What happened
On November 12, 1997, a loss of vertical separation occurred in the Helsinki approach area involving two Finnair aircraft. The first aircraft, a SAAB SF3．40 (registration OH-FAD), operating flight FIN 253 from Helsinki to Pori, was cleared to climb to flight level 100. The second aircraft, an MD87 (registration OH-LMB), operating flight FIN 912 from Stockholm to Helsinki, was cleared to descend to flight level 110.
During the climb, the captain of OH-FAD incorrectly set the altitude preselector to flight level 110. Consequently, the aircraft climbed to the higher altitude, placing it on a collision course with the descending OH-LMB. When the trainee air traffic controller noticed the discrepancy, they instructed the MD87 to turn right to maintain horizontal separation. However, this instruction was issued too late to prevent the aircraft from encroaching on the required 5 NM horizontal separation, with the minimum distance between the two aircraft dropping to approximately 4 NM.
The investigation
The investigation examined radar recordings from the Tampere Area Control Center, radio communications, and the cockpit procedures of both aircraft. Investigators analyzed the actions of the trainee air traffic controller and the supervising controller, as well as the cockpit workload and altitude setting procedures on OH-FHD.
It was established that the supervising controller had briefly stepped away from the primary workstation to address a separate issue involving a Finnish Air Force HAWK trainer aircraft. During this period, the trainee controller was managing the traffic. The investigation also noted that the trainee controller failed to immediately recognize the altitude deviation and used the phrase "maintain" when acknowledging the aircraft's report of being at flight level 110, rather than correcting the error immediately.
Findings
- The primary cause of the incident was that the captain of OH-FAD incorrectly set the altitude preselector to flight level 110, deviating from the assigned clearance.
- Cockpit cross-checks were insufficient to detect the erroneous altitude setting before the aircraft reached the incorrect level.
- The air traffic controller failed to actively monitor the altitude changes on the radar display.
- The trainee controller's response to the altitude deviation was delayed, and the instruction to the OH-LMB to turn was issued too late to maintain the required horizontal separation.
- A secondary factor was the distraction of the supervising controller due to a separate altitude deviation involving a military aircraft.