NTSB Identification: ERA11FA078
14 CFR Part 91: General Aviation
Accident occurred Sunday, December 05, 2010 in Leverett, MA
Probable Cause Approval Date: 02/16/2012
Aircraft: CESSNA U206G, registration: N9742Z
Injuries: 2 Fatal,2 Serious.

NTSB investigators either traveled in support of this investigation or conducted a significant amount of investigative work without any travel, and used data obtained from various sources to prepare this aircraft accident report.

The airplane was in cruise flight at 3,000 feet above ground level when the engine developed vibrations and, shortly after, lost total power. The pilot conducted a forced landing in an open field. The pilot cleared the trees adjacent to the field but did not see the electrical wires on the other side. The airplane’s main landing gear contacted the wires, and the airplane flipped and impacted the ground inverted. Examination of the engine revealed that several incorrect nuts for the cylinder hold-down studs were installed, and the breakaway torque values measured for the cylinder hold-down nuts were consistent with improper torque having been applied. 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 through-bolts that clamp the No. 2 crankcase saddle faces are secured with the hold-down nuts for the Nos. 2 and 3 cylinders. 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 Nos. 2 and 3 cylinders having been replaced 692 and 549 hours before the accident, respectively.

The National Transportation Safety Board determines the probable cause(s) of this accident to be:

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.

Full narrative available

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