Investigation and causes of Wenzhou air crash accident

According to the information analysis provided by flight data recorder (FDR) and cockpit voice recorder (CVR), the flight was normal before the pin fell off, and after the pin fell off, the control of the elevator by the control lever was abnormal whether it was automatic driving or manual control flight, and then the pilot felt that the pitch control of the aircraft was abnormal. Because the center of gravity of the aircraft has not changed much at this time, the crew can barely keep the aircraft descending after taking measures such as moving passengers forward and releasing spoilers. With the release of the landing gear, the aircraft produces a pitching moment, and the pilot pulls the pull rod to try to keep the aircraft in this state. However, due to the abnormal operation of the elevator, the plane continued to dive. Abnormal manipulation; The pilot increased the number of tie rods. At this time, as shown in the ground test, because the π3 tie rod is in contact with the 135 rocker arm, the elevator suddenly inclines upward, and the aircraft inclines upward sharply. In order to overcome this violent upward trend, the pilot quickly put down the putter. Because the pitch control lost its linear variation law, the elevator quickly deflected downward to the maximum, and the plane swooped to the ground greatly. Finally, although the pilot tried his best to pull the rod, there was no corresponding change in the rudder surface, and the plane failed to get out of the dive state.

Through investigation and evidence collection, experimental analysis of the wreckage, ground test and simulator verification, and information provided by flight data recorder (FDR) and cockpit sound recorder (CVR), the following points can be confirmed:

65438 +0. B-2622 crashed on the ground due to the loss of control of the pitch channel during the descent approach at Wenzhou Airport.

2. The reason why the aircraft pitch channel is out of control is that the π3 pull rod of the aircraft elevator control system is disconnected from the 135 rocker arm in flight, and the elevator control fails.

3. According to the test and analysis, the most likely reason for the separation of π3 tie rod and 135 lattice arm is that the self-locking nut is installed on the connecting bolt of tie rod and rocker arm, instead of the flower nut with split pin specified in the specification, and the nut is larger than the bolt, which can not guarantee the function of restricting movement.

Although a lot of investigation work has been done, it is still uncertain whether the self-locking nut was installed at the connection between the pull rod and the rocker arm during the overhaul in Russia or after the maintenance in Southwest Airlines.