What happened
While the aircraft was in the descent phase of flight, the AFCAS stabilizer trim and the autopilot disengaged. The flight crew attempted to reset both systems; however, a second disengagement occurred. Following these events, the crew encountered nose-down control pressures. In an attempt to rectify the situation, the crew utilized the manual trim, which subsequently triggered a jammed stabilizer master warning.
In accordance with established abnormal procedures, the AFCAS stabilizer trim was left in the off position.
The investigation
Flight Data Recorder (FDR) analysis revealed that the horizontal stabilizer then crept to a full nose-down position. As part of the aircraft's design, hydraulic pressure to the stabilizer actuator was automatically removed during this event.
Subsequent testing of the horizontal stabilizer control unit assembly identified a mechanical failure mechanism. When the servo valve was sprayed with water and subjected to a cold soak, ice formed inside the end cap, resulting in a jammed servo valve condition. Investigators also found that the end cap and spring exhibited substantial rust corrosion. Additionally, it was determined that crew movement of the manual trim contributed to the activation of the jammed stabilizer warning.