What happened
On August 24, 2000, at approximately 10:27 UTC, an airprox incident occurred 12 NM south of CANNE involving two aircraft operating in Class C airspace. The first aircraft, a Swissair Airbus A321 with registration HB-IOA, was climbing toward flight level 270. The second aircraft, a Crossair Saab 2000 with registration HB-IZL, was flying north at a stable flight level 240.
The two aircraft encountered each other with a lateral separation of only 3.8 NM and a vertical separation of approximately 500 feet. The crew of the HB-IZL reported receiving a TCAS Traffic Advisory followed by a TCAS Resolution Advisory commanding a descent. The crew elected not to follow the descent command because they had visually identified the opposing aircraft and could monitor the crossing maneuver manually.
The investigation
The investigation by the Swiss Transportation Safety Investigation Board (SUST) focused on the coordination between the Zurich and Milan air traffic control centers. It was established that the Zurich radar controller instructed the HB-IOA to contact Milan Control but failed to complete the required transfer of control according to established procedures.
Investigators also examined the radar data display technology at the Zurich center. The system was found to be unable to process the "previous code" from incoming aircraft, meaning the HB-IDL was not visible on the Zurich radar monitor until after the encounter had occurred. Furthermore, the investigation looked into why flight plan data for the northbound HB-IZL was missing from the South sector workstation, noting that a technical failure was the most likely cause.
Findings
- The primary cause was the failure of the South sector radar controller to transfer the southbound HB-IOA to Milan Control as per the agreed-upon coordination procedures.
- A significant contributing factor was the absence of flight plan data for the northbound HB-IZL at the South sector workstation, which prevented controllers from identifying the potential conflict.
- The radar technology in use prevented the correlation of the aircraft on the monitor because the system could not handle the transition of transponder codes between sectors.
- Operational difficulties were noted regarding the reliability of telephone coordination between the Zurich and Milan centers.