What happened
On September 16, 1999, an air traffic incident occurred approximately 15 nautical miles from the touchdown point of runway 14 at Zurich Airport. A Swissair Airbus A319, operating flight SWR467, was arriving from Budapest using an FMS RNAV transition. To optimize the arrival sequence, the arrival controller (APE) instructed the aircraft to take a shortcut directly to waypoint ZH 993.
Shortly after, the aircraft was handed over to the final approach controller (FINAL). Simultaneously, a Crossair MD-80, operating flight CRX8 855, was approaching from the west to intercept the ILS runway 14. As both aircraft converged on the final approach path from opposite directions, the separation between them rapidly decreased. The final approach controller recognized the impending conflict and immediately ordered the Swissair aircraft to reduce speed and turn to a heading of 250 degrees to reposition it behind the Crossair flight. Despite these interventions, the aircraft came within approximately 1.5 nautical miles of each other with only 100 feet of vertical separation before the conflict was resolved.
The investigation
An investigation by the Swiss Transportation Safety Investigation Board (SUST) established that both aircraft were operating under Instrument Flight Rules (IFR) within Class C airspace and were in continuous contact with air traffic control. The investigation examined the coordination between the different arrival sectors (APE, APW, and FINAL) and the impact of then-newly implemented FMS-based approach procedures.
Investigators found that while the arrival sequence was correctly displayed on the controller's coordination screen, the controller in the East Sector (APE) had misunderstood the actual order of arriving traffic. The controller mistakenly used flight SWR 3969 as the reference for the arrival sequence rather than the actual preceding aircraft, CRX 855.
Findings
- The primary cause of the incident was that the arrival controller in the East Sector failed to recognize the actual arrival sequence.
- The controller's error was compounded by the use of FMS-based approach procedures, which required increased workload to verify the correct following of transition routes.
- High traffic density in both arrival sectors contributed to the complexity of the situation.
- The separation between the two aircraft reached levels as low as half of the required minimums during the encounter.
Safety action
Following the investigation, the use of FMS-based approach procedures was suspended to prevent similar occurrences during periods of heavy traffic.