What happened
On February 13, 1997, at approximately 20:20 local time, two aircraft experienced a loss of separation in controlled airspace south of Bromma, Sweden. The first aircraft, a Fokker F28 MK 4000 (registration SE-DGH) operating a scheduled flight from Ronneby, was descending toward Stockholm/Arlanda. The second aircraft, a Learjet 60 (registration TC-MEK) on a non-scheduled passenger flight from Bromma to Istanbul, was climbing through the same sector.
Air traffic controllers had coordinated an altitude separation of 1,000 feet between the two flights, instructing the Fokker F28 MK 4000 to maintain at least flight level 90 and the Learjet 60 to climb only to flight level 80. However, as the aircraft approached each other, radar data indicated that the Learjet 60 had climbed through its assigned altitude. At the point of closest approach, the aircraft were separated by only 0.4 nautical miles laterally and 500 feet vertically, failing to meet the required separation standards.
The investigation
The Swedish Board of Accident Investigation examined the flight paths, radar recordings, and cockpit communications. The investigation focused on why the Learjet 60 exceeded its cleared altitude and why the controllers were unable to maintain separation.
Investigators reviewed the technical status of the Learjet 60's instruments and the actions taken by the pilots of both aircraft. The investigation also looked into the impact of the aircraft's performance characteristics on the ability of the crew to respond to altitude deviations during the climb.