HS-121 Trident G-ARPY departed Hatfield at 16:52 to carry out the first of a series of production test flights to qualify for a Series Certificate of Airworthiness. At 18:30, after completing a large part of the tests, the stalling tests were begun. Three approaches to stall were made to check the stall warning and stall recovery systems. The fourth stalling run was made at a height of 11600 ft in landing configuration with the stall warning and recovery systems made inoperative. The Trident entered a superstall with the nose pitched up to 30-40 degrees. The plane turned to the left, the right wing dropped and the plane then went into a flat spin to the right. PROBABLE CAUSE: “During a stalling test decisive recovery action was delayed too long to prevent the aircraft from entering a superstall from which recovery was not possible.”