What happened
During a descent phase of flight, a Dornier 328-300 aircraft encountered uncommanded rudder oscillations. At the onset of these vibrations, the autopilot system was engaged. The crew disengaged the autopilot, but the oscillations continued despite this action.
The investigation
Following the incident, the integrated avionics computer (IAC) was replaced as a precautionary measure. The airplane underwent a maintenance test flight to verify the repair. No problems were noted during this test, and the aircraft was returned to service. A subsequent examination of the removed IAC found no direct link between the rudder oscillations and the computer unit.
Findings
Seven months after the event, the issue had not reoccurred. The cause of the oscillations was not determined.