Incident Description
During descent, the AFCAS stabilizer trim and autopilot disengaged. Both were subsequently reset, followed by a second disengagement. The flight crew experienced nose-down control pressures and attempted to use manual trim. A jammed stabilizer master warning then appeared. Per abnormal procedures, AFCAS stabilizer trim was left off.
Flight Crew Actions
Flight data recorder (FDR) data showed that the stabilizer then crept to a full aircraft nose-down position. Hydraulic pressure to the stabilizer actuator was automatically removed, as designed. Crew movement of the manual trim resulted in the jammed stabilizer warning.
Post-Incident Testing
Subsequent testing of the horizontal stabilizer control unit assembly revealed that, after spraying the servo valve with water and cold soaking, ice would form inside the end cap, leading to a jammed servo valve condition. The end cap and spring exhibited substantial rust corrosion.