What happened
On May 27, 1997, at approximately 04:20 UTC, two Boeing 747 aircraft experienced a period of close proximity within Irish airspace near the 53N 15W waypoint. The first aircraft, USAF 1, was transiting from the United States toward Paris at flight level 2D90. Simultaneously, a UPS cargo flight, UPS 6080, was traveling from Europe to the United States.
While the UPS aircraft was descending from flight level 310 to its assigned oceanic crossing level of 280, it passed through the altitude of the US Air Force aircraft. At 04:18 UTC, the ground-based Short Term Conflict Alert (STCA) activated. Air traffic controllers responded immediately by issuing heading and altitude changes to both aircraft. Radar analysis later confirmed that while the aircraft were on diverging tracks, the closest distance between them was 6 nm, with the UPS aircraft at flight level 282 and the USAF 1 at flight level 290.
The investigation
The AAIU investigation involved a comprehensive review of radar and audio recordings from the Shannon control center, alongside interviews with air traffic management and the controllers on duty. The inquiry examined the functionality of both the ground-based STCA and the airborne Traffic Alert and Collision Avoidance System (TCAS) installed on the USAF 1.
Investigators also looked into the administrative handling of the event, specifically noting that the incident was not initially reported through the Mandatory Occurrence Reporting (MOR) scheme. The investigation also focused on the psychological impact on the air traffic controllers involved, who experienced significant distress following the event.
Findings
- The primary cause of the incident was attributed to human factors within the air traffic control environment, specifically an ATC clearance that failed to maintain the required separation.
- Both the ground-based STCA and the onboard TCAS functioned correctly, providing the necessary alerts to allow for timely intervention.
- There was no actual risk of a collision, as the aircraft were on diverging paths and the controllers corrected the conflict almost immediately after the alert.
- The flight crews of both aircraft were not found to be contributors to the event.
- The investigation identified a lack of a formal critical incident response program for air traffic controllers, noting the psychological trauma experienced by the staff involved.
Safety action
Following the investigation, several safety recommendations were issued to the Irish Aviation Authority (IAA) and the Department of Public Enterprise, including:
- The establishment of a Joint Airprox Working Group to independently assess proximity reports.
- The implementation of Human Factors training for all controllers.
- The creation of a Critical Incident Response Programme specifically for air traffic control personnel.
- The reinforcement of mandatory reporting procedures across the aviation system.