside down.
The post-accident investigation determined that the wrong nuts has been used for the cylinder hold-down studs, and the breakaway torque values measured for the cylinder hold-down nuts were improperly torqued. Metallurgical examination revealed that the engine’s crankshaft fractured due to fatigue cracks that developed as a result of the No. 2 main bearing shifting in the crankcase saddle. The engine had a reported total time in service of 6,629 hours, with 1,475 hours since major overhaul. All six cylinders had been replaced since the overhaul, with the No. 2 cylinder replaced 692 hours and the No. 3 cylinder replaced 549 hours before the accident.
Probable cause: A total loss of engine power due to the fatigue failure of the crankshaft, which resulted from improper tightening of the cylinder hold-down nuts.
NTSB Identification: ERA11FA078
This December 2010 accident report is provided by the National Transportation Safety Board. Published as an educational tool, it is intended to help pilots learn from the misfortunes of others.