NTSB Identification: NYC01FA141.
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Accident occurred Monday, June 11, 2001 in Honesdale, PA
Probable Cause Approval Date: 10/23/2002
Aircraft: Cessna P210N, registration: N450M
Injuries: 3 Fatal.

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 sales demonstration pilot was in the right seat and the prospective buyer was in the left seat. About 1 1/2 hours after takeoff, while in cruise flight at an altitude estimated by a witness to be between 3,000 feet and 3,500 feet, an explosion occurred in the engine compartment, and white smoke began emanating from the exhaust stacks. The airplane circled down to a forced landing in an area of rolling hills, farms, and pastures bordered by stone walls embedded with trees. As the airplane was nearing the ground, a witness saw it make a sharp left turn, then impact trees and a stone wall between two pastures. Area winds at the time of the accident were calm. The airplane was equipped with an overhauled Allison (Rolls Royce Corporation) 250-B17F2 engine which had been installed 9 months, and about 25 operating hours earlier. Teardown examination of the engine after the accident revealed a tri-burst of the third stage turbine wheel, due to an overspeed of N2 after a sudden loss of load on the power train. It also revealed a failure of the torquemeter gearshaft, from a crack which initiated in an area of intergranular separation, and subsequently progressed in high cycle fatigue. The intergranular separation resulted from "stress corrosion mechanisms during the AMS 2485 black oxide treatment [process]." The torquemeter gearshaft had been installed during the engine overhaul.

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

An improperly manufactured torquemeter gearshaft assembly, which resulted in gearshaft assembly failure, an N2 overspeed, and a subsequent uncontained engine failure.

Full narrative available

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