China Southwest Airlines flight SZ4509, a Tupolev Tu-154M, departed Chengdu Airport in China at 14:35 hours on a domestic flight to Wenzhou. The en route altitude was 11400 meters.
At 16:02 hours, when the aircraft passed 9000 meters in the descent towards Wenzhou, the crew felt that the steering column was too far forward. The crew were puzzled by the control issue, especially at 16:26 hours when the steering column was fully forward. Yet, the aircraft was not in a nose-down attitude. The captain noted that the controls felt disconnected. At 16:29 the flight was cleared to descend from 1200 m to 700 m. When the flight crew selected the first stage of flaps, an angle-of-attack warning sounded in the cockpit. The aircraft entered a stall and began a steep descent until it impacted a farm field at Rui'an.
Several farmers were injured by the flying debris.

PROBABLE CAUSE: A self-locking nut, other than castle nut with cotter pin as specified, had been installed at the bolt for connection between pull rod and bellcranck in the elevator control system. The nut screwed off, resulting in bolt loss, which led to the loss of pitch control.