Aviation safety investigations & reports

Boeing Co 747SP-38, VH-EAA

Investigation number:
Status: Completed
Investigation completed


A Boeing 747-SP38 aircraft was maintaining Flight Level (FL) 430 with autopilot `A' engaged, when the aircraft yawed abruptly to the right and rolled to a bank angle of approximately 20 degrees. The autopilot was disengaged and the aircraft stabilised in a straight and level attitude. The uncommanded yaw occurred again. The flight crew broadcast a PAN (radio code indicating uncertainty or alert, not yet the level of a Mayday) and received a descent authorisation to FL380.

The upper rudder position indicator showed a rudder displacement of 5-degrees right and the lower rudder indicator showed zero degrees deflection. The flight crew began activating and de-activating the upper and lower yaw damper switches attempting to isolate the problem. During those actions, the aircraft commenced to `Dutch roll' (lateral oscillations with both rolling and yawing components). The crew then successfully isolated the problem to the upper damper and turned the upper damper switch off. With the aircraft at FL380, normal operations ensued. Autopilot `B' was then engaged and the flight proceeded without further incident.

Investigation by company maintenance personnel confirmed an anomaly of the upper yaw damper computer. The unit was replaced and the system tested. Normal operations ensued.

Analysis of Flight Data Recorder information revealed that during the event the upper rudder displaced 4.7 degrees. The data also indicated that the maximum roll encountered was 13 degrees to the right.

System redundancy had operated as required to limit the effect of the upper yaw damper anomaly.

General details
Date: 13 November 2001   Investigation status: Completed  
Time: 1715 hours CSuT    
Location   (show map): Abeam Moomba    
State: South Australia   Occurrence type: Flight control systems  
Release date: 25 March 2002   Occurrence category: Incident  
Report status: Final   Highest injury level: None  

Aircraft details

Aircraft details
Aircraft manufacturer The Boeing Company  
Aircraft model 747  
Aircraft registration VH-EAA  
Serial number 22495  
Type of operation Air Transport High Capacity  
Damage to aircraft Nil  
Departure point SINGAPORE  
Destination Sydney, NSW  
Last update 13 May 2014