What happened
On the evening of 25 October 2010, a serious airprox incident occurred near the Trasadingen DVOR within Swiss airspace. An Airbus A320-232, operating as CFG 3HD, was climbing through flight level 370 under the control of Zurich Area Control Centre. The aircraft was flying a westward path between two other eastbound aircraft.
During the climb, the crew of the Airbus A320-232 observed another aircraft visually and received a Traffic Advisory (TA) from their TCAS. Despite receiving instructions from air traffic control to maintain their heading and turn left to maintain separation, the crew decided to initiate an unauthorized right turn and a descent to flight level 365. This maneuver caused the aircraft to converge with an Airbus A340-642, operating as QTR 020, which was flying on a direct course to waypoint MADEB. The two aircraft eventually closed to a lateral separation of only 3.1 NM and a vertical separation of 500 ft, a distance below the required 5 NM lateral or 1000 ft vertical minimums.
The investigation
The Swiss Accident Investigation Board (SAIB) examined the actions of both flight crews and the air traffic controllers involved. The investigation focused on why the crew of CFG 366 deviated from assigned headings and why the air traffic controller was unable to effectively intervene.
Investigators reviewed radio communications, which revealed that while a trainee controller attempted to issue corrective instructions, a technical fault in the supervising controller's headset prevented his instructions from being clearly heard via the headset. The investigation also looked into the separation concept used by the Zurich ACC, which had placed the aircraft in close proximity to the minimum separation standards.