Flight Details
A pilot and three crew members were conducting flight testing for a new Supplemental Type Certificate (STC) on a single-engine turboprop airplane. After departure, the pilot executed several maneuvers from the test card, then configured the airplane with flaps extended for an intentional accelerated stall in a 30° left bank with engine torque set to 930 ft-lb.
Stall and Departure
Analysis of ADS-B data combined with a simulation matching the recorded trajectory showed that, after the stall, the airplane rapidly rolled left to 120° while pitch decreased to 60° nose down. Airspeed quickly exceeded the maximum flaps-extended speed (Vfe) and the maximum operating speed (Vmo). Recorded engine data indicated torque increased after the stall. ADS-B data was lost at about 7,000 ft above ground level, with the final track indicating an approximate 8,700 ft/min descent rate. Witnesses observed the airplane break up in flight and spiral to the ground. Wreckage was found in a rural field distributed over about 1,800 ft.
Previous Flight Issues
Video from a test flight the day before showed a turning stall at idle power and 30° left bank with flaps extended; the airplane rapidly rolled left to 83° before the pilot recovered. After recovery, the pilot pitched nose down about 25° to gain airspeed, during which the Electronic Stability and Protection (ESP) system activated the autopilot to level the attitude. The airplane exceeded Vmo of 175 knots, reaching 183 knots indicated airspeed before the pilot arrested acceleration and disengaged the autopilot. The crew did not identify the 83° roll exceedance as a failed test, nor did they remark on the overspeed or conduct the required overspeed inspection per the maintenance manual.
Systems and Guidance
The ESP system was designed to deter exceedances by applying an opposite force that the pilot can overcome. To deactivate it, the pilot needed to navigate a specific PFD page. The accident pilot was experienced and qualified, but his familiarity with the avionics was unknown; videos suggested unfamiliarity, as he did not deactivate ESP before the flight or discuss its forces. Simulation indicated that reducing engine power to idle after the nose dropped could have slowed airspeed increase and given more recovery time. FAA guidance warns against performing accelerated stalls with flaps deployed due to increased risk of exceeding limitations.
Probable Cause
Official findings stated the probable cause as: The pilot’s improper recovery following a departure from controlled flight after an intentional aerodynamic stall, which resulted in an exceedance of airspeed limitations, airframe overstress, and a subsequent inflight breakup.