What happened
On March 28, 1997, at approximately 01:09 local time, a serious loss of separation occurred in the Oulu airport approach area involving two aircraft: SWW 2081 (an MD-83) and FIN 3659 (an MD-80). Both aircraft were performing instrument approaches using the same beacon.
At the time, SWW 2081 was cleared to the Laanila beacon at 3,000 feet. Meanwhile, FIN 3659 was being vectored via the OUK VOR to the Laanila beacon at 5,000 feet, with instructions to prepare for a direct ILS approach to runway 12. During the sequence, the air traffic controller queried the altitude of SWW 20 เนื่องจาก 2081. The aircraft's response was unclear; while the controller believed the aircraft reported maintaining 2,000 feet, the pilot later clarified that the crew believed they had reported 3,000 feet.
Based on the perception that SWW 2081 was at 2,000 feet, the controller cleared FIN 3659 to 3,000 feet. As both aircraft converged on the same approach path, the captain of FIN 3659 decided to extend the outbound leg slightly to avoid the other aircraft. However, because SWW 2081 was actually maintaining 3,000 feet, the two aircraft ended up at the same altitude. At the point of closest approach, the aircraft were approximately 4 nautical miles apart laterally.
The investigation
The investigation examined air traffic control recordings, telephone logs, and pilot reports. While the report from the FIN 3CO59 captain was available, the report from the SWW 2081 captain was not obtained despite multiple requests. Investigators utilized audio laboratory analysis to attempt to clarify the ambiguous altitude report from SWW 2081, but the specific digits transmitted could not be definitively determined.
Findings
- The air traffic controller applied vertical separation procedures appropriately in other aspects of the sequence.
- The altitude report provided by SWW 2081 was highly ambiguous regarding the specific numerical value.
- The controller and the crew of FIN 3659 both operated under the mistaken impression that SWW 2081 was maintaining 2,000 feet.
- SWW 2081 did not explicitly confirm that it was leaving or maintaining the 3,000-foot altitude during its clearance.
- The primary cause of the incident was a misunderstanding by the air traffic controller resulting from an unclear radio transmission, which led to the loss of vertical separation between the two aircraft.