What happened
On 21 February 2004, a Boeing 737-59D, registration G-BVKC, was completing a passenger flight from Malaga to Cardiff Airport. Following a normal touchdown on Runway 30, the crew applied manual braking, which immediately triggered a violent yaw oscillation, known as a shimmy, in the left main landing gear. The crew noted the aircraft pulling sharply to the left and experienced intense vibrations that were severe enough to dislodge items from an overhead locker.
As the aircraft decelerated, the crew attempted to manage the vibration by reducing braking pressure, which temporarily eased the oscillation. However, when braking was increased again at approximately 60 kt, an even more intense period of vibration occurred. This second episode caused the left main landing gear lower torsion link to fracture, leading to significant damage to the tyre, wheel, and brake assembly. The aircraft eventually came to a halt on the runway, approximately 439 metres from the end of the runway. There were no fatalities and no injuries to the 6 crew members or 114 passengers on board.
The investigation
The AAIB investigation focused on the mechanical failure of the landing gear components and the maintenance history of the aircraft. Investigators examined the damaged left main landing gear, finding that the lower torsion link had broken due to overload. The examination also revealed that the shimmy damper piston rod had bent and the apex joint bushings had suffered compression collapse.
Investigators reviewed the aircraft's maintenance records and the manufacturer's technical documentation. They noted that while the aircraft had been maintained according to the manufacturer's recommendations, the procedures within the Aircraft Maintenance Manual (AMM) were inconsistent and difficult to follow. Specifically, the manual did not provide clear instructions for measuring the critical gap in the apex joint or a definitive requirement to replace components once wear was detected.
Findings
- The primary cause of the incident was excessive wear in the torsion link apex joint, which reduced the effectiveness of the shimmy damper.
- The initial shimmy caused the initial damage to the torsion link, and the subsequent fracture of the link led to even higher amplitude oscillations.
- The intense vibrations resulted in substantial damage to the left main landing gear tyre, wheel, and brake unit, as well as parts of the damper body.
- Maintenance procedures in the AMM were found to be difficult to follow and lacked the clarity needed to ensure the apex joint gap remained within the required 0.005-inch limit.
- Evidence of in-service wear was present on the thrust washer and piston rod shoulder, contributing to the reduction in damping effectiveness.