What happened
On May 10, 2007, at 15:06 UTC, a serious airprox incident occurred approximately 9 NM east of the TRA DVOR near Zurich. The incident involved an Airbus A340-300, registration CC-CQC, operated by LAN Airlines S.A. (flight LAN 704), and an Airbus A321-100, registration OE-LBA, operated by Austrian Airlines (flight AUA 415W).
At the time, the Airbus A340-300 was flying under IFR from Madrid to Frankfurt. The air traffic controller at Zurich Sector Upper M4 instructed the crew to descend to flight level 320. Simultaneously, the Airbus A321-100, flying from Vienna to Paris, was cruising at flight level 340. As the LAN 704 aircraft descended, it crossed the path of the AUA 415W. The aircraft passed within 1.1 NM horizontally and 900 ft vertically of each other. The crew of the LAN 704 aircraft eventually identified the passing aircraft visually in clear weather conditions.
The investigation
The investigation focused on the communications and coordination within the Zurich Upper Area Control Centre. It was established that the sector controller (a trainee) was under significant pressure due to high traffic complexity and a recent unauthorized descent by another aircraft in the sector. The investigation examined the coordination between the Radar Executive and the Radar Planner, as well as the handover process to Rhine UAC.
Investigators found that while the Radar Planner had coordinated a descent to flight level 350 for the Airbus A340-300 to avoid the Airbus A321-100, this instruction was not clearly communicated to or understood by the Radar Executive. Furthermore, the controller issued a descent clearance to flight level 320 without the necessary coordination with the adjacent M3 and M2 sectors.
Findings
- The primary cause was the failure to coordinate a descent clearance to flight level 320 with the relevant adjacent sectors.
- A breakdown in communication occurred between the Radar Executive and the Radar Planner, where a verbal confirmation of "good" was misinterpreted, leading the controller to believe a lower altitude was authorized.
- The air traffic controller was operating at the limits of his capacity due to high traffic volume and the complexity of managing a trainee under supervision.
- The TCAS systems on both aircraft functioned correctly; no resolution advisories were issued because the predicted minimum separation would have remained above the required threshold.
- The controller's attempt to stop the descent was unsuccessful because the aircraft had already switched frequencies to Rhine UAC.