What happened
On 4 October 2016, a Boeing 737-4Q8, registration OE-IAG, was performing a commercial cargo flight from East Midlands Airport to Belfast International Airport. Upon landing on Runway 25, the crew experienced a sudden and intense vibration that increased in severity during the landing rollout. Suspecting a tyre failure, the crew deactivated the automatic braking and thrust reversers, as there was sufficient runway remaining to bring the aircraft to a halt. The aircraft's ground track began to oscillate near the runway centreline before finally coming to a stop at the intersection with Runway 17/35. There were no injuries to the two crew members on board.
The investigation
The AAIB investigation examined the aircraft's flight data recorder, cockpit voice recorder, and the physical condition of the landing gear. Inspection of the right main landing gear revealed that both wheels were skewed, with significant damage to the tyres and wheel hubs. Investigators recovered debris from the runway, including pieces of bushings from the apex where the upper and lower torsion links meet. Metallurgical analysis of the fractured lower torsion link concluded that the component had failed due to an overload rather than fatigue. The investigation also reviewed maintenance records, which showed that recent checks of the torsion links had found no significant issues, and assessed the manufacturer's data regarding landing speeds and wind conditions.
Findings
- The fracture of the lower torsion link on the right main landing gear was caused by excessive landing gear vibration (shimmy).
- The aircraft touched down at a speed of approximately 148 kt, which was VREF + 7, slightly higher than the recommended approach speed for a tailwind.
- The presence of a 5-knot tailwind increased the groundspeed at touchdown to 153 knots, a factor known to increase the risk of shimmy events.
- It could not be determined if the vibration was triggered by a failure in the shimmy damper or excessive freeplay in the torsion link joints.
- There was no evidence of a pre-existing defect in the failed torsion link or the apex joint itself.