What happened
On June 17, 2006, at approximately 10:20 UTC, a serious airprox incident occurred near Zurich, 10 NM east of the KLO/DVOR. The incident involved an Air Berlin Boeing 737-86J, registration D-ABAP, operating flight BER 1315, and a Hapag-Lloyd Express Fokker 100, registration D-AGPP, operating flight HLX 3680.
The Fokker 100 was flying from Cologne/Bonn to Genoa at a flight level of 250, a deviation from its planned altitude of 310 for operational reasons. Meanwhile, the Boeing 737 was descending through flight level 300 toward flight level 250. As the two aircraft approached a crossing point, the air traffic controller in the Zurich Upper Sector issued a descent instruction to the Boeing 737, directing it to descend to 250 with a high rate of descent.
Shortly before the Short Term Conflict Alert (STCA) triggered, the controller provided traffic information to the Fokker 100 crew. Simultaneously, the ACAS/TCAS systems on both aircraft generated Resolution Advisories (RA). The crew of the Boeing 737 performed a descent, while the Fokker 100 crew executed a climb to flight level 254. The aircraft crossed paths at approximately 10:19:59 UTC, with the Boeing 737 passing through 238 and the Fokker 100 at 253.
The investigation
SUST examined the communications and radar data from the Zurich Upper and Lower East sectors. The investigation focused on the air traffic controller's actions and the coordination between sectors. The investigation found that the controller in the Upper Sector was managing a high volume of traffic and was heavily focused on maintaining separation with another aircraft.
Radar data confirmed that the Boeing 737 was descending through the flight level of the Fokker 100. The investigation also reviewed the role of the Radar Planner, who was occupied with coordination tasks and failed to notice the developing conflict. The investigation concluded that the pilots of both aircraft responded immediately and correctly to the ACAS instructions, which prevented a collision.
Findings
- The primary cause of the incident was that the air traffic controller failed to consider one of the aircraft in the traffic flow.
- The controller was preoccupied with maintaining separation with a different aircraft, which led to overlooking the flight path of the Fokker 100.
- The Radar Planner was engaged in heavy coordination work and did not provide the necessary redundancy to detect the conflict.
- A high frequency of radio transmissions (14 exchanges in 95 seconds) may have contributed to the controller's loss of situational awareness regarding the Fokker 100.
- The ACAS/TCAS resolution advisories were critical in preventing the collision.