What happened
On 12 January 2006, a Swiss Air Force Learjet 35A, operating flight SUI 555, was conducting a military flight from Zurich-Dübendorf to Banja Luka. While navigating the airspace near the Monte Ceneri region, the aircraft was directed by Zurich air traffic control to a new heading to maintain separation from other traffic. Simultaneously, a Tunisair Boeing 737-60 and (registration TS-IOK) was on a scheduled flight from Tunis to Zurich.
During the transition of communication between Zurich and Milan air traffic control, the Milan controller issued a clearance to the Learjet to climb to FL 280 and fly direct to waypoint OSKOR. At the same time, the Boeing 737 was flying on a reciprocal track. As the Learjet climbed through FL 274, the two aircraft converged. The Boeing 737's TCAS system triggered a Resolution Advisory (RA), prompting the crew to climb to FL 289 to avoid a collision. The aircraft eventually crossed with a lateral separation of only 0.5 NM and an altitude difference of 700 ft.
The investigation
The investigation focused on the coordination between Zurich and Milan air traffic control units. Investigators examined radar recordings, which showed that the Short Term Conflict Alert (STCA) was triggered just seconds before the encounter. The probe looked into the specific instructions issued by the Milan controller, noting that the clearance to climb and change course was given while the Learjet was still within the Zurich Sector U area of responsibility. The investigation also reviewed the equipment capabilities of both aircraft, noting that the Learjet was not equipped with ACAS II, whereas the Boeing 737 was.
Findings
- The primary cause of the serious incident was the issuing of instructions by ATC to an aircraft outside its own area of responsibility.
- The Milan controller issued a climb and heading clearance to the Learjet without coordinating the change with Zurich air traffic control.
- A loss of situational awareness by the Milan controller contributed to the error.
- The Learjet was not equipped with an airborne collision avoidance system (ACAS II).
- The separation between the two aircraft was dangerously low, with only 0.5 NM of lateral distance and 700 ft of vertical distance at the point of closest approach.