What happened
During a descent from cruise altitude in preparation for landing, the flight crew observed that the horizontal stabilizer failed to respond to trim inputs. In response to the malfunction, the crew disengaged the autopilot and attempted to use both normal and alternate modes of stabilizer trim, but neither method was successful.
The crew implemented the jammed stabilizer approach checklist. They managed to maintain pitch control by having both crew members exert physical force on their respective control yokes to effect necessary pitch changes.
The investigation
Post-incident inspection of the stabilizer drive actuator revealed no evidence of hydraulic leaks. Upon draining the actuator's gearbox, investigators found only approximately one ounce of murky, sludge-like hydraulic fluid. The gearbox is specified to require a total of seven ounces of hydraulic fluid. It was noted that actuators are delivered from the manufacturer essentially empty, with specific instructions requiring them to be filled prior to being placed into service.