What happened
On September 4, 2002, at approximately 10:11 local time, two aircraft operating in the Oslo Terminal Control Area (TMA) experienced a loss of separation. The first aircraft, a British Airways Citiexpress Embraer ERJ 145 (registration LN-RMN), was being radar vectored for an ILS approach to runway 19L. Simultaneously, a Scandinavian Airlines System flight was descending to 5,000 feet on the left downwind leg for the same runway.
During the descent, the SAS crew received updated instructions from TMA E to descend further to 4,000 feet. However, the air traffic controller noticed the crew had initiated a left turn toward the base leg that did not align with their assigned clearance. When questioned about their intentions, the crew stated they had traffic in sight. The controller immediately ordered the aircraft to level off at 4,000 feet and turn right to a heading of 360 degrees, which the crew executed.
At the closest point of the encounter, the separation between the two aircraft was reduced to 2 nautical miles and 600 feet. This proximity triggered a TCAS/RA (Resolution Advisory) on the Embraer ERJ 145 during its final approach.
The investigation
The Norwegian Safety Investigation Authority (NSIA) analyzed data from the Oslo ATCC and conducted an independent investigation. The inquiry focused on the flight paths of both aircraft and the communications between the crew and the controller to determine why the separation minima were breached.