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At approximately 0955 Central Daylight-saving Time on 8 Jan 2007, the flight crew commenced the take-off roll on runway 23 in a Boeing Company 737-838 aircraft, registered VH-VXG, on a scheduled passenger service from Adelaide, SA to Alice Springs NT.

At a speed of approximately 140 kts, the crew reported an abrupt, uncommanded yaw. Corrective action was applied, engine parameters checked, and the takeoff was continued without further incident. The crew advised Air Traffic Control of the uncommanded yaw and contacted the operator's maintenance watch for advice. The crew subsequently returned the aircraft to Adelaide Airport. The wind at the time was reported to be light (approximately 3 kts) from the east.

Data from the aircraft's Flight Data Recorder was recovered and downloaded by the Australian Transport Safety Bureau (ATSB) for review. That review indicated that the input to the aircraft rudder was not uncommanded and that the rudder pedals moved proportionally to the rudder surface deflection at all times. An engineering examination of the aircraft did not identify any reason for the uncommanded yaw and the aircraft was released back to service.

Due to a previous, similar, event at Adelaide Airport on 15 Dec 2006 (ATSB occurrence 200607627), the aircraft operator sought advice from the aircraft manufacturer. The aircraft manufacturer reviewed the data from the Flight Data Recorder and concluded that the recorded event was not a result of an uncommanded aircraft rudder input, asymmetric thrust, nose-wheel steering or asymmetric brake application.

While the nature of the uncommanded yaw could not be positively identified, it is likely that the event was related to an atmospheric disturbance during the take-off run.

The ATSB continues to monitor such reported uncommanded yaw events and has reported similar events in the past (see occurrence reports 200607627, 200500994 and 199703237 available on the ATSB website: www.atsb.gov.au).

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