What happened
On 1 March 2010, a scheduled passenger flight operated by a Boeing 737-8AS, registration EI-DYI, was traveling from Eindhoven to Madrid when the crew detected unusual vibrations throughout the airframe. Due to the instability, the flight crew elected to divert the aircraft to Charleroi Airport in Belgium. There were no fatalities and no injuries among the 140 passengers or 6 crew members on board.
Upon landing, engineering inspections revealed that the left-hand stabilizer tab control mechanism had sustained damage. Specifically, certain frame assemblies had broken away from the tab mechanism assembly, and the oscillation of the elevator tab had caused accelerated wear to the mechanism.
The investigation
The investigation, monitored by the AAIU, involved the aircraft manufacturer and the US National Transportation Safety Board. Analysis of Flight Data Recorder (FDR) data indicated that the most intense vibrations occurred at an indicated airspeed of approximately 300 knots.
Subsequent inspections of other aircraft in the fleet were conducted to identify similar wear. The investigation established that the failure originated from the wear of the bearing swage ring at the aft attach lugs of the elevator tab control mechanism. This degradation allowed the bearing to migrate or caused the lugs to separate, leading to the severe oscillations experienced during flight.
Findings
- The aircraft sustained damage to the left-hand stabilizer tab control mechanism.
- The mechanical failure resulted in significant in-flight vibrations, necessitating an emergency diversion.
- The root cause was identified as bearing swage wear resulting from improper tool usage and workmanship escapement during the manufacturing process of the component.
- The design specification may have been inadequate for handling multiple layer clamp-up, contributing to unexpected wear at the joint.