Incident Overview
On 24 August 2000 at 1027 UTC, an Airprox (near midair collision) occurred 12 nautical miles south of CANNE between Swissair Flight 302 (SWR302) and Crossair Flight 3633 (SWR3633). Both aircraft were operating under Instrument Flight Rules (IFR) in Class C airspace.
Aircraft and Flight Information
SWR302, an Airbus A321 (registration HB-IOA), was operating Swissair flight from Zürich to Athens. SWR3633, a Saab 2000 (registration HB-IZL), was operating Crossair flight from Genoa to Zürich.
Sequence of Events
SWR302 departed Zürich and climbed southbound along ATS route N/UN 850. It received clearance to climb to flight level 270 (FL270). At 10:24:45, when approximately 10 NM north of ODINA and passing FL226, the Zürich south sector radar executive controller instructed SWR302 to contact Milano Control. SWR302 subsequently changed frequency and received a new transponder code (0451) from Milano.
At 10:27:42, SWR302 appeared on the radar monitor with the new code, 15 NM south of ODINA climbing through FL254. At the same time, SWR3633 was approximately 7 NM north-northeast of SWR302, still displaying the Milano transponder code. The two aircraft had crossed paths with a lateral separation of 3.8 NM and a vertical separation of about 500 feet.
SWR3633 was flying northbound at a stable FL240. According to its Air Traffic Incident Report (ATIR), the crew received a TCAS Traffic Advisory (TA) about 10 NM before the crossing, followed by a Milano controller instruction to turn right 20°. Seconds later, a TCAS Resolution Advisory (RA) "descend!" was issued, but the crew did not comply because they visually sighted the other aircraft and controlled the crossing visually.
SWR3633 first contacted Zürich sector south at 10:31:11, about four minutes after the incident.
Coordination and Data Exchange
The automatic OLDI (On Line Data Interchange) system recorded that Zürich sent an activation (ACT) message for SWR302 to Milano at 10:11:49, estimating ODINA at 10:24 and FL270. Milano sent an ACT for SWR3633 to Zürich at 10:16:33, estimating CANNE at 10:29 and FL240. Despite this, the flight plan data for SWR3633 were not available at the Zürich south sector radar planner and radar executive positions until after the incident. The controller assistant (CA) workstation, responsible for printing and distributing flight progress strips, sometimes experienced delays or printing failures. The system manager indicated that such irregularities had been reduced by late 1999 but not eliminated. System log recordings for the day were not preserved.
Technical Limitations of Radar Display
The radar data display system at Zürich (MV9800) processed only the current transponder code for an aircraft; the previous code used in another sector was not passed to the radar computer. Therefore, SWR3633 remained invisible on the Zürich radar display while still using the Milano transponder code. As of 19 February 2001, a request to enable previous code transmission had been rejected multiple times by skyguide technical division.
Controller Actions
The south sector radar executive controller responsible for SWR302 was aware of the requirement under the Letter of Agreement with Milano Control that flights from Zürich capable of crossing ODINA at FL270 should do so. To avoid an uneconomical high climb rate and to prioritize passenger comfort, the controller did not issue an expedite climb instruction. Instead, he transferred SWR302 to Milano early, believing this best served Milano's traffic flow. He was unaware of SWR3633 due to the missing flight progress strips, which eliminated any perceived conflict.
Conclusion of Investigation
The investigation determined that the incident was caused by the south sector air traffic controller not transferring SWR302 to Milano control according to agreed procedures. A contributing factor was the lack of flight plan data for SWR3633 at the south sector workstation, likely due to a technical problem with strip printing.