Background
The accident occurred during a repositioning flight intended to pick up a patient for aeromedical transport. The aircraft was crewed by a pilot and two medical personnel. Dark night instrument meteorological conditions (IMC) prevailed for the flight.
Accident Sequence
Radar data indicated that after departure, the airplane climbed to 14,000 feet mean sea level and proceeded directly toward the destination airport. Subsequently, it began a right descending turn. The airplane then broke up in flight and impacted terrain. No distress calls were received from the pilot prior to the accident.
Wreckage Examination
The debris field extended 2,500 feet, and the distribution of wreckage confirmed that both wings, the horizontal stabilizer, both elevators, and both engines separated from the airplane before ground impact. Examination revealed that the initiating failure was the failure of the wing where it passed through the center of the airplane. The three wing spars exhibited S-bending deformation, indicative of positive overload producing compressive buckling and fracture. Impact signatures, such as black paint transfers and gouged aluminum, were consistent with the left outboard wing separating after being struck by the right engine, following the wing spar failures. No evidence of pre-existing conditions that would have degraded the structural strength at the fracture locations was found. Flight control continuity was confirmed. Examination of the engines, propellers, and systems showed no mechanical malfunctions or failures that could have contributed to the accident.
Analysis of Events
Although weather conditions were conducive for inflight icing, no evidence of structural icing was identified at the scene. The descending right turn was inconsistent with the intended flight track and air traffic control (ATC)-provided clearance. However, there was insufficient information to determine how the turn was initiated and when the pilot became aware of the airplane's state in the dark night IMC conditions. The absence of a distress call or communication with ATC about the deviation suggests that the pilot was not initially aware of the change in state. The structural failure signatures indicated that the wings failed in positive overload, consistent with the pilot initiating a pullup maneuver that exceeded the airplane spars' structural integrity during an attempted recovery from a spiral dive.
Probable Cause
The probable cause, as stated by the investigation, is the pilot's failure to maintain control of the airplane in dark night conditions that resulted in an in-flight positive overload failure of the wings and the subsequent in-flight breakup of the airplane.