What happened
On 8 June 2009, a serious airprox incident occurred near the Trasadingen beacon in Swiss airspace. An Airbus A318-111, registration F-GUGK, operated by Air France, was cruising at flight level 380. Simultaneously, a Boeing 737-8AS, registration EI-DHK, operated by Ryanair, was traversing the same area at flight level 370.
During the operation, a trainee air traffic controller at the Zurich Area Control Centre instructed the Air France crew to descend to flight level 360. This instruction placed the aircraft on a collision course with the Ryanair flight. The supervising coach, who was monitoring the trainee, failed to notice this specific clearance. The impending conflict triggered a short-term conflict alert on the radar display. Following the alert, the coach took control of the sector and instructed the Ryanair aircraft to also descend to flight level 360. Both aircraft subsequently received resolution advisories from the airborne collision avoidance system (TCAS), which the crews followed. The two aircraft eventually crossed paths with a lateral separation of only 1.4 NM and a vertical separation of 725 ft.
The investigation
The investigation examined the actions of the air traffic controllers, the flight paths of both aircraft, and the technical capabilities of the Zurich Area Control Centre. Investigators reviewed radar data, radio communications, and the training status of the personnel involved. It was noted that the trainee controller was in the early stages of a practical training phase. The investigation also compared the technical systems in the Zurich center to those in the Geneva Area Control Centre, specifically regarding conflict monitoring capabilities.
Findings
- The primary cause was that air traffic control cleared an aircraft to descend into the flight path of another without accounting for the crossing traffic.
- The supervising coach failed to intervene in a timely manner, as they did not realize the trainee was overwhelmed by the high traffic volume.
- The Zurich Area Control Centre lacked a system to monitor planned clearances for potential conflicts, a feature that was already operational in the Geneva center.
- The supervising coach was monitoring the trainee without using a headset, which hindered situational awareness.
- The Air France crew did not confirm the required high rate of descent, and the controller did not receive a response to the instruction.