What happened
On June 16, 2000, a Boeing 757-300 operating a charter flight to Cologne, Germany, experienced severe directional instability during its takeoff roll at Antalya, Turkey. As the aircraft accelerated, it underwent several unintended changes in heading, deviating up to 6 meters from the runway centerline. The pilot experienced physical jolts through the rudder pedals while attempting to correct the aircraft's path.
As the aircraft reached speeds between 85 and 155 knots, the heading fluctuated repeatedly. The pilot applied significant rudder pedal inputs—reaching up to 40% pedal travel—to counteract the deviations, but the aircraft failed to stabilize. Because of the potential for compromised controllability during landing, the crew declared an emergency and prepared the 249 passengers for an abnormal landing. The aircraft landed safely and taxied to its parking position without further incident.
The investigation
Following the emergency declaration, the BFU investigated the incident in coordination with the airline. Initial inspections of the rudder, brakes, engines, and wheels at the destination airport in Cologne were inconclusive. The aircraft was subsequently moved to Frankfurt for a more detailed technical examination.
Investigators analyzed the Flight Data Recorder (FDR), which revealed a pattern of increasing directional deviations and lateral accelerations (ranging from -0.2g to +0.15g) as the takeoff speed increased. The investigation also included a review of the nose wheel steering system and the yaw damper/stabilizer trim control units (YSM). During the inspection of the nose wheel steering, investigators found that the cable tension was only 45 lbs, significantly lower than the Boeing-mandated 62 lbs, and that the hydraulic neutral position was off-center.
Findings
- The primary cause of the instability was insufficient cable tension in the nose wheel steering system.
- The low tension caused a delay (lag) in the nose wheel's response to rudder pedal inputs. While the rudder deflected immediately, the nose wheels followed with a delay, creating a mismatch in steering forces.
- This lag made the steering system highly sluggish, preventing the pilot from applying the precise, modulated inputs required to maintain the centerline at high speeds, leading to overcorrections.
- The investigation identified a manufacturing defect: Boeing had not pre-tensioned the control cables in certain Boeing 757-300 aircraft produced between November 15, 1998, and June 20, 2000.
- FDR data indicated that previous flights by the same aircraft had also exhibited unstable directional control during takeoff, though the limits of controllability were only exceeded during this specific event.