What aircraft was involved and where did it happen?

The accident on 2010-07-02 involved a aircraft, registration LN-RRN, at Serious incident between two aircraft LN-RRN and OH-LBT in the airspace southwest of Östersund, SE.

What was the probable cause of the accident?

The loss of separation was caused by the aircraft flying at an irregular flight level, compounded by the air traffic controller's failure to identify the conflict due to divided attention, single-manning of the sector, and the lack of effective monitoring of automated safety alerts.

TCAS Activation Avoids Mid-Air Collision in Swedish Airspace

A near-miss between a SAS Boeing 737 and a Finnair Boeing 757 occurred southwest of Östersund after an air traffic controller failed to notice a conflict.

What happened

On 2 July 2010, at approximately 12:15 local time, two commercial aircraft were cruising at 11,000 meters in the airspace southwest of Östersund, Jämtland. The first aircraft, a Boeing 737-700 with registration LN-RRN operated by SAS, was traveling on a heading of 200 degrees. Simultaneously, a Boeing 757-200 with registration OH-LBT operated by Finnair was traversing the same altitude on a heading of 100 degrees.

The incident began when the Finnair flight was cleared to an irregular cruising altitude that placed it on a conflicting semicircular level. During a shift handover at the Stockholm air traffic control centre, the potential for conflict was not properly noted or highlighted in the radar system. As the aircraft approached an intersecting path, the air traffic controller failed to notice automated conflict alerts on the radar screen. The situation only resolved when the onboard Traffic Collision Avoidance System (TCAS) triggered Resolution Advisories (RA) in both cockpits, prompting the crews to perform emergency avoidance maneuvers. The closest the aircraft came to each other was a separation of 3.1 nautical miles and 1,000 feet.

The investigation

The Swedish Accident Investigation Board (SHK) examined the operational environment at the Stockholm air traffic control centre, specifically focusing on sector N/K. The investigation looked into the handover process between controllers, the functionality of the Conflict and Risk Display (CARD) and the Short Term Conflict Alert (STCA) systems, and the manning levels of the sector. Investigators also reviewed the controller's recent duty schedule, noting a transition from evening shifts to a morning shift, and evaluated the impact of administrative duties on operational focus.

Findings

Several factors contributed to the loss of separation:

The Finnair aircraft was flying at an incorrect semicircular flight level.
The air traffic controller was performing both executive and planner duties alone, leaving the sector without a secondary safety net to monitor conflicts.
The controller's attention was divided between the southern conflict area and traffic in the northern part of the sector.
The handover process failed to officially mark the irregular altitude of OH-LBT, leading to the oversight.
The design and placement of the CARD tool and the visual-only nature of the STCA alarm made them difficult to detect during periods of high workload or divided attention.
Potential fatigue and the cognitive load of transitioning from administrative tasks to active radar monitoring played a role.

Safety action

Following the incident, the air traffic service provider, LFV, implemented changes to the air traffic control system to provide a more pronounced safety net. Additionally, LFV adjusted the manning requirements and opening hours for the planner controller position to ensure better coverage and prevent single-manning risks in the area.