What happened
On 15 January 2003, a McDonnell Douglas MD-11F, registration N583FE, was conducting a cargo flight from Paris, Charles De Gaulle Airport, to Stansted Airport, with a subsequent leg to Newark. The flight crew, which included a training captain and a trainee first officer, was preparing for landing on Runway 23. To facilitate the trainee's learning, the crew had planned to use a full flap setting of 50°.
During the approach at approximately 4,000 feet, the crew initially selected flap 35°. The handling pilot noted a requirement for minor rudder trim to maintain balance. Upon transitioning to flap 50°, the captain reported a physical impact sensation, similar to a birdstrike. Immediately following this, the aircraft exhibited a significant roll tendency, requiring the pilot to apply substantial right aileron to maintain level flight. The crew attempted to rectify the imbalance by reverting to flap 3ally 35°, which mitigated the roll tendency. After a brief assessment, the crew re-selected flap 50°, which caused the roll tendency to return. The aircraft subsequently completed a normal landing at Stansted.
During the post-landing taxi, the crew retracted the flaps. Shortly after, the aircraft's electronic instrument system issued warnings regarding low quantity and failure in the No 2 hydraulic system. A subsequent inspection revealed that a large portion of the left inboard flap vane had been lost during the flight.
The investigation
The investigation focused on the structural integrity of the flap vane and the mechanical cause of its detachment. Physical examination of the McDonnell Douglas MD-11F showed that the outboard half of the left inboard flap vane, including its attachment fittings, was missing. A portion of the vane was later recovered from a village approximately 6 miles northeast of the airfield.
Analysis of the remaining vane structure revealed significant distortion to the inboard and centre tracks. Investigators found a locking plate and nut inside the flap, suggesting these components had detached while the flaps were in a retracted position. Microscopic analysis of the fractured lock wire showed evidence of a brittle fracture caused by fatigue, likely initiated by a nick or groove in the wire which may have been caused by contact with a nut corner.
Findings
- The outboard rail of the flap vane had likely detached prior to the approach, causing the initial trim imbalance at flap 35°.
- Aerodynamic forces caused the remaining attached sections of the vane to fail once the flaps were extended to 50°.
- The loss of the vane created an asymmetry of lift, necessitating heavy aileron input to maintain wings level.
- The failure of the vane attachment was linked to the fatigue fracture of the locking wire.
- The aircraft was using a pre-modification design for the vane attachment, whereas a Service Bulletin (MD11-57-034) had previously recommended an optional modification to use bolts and cotter pins to prevent similar occurrences.