What happened
On 13 July 1999, a BAe 146-300, registration G-BSNS, was performing a scheduled passenger flight from Stansted to Edinburgh. While climbing through 10,500 feet and executing a gentle right-hand turn, the aircraft experienced a sudden and violent yaw to the right. This movement was accompanied by a significant roll, which caused a cabin trolley to tip and meal trays to slide off passenger tables.
As the yaw occurred, the yaw damper disengaged, followed shortly by the disengagement of the autopilot. The aircraft rolled to a bank angle of approximately 50 degrees before the pilot was able to arrest the motion and regain level flight. There were no injuries to the 103 passengers or 5 crew members on board, and the aircraft sustained no damage. Following the incident, the pilot elected to return to Stansted, flying with gentle bank angles to ensure safety. The aircraft landed normally.
The investigation
An investigation involving the aircraft manufacturer analyzed Flight Data Recorder (FDR) information to reconstruct the event. The data revealed that a 1/5-degree rudder 'step input' to the left occurred during the turn, which triggered the immediate disconnection of the yaw damper. While the autopilot initially attempted to correct the heading using aileron, it disconnected approximately six seconds later, likely due to the excessive roll rate.
Technical examinations focused on the Vertical Reference Units (VRUs), which provide essential attitude signals to the flight computers. Investigators found that VRU No 1 had worn rotor bearings, while VRU No 2 exhibited fluid contamination on its secondary output circuit board. It was determined that the contaminated VRU No 2 provided incorrect roll attitude signals to the No 2 yaw damper computer, creating the rudder input that destabilized the aircraft.
Findings
- The primary cause of the yaw damper disconnection was an erroneous 1.5-degree rudder input caused by incorrect roll attitude signals from the No 2 Vertical Reference Unit.
- The No 2 VRU had suffered from fluid contamination on its secondary output circuit board.
- The pilot's subsequent manual inputs, while intended to stabilize the aircraft, likely temporarily exacerbated the Dutch roll tendency.
- The aircraft's aileron control cable tensions and left spoiler deployment angles were found to be outside of acceptable limits and were corrected during maintenance.
Safety action
- The aircraft manufacturer issued Service Information Letter SIL 25/23 to remind operators of an existing Service Bulletin regarding the installation of protective splash covers on VRUs.
- The aircraft involved was retrofitted with the protective splash cover to prevent future fluid ingress from the galley or toilet drains.