What happened
On August 14, 1999, a Boeing 757-300 operating a flight from Leipzig to Ibiza, Spain, experienced a serious malfunction during its landing roll on runway 24. After touchdown, the crew applied autobrake 2 and reverse thrust. As the aircraft decelerated to approximately 80 knots, it suddenly veered to the left. Despite the pilot's immediate efforts to correct the deviation using full right rudder and applying the right wheel brakes, the aircraft could not be maintained on the runway and exited the paved surface. The excursion resulted in the destruction of a runway light and minor damage to the aircraft's tires, but there were no injuries among the 234 passengers and 8 crew members.
The investigation
The BFU conducted the investigation in coordination with Spanish authorities. Investigators analyzed the flight data recorder and cockpit voice recorder, which had been transported to Germany for evaluation. The analysis of the flight data provided critical insights into the aircraft's parameters, specifically the magnetic heading and lateral acceleration. The investigation also involved a technical examination of the aircraft's braking system, thrust reversers, and nosewheel steering. A specific component, the steering control valve, was removed and sent to Boeing for detailed inspection. Additionally, investigators reviewed tire marks on the runway and analyzed the aircraft's steering geometry.
Findings
Data from the flight recorders confirmed that the aircraft's nosewheel steering became ineffective during the landing roll. While the manufacturer's initial report suggested the steering control valve might have blocked in an open position, the BFU investigation concluded that the steering control valve blocked in a closed position while the nosewheel was deflected approximately 1.5 degrees to the left. This blockage prevented the pilot from correcting the leftward turn. The investigation also noted that the effectiveness of the steering was further reduced by the specific load distribution on the wheels caused by the use of reverse thrust and braking. Although a 90-degree misalignment of the nosewheel steering tiller was noted, no direct link was found between this installation error and the specific failure.
Safety action
The investigation highlighted a critical vulnerability: a single system failure was sufficient to transition the aircraft into an uncontrolled state, which could have resulted in a much more serious accident. This finding contradicts fundamental flight safety design concepts, necessitating a review of system redundancy for steering components.