Flight Crew Encounters Jammed Horizontal Stabilizer Trim During Descent; Post-Incident Inspection Reveals Low Hydraulic Fluid in Actuator Gearbox (N479AU)

Details

During a descent from cruise altitude in preparation for landing, the flight crew observed that the horizontal stabilizer was not responding to trim inputs. The autopilot was disengaged at the time. Both normal and alternate modes of stabilizer trim were attempted without success. The crew then followed the jammed stabilizer approach checklist. They were able to effect pitch changes by both crew members exerting force on their control yokes.

Inspection Findings

After the incident, an inspection of the stabilizer drive actuator was conducted. No hydraulic leaks were found on the actuator. The actuator's gear box was drained, yielding approximately one ounce of murky, sludge-like hydraulic fluid. The gear box is required to be filled with seven ounces of hydraulic fluid. According to the report, the actuators are shipped from the manufacturer essentially devoid of hydraulic fluid, with instructions that they must be filled prior to service.