Background
The accident occurred during a flight test to evaluate lateral and directional stability. The test incorporated several changes from previous trials, including a new leading edge fairing, a new flap setting, a lower reference airspeed, and trial settings for the stall protection system (stick shaker and pusher). Engineers briefed the crew that sufficient data would be obtained if the steady heading sideslip (SHSS) maneuver ended at a 15° sideslip or at the onset of stall warning. The crew agreed to terminate the maneuver at the first indication of stall warning.
Test Maneuver
During the test flight, the captain conducted the SHSS maneuver. Contrary to the agreed plan, the captain continued beyond the onset of stall warning, reaching a sideslip angle of 21° with full rudder applied. The airplane then experienced a rapid 360° roll and entered a deep stall condition.
Accident Sequence
Upon entering the deep stall, the copilot attempted to deploy the anti-spin parachute. However, the cockpit switches for the chute system had not been properly preset. As a result, instead of aiding recovery, the chute separated from the airplane. The chute system design permitted deployment even when the hydraulic lock switch was in the unlocked position, which left the hooks clasping the chute shackle to the airframe open. Although the system had tested normally before the flight, the design flaw contributed to the chute's detachment. Full control of the aircraft was not regained before impact, and all three crew members were killed.
Findings
The official investigation identified the probable cause as the captain's failure to adhere to the agreed-upon flight test plan by continuing the maneuver past the prestall stick shaker activation, and the flightcrew's failure to ensure all required switches were properly positioned for anti-spin chute deployment. A contributing factor was the inadequate design of the anti-spin chute system, which allowed deployment with the hydraulic lock switch in the unlocked position.