A Boeing 747-SP38 aircraft was maintaining Flight Level (FL) 430
with autopilot `A' engaged, when the aircraft yawed abruptly to the
right and rolled to a bank angle of approximately 20 degrees. The
autopilot was disengaged and the aircraft stabilised in a straight
and level attitude. The uncommanded yaw occurred again. The flight
crew broadcast a PAN (radio code indicating uncertainty or alert,
not yet the level of a Mayday) and received a descent authorisation
to FL380.
The upper rudder position indicator showed a rudder displacement
of 5-degrees right and the lower rudder indicator showed zero
degrees deflection. The flight crew began activating and
de-activating the upper and lower yaw damper switches attempting to
isolate the problem. During those actions, the aircraft commenced
to `Dutch roll' (lateral oscillations with both rolling and yawing
components). The crew then successfully isolated the problem to the
upper damper and turned the upper damper switch off. With the
aircraft at FL380, normal operations ensued. Autopilot `B' was then
engaged and the flight proceeded without further incident.
Investigation by company maintenance personnel confirmed an
anomaly of the upper yaw damper computer. The unit was replaced and
the system tested. Normal operations ensued.
Analysis of Flight Data Recorder information revealed that
during the event the upper rudder displaced 4.7 degrees. The data
also indicated that the maximum roll encountered was 13 degrees to
the right.
System redundancy had operated as required to limit the effect
of the upper yaw damper anomaly.