What happened
During a sales demonstration flight, the pilot was seated in the right seat with a prospective buyer in the left seat. Approximately 1.5 hours after takeoff, while 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 within the engine compartment. Following the explosion, white smoke began emitting from the exhaust stacks.
The aircraft entered a descent toward a forced landing in an area characterized by farms, pastures, and rolling hills. A witness observed the airplane make a sharp left turn as it approached the ground, eventually impacting trees and a stone wall located between two pastures. At the time of the accident, winds in the area were calm.
The investigation
The aircraft was equipped with an overhauled Allison 250-B17F2 engine, which had been installed approximately nine months and 25 operating hours prior to the accident. A teardown examination of the engine revealed a tri-burst of the third-stage turbine wheel. This failure was caused by an overspeed of N2 following a sudden loss of load on the power train.
Additionally, investigators found a failure of the torquemeter gearshaft. The investigation determined that a crack had initiated in an area of intergranular separation and progressed through high cycle fatigue. This intergranular separation was attributed to stress corrosion mechanisms occurring during the AMS 2485 black oxide treatment process. The torquemeter gearshaft had been installed during the engine's previous overhaul.