What happened
On September 6, 2002, at 09:52 UTC, a loss of vertical separation occurred near the BALTI waypoint over the Gulf of Finland involving two Finnair aircraft: a DC-9 (registration OH-LYV) and a Boeing 757 (registration OH-LBX).
The incident began within the Tallinn Approach (TLL-APP) area of responsibility. The DC-9 was descending from flight level 270 to 110, while the Boeing 757 was descending from flight level 277 to 130. To maintain separation, the Tallinn controller issued specific descent rate restrictions: the DC-9 was instructed to maintain a descent rate of 2000 feet per minute or more, while the Boeing 757 was instructed to maintain 2000 feet per minute or less.
As the aircraft transitioned from Tallinn to Helsinki Approach (HK-APP) control, the vertical separation between the two aircraft rapidly decreased. While the DC-9 eventually slowed its descent, the Boeing 757 continued to descend at an average rate of approximately 2300–2900 feet per minute, exceeding its assigned limit. Consequently, the aircraft's flight paths intersected vertically at flight level 137, with the DC-9 passing above the Boeing 757 at a horizontal distance of approximately 2.7 nautical miles. The Helsinki controller eventually recognized the conflict and issued an avoidance instruction to the Boeing 757.
The investigation
The investigation examined radar recordings, radio communications, and telephone logs from both Tallinn and Helsinki approach centers. Investigators also reviewed the flight logs and operational flight plans of both aircraft. The inquiry focused on the adherence to descent rate instructions, the coordination between the two air traffic control centers, and the cockpit management of the aircraft during the descent.
Findings
- The primary cause of the loss of separation was that neither aircraft adhered to the assigned and acknowledged descent rate restrictions.
- The Tallinn controller did not inform the Helsinki controller of the specific descent rate restrictions previously issued to the aircraft during the transfer of control.
- The Helsinki controller did not notice the loss of vertical or radar separation upon the aircraft entering their frequency.
- The crew of the Boeing 757 failed to use the most appropriate vertical speed (V/S) mode or speed settings to maintain the required descent rate.
- There was a lack of effective coordination regarding the specific vertical constraints during the handoff between the two control sectors.
- Flight logs for both aircraft were incomplete; the Boeing 757 log did not record the descent rate restriction, and neither crew could clearly recall the specific division of duties between the pilot flying and the pilot monitoring during the descent.