What happened
On January 30, 2006, at approximately 13:36 UTC, a serious airprox event occurred near the SUKUN waypoint, an intersection of air routes UM 622 and UM 603, located roughly 76 NM east of Alghero. Two large aircraft, a Boeing 767 (registration G-BNWU) and a Boeing 7 57 (registration 4X-EBU), were both operating at flight level 360.
The aircraft were under the radar control of Roma ACC. The Boeing 767 was traveling from Entebbe to London Heathrow, while the Boeing 757 was flying from Barcelona to Tel Aviv. As the aircraft converged, the radar system triggered a Short Term Conflict Alert (STCA) at 13:35:43. However, neither the Executive Air Traffic Controller (ATC) nor the Planner noticed the visual alert on their screens.
The conflict was only identified when the pilot of the 4X-EBU flight contacted the controller to ask if there was any traffic ahead at their level. Upon realizing the imminent danger, the controller immediately issued instructions for the G-BNWU flight to climb to level 370 and for the 4X-EBU flight to descend to level 350. These instructions were consistent with the Resolution Advisories (RA) issued by the aircraft's onboard TCAS. The aircraft passed with a vertical separation of approximately 1,000 feet.
The investigation
The ANSV investigation examined radar data, radio communications, and the performance of the air traffic controllers. The investigation reviewed the handover process between the outgoing and incoming controllers, noting that while procedures were in place, the specific conflict was not highlighted during the shift change.
Investigators also analyzed the radar screens, which showed that the STCA alert remained active for 47 seconds without being noticed by the controllers. The investigation looked into the frequency of STCA alerts at the center, noting that false or momentary alerts occur frequently (between five and ten times per shift), which may contribute to operator desensitization.
Findings
- The primary cause of the event was a momentary loss of operational attention by the Executive Air Traffic Controller.
- The STCA visual alert was completely disregarded by both the Executive and Planner controllers for 47 seconds.
- The controllers were managing a normal traffic load, with approximately 12 to 13 aircraft in the area.
- The frequency of STCA alerts, including false alarms, may have led to a state of desensitization among the controllers.
- The controller had been engaged in a radio communication with another aircraft just 19 seconds before the STCA alert appeared.