What happened
On May 7, 1999, at Vaasa Airport, a separation loss occurred between two scheduled commercial flights during the arrival and departure phases. The first aircraft, a Finnair DC-9-51 (OH-LYV), was departing for Helsinki with a delay. The second aircraft, an Air Botnia Saab 340 (OH-SAD), was arriving from Oulu on a visual approach to runway 34.
During the sequence, the air traffic controller implemented a vertical separation of 2,000 feet, instructing the departing DC-9-51 to maintain this altitude until reaching a specific radial. However, as the arriving Saab 340 progressed on its approach, the crew of the DC-9-51 reported seeing the other aircraft. This informal report led to a misunderstanding between the flight crew and the controller.
As the aircraft converged, the crew of the DC-9-51 initiated a sharp 90-degree left turn to avoid a collision, noting to the controller that the maneuver "did not go quite right." At the time of the maneuver, radar data indicated the aircraft were less than 1 nautical mile apart with a vertical separation of only 400 feet. The controller subsequently released the vertical separation, relying on visual observations to maintain safety, which proved insufficient to accurately judge the lateral distance and speed differential between the two aircraft.
The investigation
The investigation examined the sequence of events, the controller's instructions, and the communication between the aircraft and the tower. The investigation focused on the controller's use of visual separation and the impact of the informal radio exchange. The investigation also reviewed the radar data, which showed the aircraft were on converging tracks, making the closing speed and lateral distance difficult for the controller to perceive accurately from the tower.
Findings
- The air traffic controller failed to adequately anticipate the developing situation and could have provided a left-hand departure clearance to avoid the conflict.
- A misunderstanding arose from the informal radio communication; the controller assumed the departing crew would manage the separation through their own actions, while the crew was actually reacting to an immediate perceived threat.
- The controller relied on visual observations to waive vertical separation but was unable to reliably assess the lateral separation and the closing speed of the two aircraft.
- The controller's attempt to provide flexible service led to the decision to relinquish the established altitude separation.