What happened
On January 18, 2000, at approximately 15:43 UTC, an air traffic incident occurred in the SAFFA region involving two Swissair flights. The first aircraft, an Airbus A320 with registration HB-IJQ (SWR471), was operating a scheduled flight from Kiev to Zurich. The second aircraft, a McDonnell Douglas MD11 with registration HB-IWG (SWR169), was operating a scheduled flight from Tokyo to Zurich.
As SWR471 approached the SAFFA intersection, the Zurich Area Control Centre (ACC) instructed the crew to descend to flight level 140 and to maintain that altitude upon arrival. The crew acknowledged the instruction and reported having other traffic in sight. However, instead of leveling off at the assigned altitude, the aircraft continued its descent, passing through flight level 131. At the same time, SWR169 was descending through flight level 130.
Upon realizing the altitude deviation, the pilot of SWR471 queried their assigned altitude. The controller confirmed the requirement to maintain flight level 140 and subsequently issued a heading change to 240 degrees to avoid a conflict with SWR169. Although the crew of SWR471 immediately engaged corrective maneuvers, including disabling the autopilot to initiate the turn and climbing back to flight level 140, the horizontal separation between the two aircraft had already decreased to 5 nautical miles. The aircraft eventually passed each other with a minimum vertical separation of only 200 feet and a horizontal distance of 3.3 nautical miles.
The investigation
The investigation by the Swiss Transportation Safety Investigation Board (SUST) focused on the altitude deviation and the breakdown in separation. Investigators examined radio transcripts and radar data from the Zurich ACC East Sector. The investigation established that while both aircraft were in contact with radar controllers, the primary issue stemmed from the unauthorized descent of SWR471.
Investigators also looked into why the Traffic Collision Avoidance System (TCAS) did not provide an alert. It was confirmed that neither aircraft received a TCAS warning during the encounter. Furthermore, the investigation clarified that the traffic the crew of SWR471 initially reported seeing was not SWR169, but likely another aircraft (SWR3439) operating at a different altitude.