Event Overview
On 14 August 1999, a serious incident occurred involving a Boeing 757-300 on a flight from Leipzig to Ibiza, Spain. During the landing roll on runway 24, after touchdown about 500 meters past the threshold, the aircraft decelerated using autobrake setting 2 and thrust reversers. At approximately 80 knots, the aircraft unexpectedly turned left. Despite full right rudder input and application of the right wheel brakes, the crew could not prevent the aircraft from departing the runway. All 8 crew members and 234 passengers remained uninjured. The aircraft sustained no damage beyond tire wear, and a runway edge light was destroyed.
Flight Data Recorder Analysis
Analysis of the flight data recorder (FDR) and cockpit voice recorder (CVR) confirmed the crew's account. The FDR data showed that from about 25 seconds before the aircraft came to a stop, the magnetic heading began to change, indicating a left yaw. The pilot flying applied full right rudder, right brake, and right aileron, but the yaw momentarily increased before halting for about two seconds as the aircraft exited the paved surface. The nose wheel tire marks on the runway showed a curved path consistent with a constant steering angle, with no evidence of a large deflection. Reconstruction of the ground track using groundspeed and heading indicated a turn radius corresponding to a nose wheel steering angle of approximately 1.5 degrees.
Nose Wheel Steering Investigation
The investigation focused on the nose wheel steering system. The steering control valve was removed and sent to Boeing for examination, but no mechanical cause for blockage was found. However, the report concluded that the nose wheel steering was blocked in a slightly deflected position, likely from a closed control valve after a deflection of about 1.5 degrees left. This blockage caused the aircraft to roll on a constant-radius curve, and the effectiveness of the steering was further reduced by variations in nose wheel load during braking. The report noted that the tiller (nose wheel steering handle) in this aircraft had been installed 90 degrees offset from its normal orientation, but no connection between this misinstallation and the incident could be established.
Probable Cause
The investigation determined that a blockage of the nose wheel steering control valve was the cause of the runway excursion. The blockage likely occurred in the closed position, resulting in a steady left deflection of the nose wheel of approximately 1.5 degrees, which the crew could not overcome despite full corrective inputs.