China Southwest Airlines Flight 4509

China Southwest Airlines Flight 4509 (SZ4509) was a domestic flight in China from Chengdu Shuangliu International Airport, Sichuan to Wenzhou Yongqiang Airport, Zhejiang. On February 24, 1999, the Tupolev Tu-154M operating the flight crashed while on approach to Wenzhou Airport, killing all 61 passengers and crew members on board.

Aircraft and crew
The aircraft was a 1990-built Tupolev Tu-154M (serial number 90A-846, serial 0846) airliner powered by three Soloviev D-30 turbofan engines from UEC Saturn. It was initially registered in the Soviet Union as CCCP-85846. It was delivered to the Civil Aviation Administration of China (CAAC) in April the same year, and was registered as B-2622.

The flight crew consisted of captain Yao Fuchen, first officer Xue Mao , navigator Lan Zhangfeng , and flight engineer Guo Shuming. There were also seven flight attendants on board.

Accident
On 24 February 1999, the crew was preparing the aircraft for landing at Wenzhou Airport. The flaps were extended at 1000 m, but seconds after, the aircraft's nose lowered abruptly, the aircraft disintegrated in mid-air and crashed into an area of high ground, and exploded. Witnesses saw the plane nose dive into the ground from an altitude of 700 m and explode. All 61 people on board were killed. Several people on the ground were injured from debris.

Cause
Incorrect self-locking locknuts had been installed in the elevator operating system, which maintenance crews failed to notice. These spun off during the flight, leaving the elevator uncontrollable. This disabled the aircraft's pitch channel, causing the crash.

Aftermath
This and the China Northwest Airlines Flight 2303 disaster contributed to the decision to remove all Tupolev Tu-154 aircraft in China from service on October 30, 2002.