What happened
On 30 January 1998, a Bae 146/RJ, registration YL-BAN, operating a scheduled flight from Stockholm/Arlanda to Tallinn, experienced a significant systems failure immediately after takeoff. Just prior to liftoff, the aircraft suffered a technical mishap that caused the failure of multiple automated functions, including the autopilot, flight guidance computer, yaw damper, and flight director. This failure also triggered several warning lights throughout the cockpit.
While the copilot continued to fly the aircraft using reserve instruments, the commander focused on troubleshooting the failed systems by attempting to restart them. During this period of high workload and distraction, the aircraft climbed beyond its cleared altitude of FL 70, reaching as high as FL 75. Simultaneously, a Fokker F28 MK0100, registration SE-DUR, was descending through the same airspace toward Arlanda.
As the flight paths of the two aircraft intersected, the air traffic controller observed the radar echoes merge, indicating that the required separation had been lost. A military radar plot later confirmed that the vertical separation between the two aircraft dropped below the 1,000-foot minimum approximately 20 seconds after their paths crossed. The aircraft eventually re-established the cleared altitude after the crew successfully reset the systems by cycling the power supply.
The investigation
The Swedish Board of Accident Investigation examined the flight data, radar plots, and crew statements. The investigation focused on the impact of the automation failure on the crew's ability to maintain flight parameters. Investigators analyzed the aircraft's performance, noting that the high-performance nature of the Bae 146/RJ and its relatively light weight allowed for a steep climb that contributed to the altitude overshoot.
Findings
- The primary cause of the separation loss was an altitude overshoot occurring in instrument meteorological conditions (IMC) while the crew was operating on reserve instruments.
- The crew's ability to monitor the flight was compromised because the flying pilot had no assistance from automated monitoring systems, and the non-flying pilot was preoccupied with troubleshooting.
- The continuous blinking of warning lights and the complexity of the technical failure served as significant distractions.
- The investigation highlighted a need for improved Crew Resource Management (CRM) and more rigorous simulator training for handling failures of highly automated flight systems.
Safety action
Following the incident, the airline issued internal recommendations regarding incident information sharing and increased simulator training for procedures involving the failure of autopilot, thrust rating panels, and flight directors. The Swedish Civil Aviation Administration was also advised to ensure that airlines operating highly automated aircraft regularly practice flight on reserve instruments during simulator sessions.