What happened
On September 24, 2014, at 17:06 UTC, a loss of separation occurred between two Airbus A320 aircraft within the Barcelona TMA. The first aircraft, HA-LPL, operated by Wizz Air, was on an arrival route from Poznan to Barcelona. The second aircraft, EC-LZF, operated by Vueling, was departing Barcelona for Amsterdam/Schiphol.
The incident began when the air traffic controller (ATC) instructed the crew of HA-LPL to perform a 360-degree turn due to traffic near the Sabadell (SLL) VOR. However, the controller used non-standard phraseology, stating, "Wizzair niner five one due to traffic over sierra lima lima make a tree sixty on your right." The crew of HA-LPL read back the instruction as a command to execute the turn immediately rather than upon reaching the SLL waypoint.
As the aircraft performed the turn, the separation between the two Airbus A320 jets decreased significantly. At the closest point, the aircraft were separated by only 400 ft vertically and 1.3 NM horizontally. While the pilots of HA-LPL observed the conflicting traffic on their navigation displays, no TCAS Resolution Advisory (RA) was triggered, though a Traffic Advisory (TA) was noted. The conflict was eventually resolved after the crew of HA-LPL requested new vectors and the controller instructed them to turn left toward the SLL VOR.
The investigation
The CIAIAC investigation focused on the communications between the aircraft and the Barcelona ACC Sector T1. The investigation examined radar data, which confirmed that the aircraft trajectories converged due to the misunderstood maneuver. The investigators also reviewed the controller's adherence to standard phraseology and the read-back/hear-back protocols.
It was established that the controller failed to identify the discrepancy between the original instruction and the crew's incorrect read-back. Furthermore, the investigation found that once the conflict was identified, the controller engaged in a prolonged radio discussion with the crew—lasting approximately one minute—rather than providing immediate corrective instructions to ensure separation. The controller also noted that they waited for a potential TCAS RA rather than proactively intervening with new vectors.
Findings
- The primary cause of the incident was the lack of adherence to standard phraseology by the ATC regarding maneuver instructions, which led the crew of HA-LPL to believe the turn should be executed immediately.
- A contributing factor was the lack of oversight by ATC personnel, as the discrepancy between the issued instruction and the pilot's read-back went undetected.
- The management of the conflict was inadequate or delayed, as the controller insisted on the original instruction rather than providing immediate separation instructions once the error was realized.