While on final approach to Brisbane at about 1,000 ft, the crew of the Boeing 737 reported feeling a 'kick' in the rudder pedals accompanied by a minor aircraft yaw oscillation. It was reported that there was no aircraft ahead to cause wake turbulence.
Investigation by the aircraft operator suggested a problem with the rudder power control unit. The aircraft manufacturer recommended that the standby rudder actuator and the standby input rod bearings should also be examined for wear; no wear was evident.
The rudder power control unit had accumulated 3,064 hours since its overhaul by the manufacturer in May 1999. It was removed from the aircraft and forwarded to the USA for examination.
The examination was conducted by the component manufacturer and supervised by the National Transportation Safety Board of the USA on behalf of the Australian Transport Safety Bureau. No discrepancies that may have led to the anomaly in the operation of the rudder power control unit were found. The internal and external components contained no evidence of excessive wear, damage or overtravel and met the manufacturer's standards for in-service units.
In support of the investigation, the National Transportation Safety Board also conducted a performance simulation study based on the actual aircraft configuration data at the time of the incident. The study concluded that the rudder had oscillated.
The investigation was unable to determine why the rudder reportedly oscillated.