What happened
During a flight test intended to certify a new Supplemental Type Certificate, a crew of four was performing maneuvers in a single-engine turboprop airplane. The flight involved executing a specific test card, which included an intentional accelerated stall. During this maneuver, the aircraft was configured with flaps extended in a 30-degree left bank with engine torque set to 930 ft-lb.
Following the stall, the aircraft entered a rapid left roll, reaching an angle of 120 degrees, while the nose pitched down to 60 degrees. As the aircraft descended, the airspeed increased rapidly, surpassing both the maximum flaps-extended speed and the maximum operating speed. Witnesses reported seeing the aircraft break apart in mid-air before spiraling toward the ground. The wreckage was recovered from a rural field, spread across a distance of approximately 1,800 feet. The accident resulted in 4 fatalities.
Findings
Investigations revealed that the aircraft's airspeed exceeded its structural limits, leading to an in-flight breakup. Analysis indicated that the wing design stress limits would be exceeded at high speeds when the flaps are fully extended. A critical factor was the failure to reduce engine power to idle after the nose dropped, which allowed the airspeed and aerodynamic loads to increase at a rate that prevented recovery.
Additionally, the aircraft was equipped with an Electronic Stability and Protection (ESP) system. While the pilot was experienced, there were indications of unfamiliarity with the ESP system's interface. The system's activation and deactivation during the maneuver may have contributed to the loss of control. Furthermore, previous test flights had already demonstrated exceedances of roll and speed limits that were not properly addressed according to risk mitigation procedures, including the failure to perform required overspeed inspections.