What happened
On July 19, 2002, an Antonov-24, operated by Arkhangelsk Airlines, was flying en route to Tromsø airport. At the same time, a Fokker 50, operated by SAS Commuter with registration LN-RND, was proceeding from Lakselv to Tromsø. The two aircraft were operating in the same airspace, with the Fokker 50 positioned approximately 4 NM behind the Antonov-24 but traveling at a higher indicated airspeed.
The air traffic controller at Tromsø APP/TWR cleared the Antonov-24 to descend from FL 180 to FL 160. Simultaneously, the Fokker 50 was cleared to descend from FL 160 to FL 100. During this maneuver, the controller observed the Antonov-24 descending through FL 156, well below its assigned altitude. Despite repeated instructions from ATC to stop the descent, the Antonov-24 crew provided inaccurate altitude reports, claiming they were at FL 170 and later FL 160, while they were actually descending through FL 150.
The incident reached a critical point when the Fokker 50 crew reported that the Antonov-24 was closing in on them. The closest horizontal separation between the two aircraft was approximately 0.5 NM, with a vertical separation of only 300 - 400 ft. The Antonov-24 eventually climbed back to FL 160 without acknowledging its deviation to the controller.
The investigation
The investigation examined radar data, air traffic controller reports, and statements from the flight crews. The investigation noted that while the Fokker 50 crew maintained visual contact with the other aircraft, the Antonov-24 crew provided conflicting information regarding their actual flight level. The investigation also considered technical factors, including the potential for altimeter discrepancies and the lack of a TCAS resolution advisory during the encounter.