Incident Overview
During a climb to cruise transition, a mis-installed water filter housing lower seal allowed water to seep out and run down the housing case. Over time, water droplets fell onto a lavatory drain line and followed it to a low point, where they collected on the stabilizer augmentor (stab aug) computer box. The fluid accumulation caused corrosion on one of the circuit boards within the computer box, resulting in a power imbalance within the yaw damper system and producing an uncommanded rudder signal.
Sequence of Events
In the climb phase, a Mach trim failure occurred. The failure necessitated the use of the autopilot for speeds in excess of 0.7 Mach. However, operation of the autopilot resulted in activation of the yaw damper system, which had been producing uncommanded rudder inputs. As the aircraft was leveled at flight level 330 (FL330) and the airspeed allowed to exceed Mach 0.7, the copilot activated the autopilot. This powered the malfunctioning yaw damper system, and a Dutch roll maneuver ensued. The pilot-in-command disconnected the autopilot, executed a descent, and recovered the aircraft.
Mechanical Findings
The investigation identified that the lower seal of the water filter housing was mis-installed. This mis-installation permitted water to escape from the filter housing. The water then traveled via the lavatory drain line and accumulated on the stabilizer augmentor computer box. Corrosion developed on a circuit board within the computer, leading to a power imbalance in the yaw damper system. The yaw damper subsequently generated uncommanded rudder signals.
Operational Context
A Mach trim failure occurred during the climb. According to the investigation, this failure would have required autopilot engagement for speeds above Mach 0.7. When the autopilot was activated, it also engaged the yaw damper system, which was already producing uncommanded rudder inputs. The combination of high speed and the malfunctioning yaw damper resulted in a Dutch roll. The pilot's action of disconnecting the autopilot and descending allowed recovery.