What happened
On November 15, 2001, at 21:34 local time, a loss of separation occurred west of Helsinki-Vantaa airport involving two commercial aircraft. An ATR72, registration OH-KRB, operating flight FIN8902 from Tampere, and an Avro RJ85, registration OH-SAK, operating flight KFB641 from Helsinki to Copenhagen, passed each other at a distance of approximately 1.8 nautical miles. At the moment of passing, both aircraft were at the same altitude.
The aircraft were operating in severe weather conditions characterized by strong northwesterly winds. The Avro RJ85 had departed runway 33 into a heavy headwind and was climbing, while the ATR72 was descending. The air traffic controller, managing both approach and departure sectors, attempted to maintain separation by assigning different headings to the aircraft. However, due to the intense wind components—including a significant tailwind for the descending aircraft and a headwind for the climbing aircraft—the lateral corrections provided were insufficient to maintain the required 3-nautical-mile radar separation.
Both flight crews received Traffic Advisories (TA) and Resolution Advisories (RA) from their respective Traffic Collision Avoidance Systems (TCAS). However, neither crew followed the RA instructions, as both pilots reported seeing the other aircraft visually. Neither crew reported the TCAS alerts to the air traffic controller.
The investigation
The investigation examined radar recordings from the Helsinki-Vantaa Win Radar and noise monitoring systems, as well as interviews with the pilots of both aircraft, the air traffic controller, and airline representatives. The investigators analyzed the impact of the severe weather, specifically the high wind speeds at flight levels 50 and 100, which were estimated at 50–60 knots. The investigation also reviewed the controller's workload, noting that the evening shift was understaffed, requiring the supervisor to manage multiple radar positions simultaneously.
Findings
- The primary cause of the loss of separation was the air traffic controller's decision to attempt vertical separation by assigning climbing and descending altitudes, which failed to account for the impact of strong winds on the aircraft's climb performance.
- The lateral heading changes issued by the controller were insufficient to achieve the required 3-nautical-mile lateral separation.
- Severe northwesterly winds caused significant variations in ground speed and induced a drift angle of approximately 15 degrees for aircraft in crosswind components.
- The pilots of both aircraft failed to follow TCAS Resolution Advisories (RA) because they had visual contact with the other aircraft, and they did not communicate the TCAS alerts to the controller.
Safety action
- Airlines should emphasize in pilot training the necessity of adhering to TCAS Resolution Advisories (RA) to ensure flight safety, even when the other aircraft is visually identified.