What happened
On February 22, 2001, at approximately 09:02 local time, two commercial aircraft flying on intersecting paths bypassed prescribed vertical separation minimums near the LAKUT holding pattern. The first aircraft, a Saab SF340A (registration OH-FAE) operating as Golden Air Flyg flight GAO 251, was traveling from Pori to Helsinki. The second aircraft, an ATR 72-201 (registration OH-KRB) operating as Finnair flight FIN 222, was traveling from Turku to Helsinki.
At the time of the incident, the air traffic controller at the Tampere Area Control Center (ACC) issued a descent clearance to the Saab SF340A to descend from flight level 120 to flight level 100. Simultaneously, the controller mistakenly used the callsign FIN 251 when addressing the aircraft. During this maneuver, the ATR 72-201 received a Traffic Advisory (TA) and subsequently a Resolution Advisory (RA) from its TCAS, instructing the pilot to descend.
The two aircraft passed each other with a lateral separation of approximately 500 meters (0.27 NM). The vertical separation reached a minimum of approximately 150 meters (500 ft), with the Saab SF340A at flight level 115 and the ATR 72-201 at flight level 107. Both aircraft were flying in visual meteorological conditions (VMC) above the clouds and were able to visually identify one another.
The investigation
The investigation examined radar recordings, radio communications, and the Maestro traffic management system. Investigators reviewed the sequence of instructions provided by the Tampere ACC Feeder 1 workstation. The investigation also analyzed the operational status of the TCAS units on both aircraft, noting that the Saab SF340A was configured to 'TA ONLY' mode because the crew's training was still in progress, meaning it could provide traffic alerts but not resolution advisories.
Findings
- The primary cause of the loss of separation was incorrect altitude instructions provided by the air traffic controller, specifically instructing the Saab SF340A to descend to flight level 100 while the ATR 72-201 was already at flight level 110.
- The controller also used an incorrect callsign (FIN 251) during the critical instruction phase.
- The ATR 72-201 crew responded to a TCAS Resolution Advisory by descending, which further reduced the vertical gap between the two aircraft.
- Environmental factors, including bright sunlight and glare, hindered the crew of the ATR 72-201 from making an early visual identification of the intersecting aircraft.