What happened
On June 16, 2017, at approximately 10:58 UTC, a serious flight disturbance occurred involving two Austrian-operated aircraft approximately 11 nm east-northeast of the BALAD waypoint in Burgenland. The first aircraft, a Bombardier DHC-8-402, was operating an IFR scheduled flight approaching Vienna-Schwechat (LOWW). The second aircraft, an Airbus A319-112, was also conducting an IFR scheduled flight in the same vicinity.
As the Bombardier DHC-8-402 was transitioning toward a right-hand approach for runway 34, it was handed over to the WIEN DIRECTOR controller. Simultaneously, the Airbus A319-112 was instructed to climb to flight level 230. During this phase of flight, the vertical and horizontal separation between the two aircraft was lost. The situation escalated when the Airbus A319-112 received a Traffic Advisory (TA) followed by a TCAS Resolution Advisory (RA), forcing the crew to execute an immediate avoidance maneuver to prevent a collision.
The investigation
The Austrian Federal Safety Investigation Board (SUB) examined the flight trajectories, radar data, and cockpit voice/flight recorder information. The investigation focused on the vertical and horizontal separation breakdown, the sequence of ATC instructions, and the performance of the Traffic Collision Avoidance System (TCAS/ACAS). The SUB analyzed the coordination between the aircraft and the Vienna radar controllers, specifically looking at the timing of the climb instruction for the **Airbus A3 and the approach path of the Bombardier aircraft.