What happened
On January 14, 2000, at approximately 08:02 UTC, an airprox incident occurred within the Zurich Area Control Centre (ACC) airspace. A Swissair flight, SWR517, operating an Airbus A320 (registration HB-IJG) from Düsseldorf to Zürich, was cleared to descend to flight level 140 while entering the SAFFA holding pattern. Simultaneously, a private Dassault FA50 (registration OE-HIT) was performing a flight from Vienna to Zürich.
The aircraft with the faulty transponder, OE-HIT, was flying at flight level 150. As the Airbus A320 descended, the flight crew suddenly observed the Dassault FA50 approximately 100 meters to their front left. The crew immediately took evasive action by disconnecting the autopilot and maneuvering the aircraft to descend to flight level 146 to maintain separation. There were no fatalities or injuries resulting from this event.
The investigation
The investigation by the SUST examined the operational environment at the Zurich ACC East sector. At the time of the incident, the radar controller (Radar Executive) had just undergone a shift handover. The investigation established that the electronic data exchange system with Germany was non-functional, forcing controllers to rely on telephone coordination for all traffic data. This increased the workload for the Radar Planner, who was preoccupied with managing data transfers and supporting the Controller Assistant.
Furthermore, the investigation found that the aircraft OE-HIT was operating with a defective transponder. Because the secondary radar signal could not be maintained, the aircraft was instructed to use "Squawk Stand-by," which rendered it invisible on the controller's secondary radar screen. The investigation also noted that a Radar Coordinator was present but had not been actively engaged by the Radar Executive to monitor the specific descent clearance that led to the conflict.
Findings
- The primary cause of the incident was the inattention of the Radar Executive controller, which resulted in the temporary loss of the aircraft OE-HIT from their active control plan.
- The acceptance of an aircraft with a defective transponder during a period of relatively dense traffic contributed to the risk.
- Technical failures in the electronic data exchange with Germany significantly increased the controller's workload and reduced the effectiveness of traffic monitoring.
- A noisy working environment and high levels of coordination-related stress prevented the Radar Coordinator from noticing the erroneous descent clearance.
- The lack of a TCAS warning and the failure of the Short Term Conflict Alert (STCA) were due to the absence of transponder altitude data from OE-HIT.