What happened
During the flight, observers noted the aircraft performed an abrupt left turn or roll, eventually reaching a position where the wings were perpendicular to the earth. Following this maneuver, the plane entered a nose-down descent. The impact resulted in the total destruction of the aircraft and the deaths of all 23 occupants.
Findings
Post-accident investigations focused on the left propeller components. Analysis revealed that the blade angle on the left propeller was approximately 3°, even though the propeller control unit (pcu) ballscrew position indicated a commanded angle of 79.2°.
Technical examination identified significant wear on the teeth of the pcu quill spline. This wear occurred where the quill engaged the titanium-nitrided splines of the propeller transfer tube. Because the titanium-nitrided surface of the transfer tube was harder and rougher than the surface of the quill, it functioned similarly to a file, leading to the degradation of the gear teeth. It was determined that the wear of the quill was not accounted for during the certification process of the propeller system.