What happened
On 15 October 2001, an Avro 146-RJ85, registration D-AVRO, was preparing for a scheduled passenger flight from London City Airport to Frankfurt. During taxiing, the crew identified a yaw damper failure light. Following the airline's Minimum Equipment List, the commander determined the aircraft could proceed with the yaw damper inoperative, provided the airspeed remained below 240 knots.
After takeoff, while the aircraft was established in level flight at 3,000 feet, the crew experienced a noticeable tendency for the aircraft to yaw and roll to the right. The handling pilot disconnected the autopilot after struggling to maintain a steady heading. Upon taking control, the commander discovered that the rudder pedals felt restricted when attempting to move them to the left. To maintain a level wing attitude, the pilot had to fly with a left sideslip, and attempts to use the rudder trim provided no relief.
The crew declared an emergency and requested radar vectors to London Stansted Airport. During the final approach to Runway 23, the commander noted that the rudder restriction appeared to diminish, although the yawing and rolling tendencies persisted. The aircraft landed without further incident, and there were no injuries among the 5 crew members or 65 passengers.
The investigation
Following the landing, maintenance engineers inspected the aircraft at London Stansted. They found the rudder deflected approximately 2 to 3 degrees to the right despite the trim being set to neutral. An interrogation of the Digital Flight Guidance System revealed a fault message logged on the ground, indicating a yaw damper failure caused by a synchronization disparity.
Testing of the yaw damper actuator servo eventually led to its replacement. Subsequent analysis by the manufacturer revealed that carbon buildup from worn motor brushes had caused electrical shorts, which could trigger the application of the yaw damper actuator brake. Additionally, investigators found that the Q-pot—a component designed to restrict rudder travel at high speeds—was out of adjustment.
Findings
- The initial yaw damper failure occurred while the aircraft was on the ground, causing the actuator to lock the rudder at an offset angle.
- The mis-adjusted Q-pot caused the rudder pedals to feel restricted, as the device was effectively applying a high-speed restriction even at lower airspeeds.
- The restriction became less noticeable during the final approach as the aircraft's airspeed decreased.
- The buildup of carbon in the yaw damper actuator servo was identified as a known issue that could cause intermittent electrical faults.