What happened
On October 2, 2018, two Boeing 737-800 aircraft, registrations EI-FRY and EI-DWW, operated by Ryanair, experienced a loss of separation while cruising at FL340 near the GOSVI reporting point in Navarre, Spain. The aircraft were on converging tracks in the Madrid ACC airspace.
At approximately 14:54 UTC, the controller in the PAL sector transferred EI-DWW to the adjacent ZGZ sector, despite the aircraft still being in a conflict with EI-FRY. The conflict was not identified by the controllers in either sector; instead, it was detected by the Bordeaux ACC in France approximately one minute and 27 seconds before the closest point of approach.
Following the detection by the French controller, the air traffic controllers in the PAL and ZG and sectors issued conflicting instructions. One controller instructed an aircraft to descend, while the other subsequently instructed both aircraft to climb. These contradictory maneuvers led to a reduction in vertical separation. The aircraft reached their closest point of approach at 14:57 UTC, separated by only 2.3 NM horizontally and 334 ft vertically. The situation was resolved only after the TCAS systems on both aircraft issued Resolution Advisories (RA), prompting the crews to execute evasive maneuvers.
The investigation
The investigation focused on the failure of air traffic services to maintain separation. Investigators examined the functionality of the Short-Term Conflict Alert (STCA) system and the communications between the PAL and ZGZ sectors.
It was established that the STCA system failed to generate the required PAC or VAC alerts, which should have alerted the controllers to the approaching conflict. The investigation also reviewed the controllers' actions, noting that the PAL controller had recently returned from vacation and the ZGZ controller had just commenced a shift change. The investigation confirmed that the flight crews performed their duties correctly, following all TCAS procedures and reporting the event to ATC as required.
Findings
- The primary cause of the incident was the failure of the PAL sector controller to identify the conflict and the subsequent transfer of EI-DWW to the adjacent sector without ensuring the aircraft was clear of the conflict.
- Contributing to the event was the improper handling of the conflict by controllers in both sectors, who issued similar, contradictory instructions to the two aircraft, contrary to established agreements.
- The STCA system malfunctioned, failing to provide the necessary automated warnings to the controllers.
- The flight crews of both Boeing 737-800 aircraft acted correctly, utilizing the TCAS system to prevent a mid-air collision.