What happened
During the climb to cruise transition, a Mach trim failure occurred on the aircraft. This malfunction necessitated the use of the autopilot for flight speeds exceeding Mach 0.7. As the aircraft leveled at Flight Level 330 and airspeed increased beyond this threshold, the first officer engaged the autopilot. However, the engagement also powered the yaw damper system, which was subsequently found to be producing uncommanded rudder inputs. This sequence triggered a Dutch roll maneuver. The pilot in command disconnected the autopilot and successfully executed a descent and recovery.
The investigation
The investigation determined that the root cause of the yaw damper malfunction was water ingress. A water filter housing lower seal had been mis-installed, allowing water to seep out and run down the housing case. Over time, this water dropped onto a lavatory drain line, followed the line to a low point, and collected on the stabilizer augmentation computer box. The accumulated fluid resulted in corrosion on one of the circuit boards within the computer box. This corrosion caused a power imbalance within the yaw damper system, leading to the uncommanded rudder signal.
Findings
The primary contributing factor was the mis-installation of the water filter housing lower seal. This maintenance error allowed water to reach sensitive avionics components. The subsequent corrosion of the circuit board created a power imbalance in the yaw damper system. When combined with the Mach trim failure, which required autopilot usage at high speeds, the malfunctioning damper system became active during a critical phase of flight, resulting in the Dutch roll.
Safety message
Proper installation of seals and gaskets is critical to prevent fluid ingress into avionics compartments. Maintenance procedures must ensure that water filter housings are sealed correctly to protect downstream electrical components from corrosion and short circuits.