What happened
On July 16, 2005, at approximately 09:47 UTC, a serious airprox incident occurred 36 NM south of the Kloten DVOR. An Eurocypria Airlines Boeing 737-800, registration 5B-DBW, was climbing through the Zurich upper airspace toward RESIA. Simultaneously, a Speedwings S.A. Cessna 560 Citation V Ultra, registration HB-VNA, was descending through the same airspace near KELIP.
Due to significant thunderstorms in the region, the flight path for the 5B-DBW had been modified. As the aircraft climbed, the air traffic controller (RE-U) attempted to manage vertical separation by instructing the 5B-DBW to turn right. However, the crew of the 5B-DBW declined the turn due to heavy convective activity (CB clouds) on the right side of their path.
As the aircraft continued its climb, the vertical separation between the two planes decreased rapidly. The crew of the HB-VNA received a TCAS Resolution Advisory (RA), prompting a descent. The 5B-DB7 crew also subsequently experienced a TCAS descent command. The aircraft eventually passed each other with a vertical separation of approximately 700 feet. The crew of the 5B-DBW reported seeing the other aircraft visually only after the crossing had occurred, as cloud cover obscured the view during the approach.
The investigation
SUST examined the coordination between the different radar sectors (West, South, and Upper) and the impact of the prevailing weather. The investigation focused on the "silent transfer" procedures used for aircraft transitioning between sectors without verbal coordination. Investigators also reviewed the radar update rates, noting a 12-second refresh interval, which meant the controller's display showed aircraft positions and altitudes that were slightly outdated.
Findings
- The primary cause was a failure in effective coordination between the radar executives of the Upper and South sectors regarding the vertical levels of the aircraft.
- The controller in the Upper sector (RE-U) made a heading instruction based on an incomplete analysis of the traffic situation, influenced by high workload and pressure.
- A lack of precise communication regarding the intended flight levels of the HB-VNA led the controller to believe vertical separation would be maintained through climbing/descending profiles that were not actually being executed.
- The presence of heavy thunderstorms increased the complexity of the airspace and limited the maneuvering options for the 5B-DBW crew.
- The 12-second radar update latency contributed to the controller seeing the 5B-DBW at a lower altitude than its actual position during its rapid climb.