What happened
On 11 May 2010, an Airbus A330-301, registration EI-ORD, was performing a scheduled flight from Dublin to Chicago O’Hare. While cruising at 33,000 feet over Iceland, the aircraft encountered moderate turbulence. During the autopilot's attempt to stabilize the aircraft, a structural failure occurred on the right-hand wing. Specifically, the outer bracket supporting the outside servo-control of the right-hand inboard aileron fractured, followed by the failure of the inner bracket due to overload. This caused the right-hand inboard aileron to oscillate significantly and settle in a deflected position.
Despite the asymmetric control surface configuration, the flight crew observed that the aircraft remained controllable via the autopilot and manual inputs. After troubleshooting the performance loss and verifying the aircraft's handling, the crew decided to proceed to their destination. The aircraft landed at Chicago O’Hare without further incident, though maintenance personnel later discovered a crack in a corresponding bracket on the left-hand wing.
The investigation
The AAIU examined the mechanical failure of the aileron assembly and the effectiveness of previous maintenance inspections. The investigation focused on the fractured brackets (part number F575-50986) and the underlying cause of the metal fatigue. Investigators analyzed the manufacturing history of the components, specifically looking at the surface finish of the brackets.
Testing revealed that the failure originated from a small pit, approximately 50 microns deep, on the surface of the outer bracket. This pit likely existed since the part's manufacture due to an older, manual "pickling" process used before 2003. Furthermore, the investigation looked into a previous Service Bulletin (SB) intended to detect such cracks. While the aircraft had undergone the recommended high-frequency eddy current (HFEC) inspection in December 2009, the inspection failed to identify the cracks that were likely already present.