What happened
On February 1, 1996, at approximately 06:46 local time, a serious loss of separation occurred near Riihimäki, Finland, involving two aircraft: an ATR 72-201 (registration OH-KRC) and a Saab SF 340 A (registration OH-FAB).
The ATR 72-201, operating as KAR-302, had departed Helsinki-Vantaa and was climbing toward the Kruunupyy NDB. Simultaneously, the Saab SF 340 A, operating as FAV-301, was arriving from Kruunupyy toward Helsinki-Vantaa for an ILS approach to runway 22.
During the approach phase, the air traffic controller at the Helsinki-Vantaa approach workstation (COR) issued a clearance to the ATR 72-201 that did not ensure adequate vertical or lateral separation from the oncoming Saab SF 340 A. The two aircraft crossed paths at an altitude of approximately 100 feet separation and a lateral distance of only 0.2 NM. The pilots of the Saab SF 340 A observed the encounter and initiated an evasive maneuver by increasing their rate of descent. There were no injuries or aircraft damage.
The investigation
The investigation examined radar (SSR) recordings, radio communications, and controller work practices. Investigators analyzed the reliability of the radar system, which had previously experienced functional disturbances. The probe also looked into the use of an unauthorized separation technique known as "track separation" (or "trailing separation"), where a controller intentionally allows aircraft to pass through the same point without standard separation, provided one aircraft follows the other. The investigation also reviewed personnel work schedules and the adequacy of internal quality control and training within the air traffic services.
Findings
- The primary cause of the near-miss was a separation error by the air traffic controller, which was facilitated by the use of an unauthorized and unsafe working method.
- The controller utilized a technique where aircraft on intersecting tracks were left unseparated based on the assumption that one would pass the intersection point before the other.
- This unsafe practice was driven by a desire for traffic efficiency and was a long-standing, uncorrected habit within the unit.
- Contributing factors included functional disturbances in the SSR radar system, which led controllers to develop non-standard separation applications.
- A lack of effective recurrent training and internal quality oversight allowed these non-standard procedures to become established organizational norms.
- Inconsistencies in personnel work schedules and rest periods were also noted among the controllers.