A Tupolev 154M operated by China Northwest Airlines as flight 2303 from Xian (XIY) to Guangzhou (CAN) was destroyed in an accident after takeoff from Xian. All 146 passengers and 14 crew members were killed.The aircraft took off from Xianyang Airport at 08:13. Twenty-four seconds after takeoff the flight crew reported that the aircraft was difficult to control with uncommanded banking to the left and right of about 20°, increasing to 30°.The angle of attack increased to 20°, activating the stall warning. After that, the airplane suddenly rolled to the left and the nose dropped to a pitch angle of -65°. The speed reached 747 km/h and the overspeed warning sounded in the flight deck. In 12 seconds, the altitude dropped from 4717 to 2884 meters. The airframe was overloaded beyond structural limits and the aircraft began to break up.PROBABLE CAUSE: Auto-pilot induced oscillations caused the aircraft to shake violently. It appeared that the autopilot yaw-channel had been connected to the bank control and the bank-channel to the yaw controls. This was done the previous evening 'in the field' rather than in a workshop.