What aircraft was involved and where did it happen?

The accident on 2013-03-21 involved a AIRBUS S.A.S. A330-343, registration HB-JHD, at 4.5 NM südwestlich des Flughafens Zürich, ZH, CH.

What was the probable cause of the accident?

The incident was caused by a loss of vertical separation between the two aircraft, resulting from conflicting altitude clearances and the complexity of managing multiple aircraft within the Zurich TMA.

TCAS RA Avoidance Between Two Swiss Airliners in Zurich TMA

An Airbus A330 and an Avro 146 narrowly avoided a collision in Swiss airspace after a conflict in altitude clearances led to a TCAS resolution advisory.

What happened

On March 22, 2013, at approximately 17:04 UTC, a serious airprox incident occurred within the Zurich Terminal Control Area (TMA), roughly 4.5 nautical miles southwest of Zurich Airport. The incident involved two aircraft operating under instrument flight rules: an AVRO 146-RJ110, registration HB-IYU, operated by Swiss European Air Lines, and an A330-343, registration HB-JHD, operated by Swiss International Air Lines.

At the time of the conflict, the AVRO 146-RJ100 was climbing on the DEGES 2W departure route. The air traffic controller issued a clearance for the aircraft to climb to flight level 120. Simultaneously, the A330-343 was also being managed within the same airspace. A conflict arose when the vertical separation between the two aircraft dropped significantly below the required 1,000 feet. The closest separation recorded was approximately 350 feet vertically and 1.8 nautical miles horizontally.

As the aircraft approached each other, the Traffic Collision Avoidance System (TCAS) triggered Resolution Advisories (RA). The crew of the AVRO 146-RJ100 received a command to climb, while the crew of the A330-343 received a command to descend. The air traffic controller also issued maneuvering instructions to both aircraft to avoid the traffic.

The investigation

The Swiss Transportation Safety Investigation Board (SUST) examined the flight data recorders (FDR), radar plots, and air traffic control communications. The investigation focused on the sequence of altitude clearances, the coordination between the air traffic controller and the flight crews, and the effectiveness of the surveillance systems. The investigation also reviewed the training environment, as the radar position was being supervised by an instructor (OJTI) while a trainee was handling the traffic.

Findings

The primary cause of the incident was a failure to maintain required vertical separation due to conflicting altitude instructions and flight path management.
The separation between the two aircraft fell to a minimum of 350 feet vertically.
The investigation identified complexities in the airspace involving multiple aircraft, including VFR traffic and a special flight (photography mission) in the vicinity.
The presence of a trainee controller under supervision required high levels of coordination, which was challenged by the rapid development of the conflict.
The TCAS system functioned as intended, providing the necessary resolution advisaries to prevent a collision.