Forced Landing After Engine Turbine Failure During Sales Demonstration (N450M)

Accident Overview and Flight Profile During a sales demonstration, the pilot occupied the right seat while the prospective buyer sat in the left seat. Approximately 1.5 hours after takeoff, the aircraft was in cruise flight at an altitude estimated by a witness to be between 3,000 and 3,500 feet. An explosion occurred in the engine compartment, and white smoke began coming from the exhaust stacks. The aircraft then circled down for a forced landing in an area of rolling hills, farms, and pastures bordered by stone walls with embedded trees. As the aircraft approached the ground, a witness observed a sharp left turn before it struck trees and a stone wall between two pastures. Area winds were calm at the time of the accident. ## Engine and Component Findings The aircraft was equipped with an overhauled Allison (Rolls Royce Corporation) 250-B17F2 engine that had been installed about 9 months and 25 operating hours before the accident. Post-accident teardown examination revealed a tri-burst of the third-stage turbine wheel caused by an N2 overspeed following a sudden loss of load on the power train. The examination also identified a failure of the torquemeter gearshaft. A crack had initiated in an area of intergranular separation and subsequently progressed through high-cycle fatigue. The intergranular separation was attributed to stress corrosion mechanisms during the AMS 2485 black oxide treatment process. The torquemeter gearshaft had been installed during the engine overhaul.