What happened
On 15 April 2004, a Boeing 737-600, registration SE-DNU, was performing a scheduled flight from Stockholm/Arlanda to Ängelholm Airport. The landing on runway 14 initially proceeded without issue. However, as the aircraft slowed to approximately 60 knots and the pilot in command took control of the steering via the tiller, the nose wheel suddenly deflected to the right without pilot input.
The crew attempted to correct the course using the rudder, nose wheel steering, and differential braking, but the aircraft continued to yaw. After traveling an additional 100 meters, the aircraft left the runway surface, eventually coming to a stop with its nose wheel resting just outside the right runway verge. There were no injuries among the 110 passengers or 5 crew members, and passengers were able to disembark safely via mobile stairs.
The investigation
The Swedish Accident Investigation Board (SHK) examined the aircraft's flight data recorder, cockpit voice recorder, and the physical site of the occurrence. Technical inspections of the nose wheel steering system, including the steering actuators and the steering metering valve module, were conducted. While the investigation noted that the steering metering valve module had been replaced recently due to a hydraulic leak, no specific mechanical failure or defect was identified in the components during the post-incident testing.
Findings
- The nose wheel steering deflected spontaneously without any command from the pilots.
- No specific technical fault could be established through testing or inspection.
- The aircraft manufacturer has noted similar occurrences on this aircraft type, with a reported failure rate of less than one per 100,000 flights.
- The manufacturer suggests that small debris in the hydraulic system may cause a momentary jam in the steering control loop, leading to an uncontrolled swing.
- The manufacturer and the SHK both classify this type of occurrence as a "Major Event" rather than "Hazardous," noting that the risk is considered acceptable under international regulations given its low frequency.