The pilot-in-command (PIC) reported that the flight was entering a stall series in accordance with the planned flight test procedures. The PIC was the pilot flying at the time of the incident. Flight crew calculations indicated that the stick shaker was expected to activate at 115 knots, with stick pusher activation at 107.5 knots. Aerodynamic stall was expected at 105.5 knots at the current operating weight. The PIC stated that "as the airplane slowed through [approximately] 126 knots [indicated airspeed], it abruptly rolled off / dropped the right wing and the nose fell rapidly." He noted that, although the autopilot was on as required by the test procedure, he was holding the control wheel and felt "no vibration or abnormal indication" prior to the event. He reported that the airplane rolled 5 to 7 times, both to the right and the left.
The PIC reported that during the resulting uncontrolled descent, the airplane entered an underlying cloud layer at 12,000 feet msl. The airplane exited the cloud layer about 10,000 feet msl and was "descending vertically." He stated: "I neutralized the ailerons with the yoke and began a higher than normal back pressure pull-out, experiencing [approximately] 4 - 5 Gs. The aircraft responded, and we stopped the descent somewhere below 7,000 [feet msl]."