What happened
On June 29, 2010, near the Basel-Mulhouse airport, two Airbus A319 aircraft experienced a serious loss of separation. The first aircraft, an easyJet Switzerland flight (HB-JZQ) arriving from Palma, was on approach to runway 15. The second aircraft, an Air France flight (F-GHRHA) departing for Paris Orly, had recently taken off.
During the sequence, an air traffic controller assigned both aircraft to the same flight level (FL110). This error triggered a Traffic Advisory (TA) on both flight decks, followed by a rapid succession of TCAS Resolution Advisories (RA). The maneuvers were intense; the vertical load factor on HB-JZQ fluctuated significantly between -0.19 g and 2.04 g. Due to the abrupt nature of these maneuvers, a cabin crew member on HB-JZQ sustained minor injuries.
The investigation
The BEA investigation focused on the sequence of events and the air traffic control environment. At the time of the incident, the Basel-Mulhouse approach sector was operating under procedural control due to an ongoing radar system failure. The investigation examined the actions of a trainee controller and their instructor, as well as the technical performance of the TCAS systems and the flight profiles of both aircraft.
Investigators analyzed the flight data from both aircraft, cockpit voice recordings, and radar data. They specifically looked at why the controller's error went undetected and how the vertical speed of the departing Air France aircraft contributed to the rapid onset of the conflict.
Findings
The investigation established that the primary cause of the loss of separation was a clerical error by a trainee air traffic controller, who assigned the same flight level to an aircraft climbing and one descending. This error was not detected by the supervising instructor.
Several contributing factors were identified:
- The radar system was officially declared inoperable, yet the controllers were using unreliable radar images to verify traffic positions.
- The air traffic control environment was complicated by weather avoidance requests, which added workload to the controllers.
- The instructor's role as an intermediary between the trainee and the coordinator hindered effective supervision.
- The high rate of climb (exceeding 3,000 ft/min) of the Air France aircraft reduced the time available between the initial Traffic Advisory and the subsequent Resolution Advisories.
- Conflicting instructions, such as a controller requesting the aircraft to maintain a specific level while the TCAS RA commanded a vertical speed change, contributed to the complexity of the maneuvers.