What happened
On November 12, 1998, at approximately 12:12 local time, a serious loss of separation occurred southwest of the Vihti VOR, roughly 55 km from Helsinki-Vantaa Airport. The incident involved two aircraft flying on nearly opposing tracks: a Scandinavian Airlines System (SAS) DC-9-81 (registration OY-KHN) arriving from Stockholm and a Finnair ATR 72-201 (registration OH-KRE) en route from Helsinki to Turku.
The SAS aircraft, operating as SAS 1708, had previously requested and received a descent to flight level 130 from Tampere Area Control. Upon contacting Helsinki Approach, the crew intercepted a transmission intended for a different Finnair aircraft. This led the SAS crew to believe they had been cleared to descend to flight level 80. Consequently, the crew maintained flight level 130 while descending through flight level 125.
Simultaneously, the Finnair ATR 72, operating as FIN 213, was maintaining flight level 120. As the two aircraft approached each other, the air traffic controller noticed the SAS aircraft had descended below its assigned level and was on a collision course with the Finnair aircraft. The controller issued immediate heading changes to both aircraft to create separation.
Radar data confirmed that the aircraft passed each other at a distance of approximately 2 nautical miles, with the SAS aircraft at flight level 124 and the Finnair aircraft at flight level 120, representing a vertical separation of only 400 feet.
The investigation
The investigation examined radio communications, radar recordings from Tampere Area Control, and flight data. Investigators analyzed the sequence of transmissions between the aircraft and Helsinki Approach to determine how the altitude misunderstanding occurred. The investigation also reviewed the cockpit actions of both crews, including the use of the Traffic Collision Avoidance System (TCAS) on the SAS aircraft, which provided a Resolution Advisory (RA) to climb.