Boeing 737-86N, VH-VOG, Melbourne, Airport, Vic, 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 takeoff 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).

Chart showing the increase in computed air speed and right lateral acceleration

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.

The ATSB investigation determined that the increase in computed
airspeed recorded at the time of the lateral acceleration was
likely due to an atmospheric disturbance, which resulted in the
aircraft veering unexpectedly. The ambient wind recorded at the
time of the occurrence did not indicate if the crosswind increased,
or decreased, during the event. Wake turbulence was considered
unlikely, as the reported landing of the other passenger aircraft
was downwind of the departure runway.

Factual Information

On 4 March 2005, at 1405 Eastern Daylight-saving Time, a Boeing Company 737-86N aircraft, registered VH-VOG, was being operated on a scheduled passenger service from Melbourne to Brisbane. The pilot in command (PIC) reported that during the take-off roll, the aircraft unexpectedly deviated to the right of the runway 27 centreline and the nosewheel felt as though it ‘had been caught in a groove’. The PIC applied left rudder to regain directional control and continued the takeoff and flight to Brisbane. The circumstances of this incident were similar to those identified in a previous Australian Transport Safety Bureau (ATSB) investigation, (see investigation report 199703237).