What happened
On November 27, 1998, a B737-400, registration EI-BXB, was performing a public transport flight on approach to runway 28 at Dublin Airport. While the aircraft was approximately 500 feet above the ground, the First Officer, who was flying the aircraft, noticed a significant aerodynamic imbalance. To maintain level wings, the pilot was forced to apply and hold 5 to 6 units of right aileron. Due to the low altitude, the crew had minimal opportunity to troubleshoot the issue before touchdown, and the aircraft landed and taxied normally.
The investigation
Following the incident, investigators examined the aircraft's spoiler and speed brake system. They identified that the outboard spoiler on the left wing (position "0") had become disconnected from its actuator. Specifically, the eye-end of the actuator had separated from the actuator shaft.
Technical analysis revealed that severe corrosion on the threaded portions of both the shaft and the eye-end had caused the separation. The investigation noted that this specific actuator had been in service since the aircraft's delivery in October 1989. While the manufacturer had previously identified similar issues in the B737-200 series and recommended applying sealant to prevent thread corrosion, this particular component had not been treated with the recommended sealant.