History
On the morning of 12 January 2006, a Swiss Air Force Learjet 35A, registration T781 and call sign SUI 555, departed Zurich-Dübendorf for a military flight to Banja Luka, Bosnia-Herzegovina. The flight route initially proceeded via GERSA, N/UN850, ODINA, UM727, and SRN. The aircraft was not equipped with an airborne collision avoidance system (ACAS II).
At the same time, a Tunisair Boeing 737-600, registration TS-IOK and call sign TAR 602, was operating a scheduled flight from Tunis to Zurich. The aircraft was equipped with ACAS II (TCAS) from Honeywell. Entry into Zurich airspace was planned via CANNE, UZ651, and KELIP.
Incident Timeline
At 08:06:57 UTC, the crew of TAR 602 reported to Milan ACC and received clearance to descend to FL 280 after crossing another aircraft. This flight level and the CANNE entry point were coordinated with Zurich air traffic control. At 08:14:01 UTC, the crew of TAR 602 was instructed to contact Zurich Sector U.
Meanwhile, after starting at Zurich Sector South, SUI 555 climbed to FL 240 and then, following instructions from Radar Executive Upper (RE-U) air traffic controller, turned to heading 165° before being cleared to climb to FL 270 at 08:12:46 UTC. At 08:13:54 UTC, the RE-U cleared SUI 555 for a direct flight to SRN, after which the crew contacted Milan ACC at 08:14:54 UTC, requesting level 390. The Milan ACC controller coordinated with Rome ACC, who noted traffic conflicts. At 08:16:15 UTC, Milan ACC cleared SUI 555 to climb to FL 280 and head direct to waypoint OSKOR, a turn that brought it toward TAR 602.
At 08:16:28 UTC, TAR 602 was at FL 280, 1.5 NM south of CANNE. Shortly after, the RE-U ATCO observed SUI 555 climbing from FL 270 to FL 274. At 08:17:00 UTC, the STCA was triggered, and the RE-U ATCO informed TAR 602 of traffic below climbing. The crew of TAR 602 received a TCAS resolution advisory (RA); the first officer switched off the autopilot and initiated a climb. The aircraft reached FL 289 before descending back to FL 280. The crew could not visually acquire the other aircraft.
The crew of SUI 555, in a left turn toward OSKOR and passing FL 276, saw TAR 602 from the 2 o'clock position. The first officer observed the aircraft about 10 seconds before crossing, and the commander about 2 seconds before. The two aircraft crossed with a vertical separation of 700 ft and a lateral separation of 0.5 NM, in the area of responsibility of Zurich Sector U, 4.5 NM north of the responsibility line between Zurich and Milan ATC. Both aircraft continued to their destinations.
Aircraft and Crews
- SUI 555: Learjet 35A, registration T781, operated by Swiss Air Force. Crew: one Swiss commander and one Swiss first officer. Flight type: IFR military flight.
- TAR 602: Boeing 737-600, registration TS-IOK, operated by Tunisair. Crew: one Tunisian commander and one Tunisian first officer. Flight type: IFR scheduled flight.
Air Traffic Control and Coordination
The incident occurred in airspace C. ATC units involved were Zurich Sector Upper U (with Radar Executive Upper and Radar Planner Upper controllers) and Milan ACC. Coordination procedures between Zurich and Milan were according to ATC Zurich operating regulations. Military flights like SUI 555 were handled as General Air Traffic (GAT) under civil control.
Weather Conditions
According to MeteoSwiss, the weather in the area at the time included broken cloud (2/8 at 13,000 ft AMSL, 6-7/8 cirrus at 25,000 ft AMSL), visibility over 30 km, wind north-north-east at 25 kt, temperature -42°C, dewpoint -56°C, and QNH 1030 hPa at Lugano. No hazards were detected.