What happened
A flight was conducted departing from Hatfield at 16:52 hours to perform a series of production tests intended for Series Certificate of Airworthiness qualification. The mission profile included specific stall testing procedures, provided environmental conditions were appropriate. Following takeoff, the aircraft climbed in a north-easterly direction.
By approximately 18:30, having completed most of the scheduled flight tasks, the crew began executing stall tests. Initial maneuvers were performed in the landing configuration to verify the functionality of the stall warning and recovery systems; logs indicated the stick shaker activated at 102 kt and the recovery system at 93 kt. During a fourth attempt at an altitude of 11,600 ft, the crew intentionally rendered the stall warning and recovery systems inoperative as per the test requirements.
After several routine radio transmissions, the pilot-in-command reported being in a "superstall" at 18:34. This was the final communication from the aircraft. Observers near Felthorpe noted the aircraft traveling slowly toward the southwest at roughly 10,000 ft. The nose pitched up between 30 and 40 degrees as the aircraft banked to port, followed by a sharp drop of the starboard wing. Despite a brief application of engine power, the aircraft type entered a flat spin toward the starboard side. The rotation continued at a rate of one turn every 6 to 8 seconds until impact with the ground approximately 90 seconds later. The accident resulted in four fatalities.
Findings
The investigation determined that the flight was being executed according to an established test plan. During the final stall maneuver, the airspeed was reduced at a rate exceeding 1 kt per second. Recovery efforts were not initiated until the speed had dropped below the limits defined by the test schedule, leading to an unrecoverable superstall.