Boeing 737-86N, VH-VOG, Melbourne Airport, Victoria, on 4 March 2005

An inspection of the aircraft by maintenance engineers on arrival in Brisbane did not find any fault with the main or standby rudder power control unit (PCU). The B737 PCU is a hydraulic mechanism that moves the rudder in response to inputs from either the pilot or the yaw damper. The aircraft operator has not reported any further rudder occurrences with the aircraft.

The aerodrome terminal information service that was valid at the time of the indicated that the surface wind was 190 degrees magnetic at 12 to 15 kts. Wind data that was recorded at the airport at the approximate time of the event indicated a varying, but generally south-south-easterly wind at a speed of 4 to 10 kts. The crew reported that another passenger aircraft had landed on the crossing runway prior to their departure.

The airport operator's inspection report included morning and midday inspections of the airport runway surfaces. The morning inspection was at 0727 and the midday inspection was carried out at 1450. Neither inspection identified any problems with the runway 27 surface.

The Flight Data Recorder (FDR) information was recovered for examination by the ATSB. The examination revealed that a right lateral acceleration, with a peak of approximately 0.17g had occurred at 125 kts, (refer Figure 1).

The ATSB referred the FDR information to the aircraft manufacturer for review.

The manufacturer reported that the recorded rudder and yaw damper inputs were in response to the acceleration, and did not initiate it. The recorded rudder data indicated that all rudder movements were commanded by the crew and/or the yaw damper. The PIC reported that he felt the rudder pedal move under his feet. The FDR recorded a 5 kt headwind increase during the take-off roll that may have been the result of an increasing crosswind.

The manufacturer conducted additional analysis with a flight simulator, to better understand the rudder pedal movements, lateral acceleration and heading data recorded during the occurrence. The simulation results indicated that a 7 kt right quartering headwind gust, followed by a 13 kt left quartering gust, would be needed to match the FDR lateral acceleration data.