What happened
On the afternoon of 10 September 2005, two commercial flights operating in Swiss airspace experienced a serious airprox incident. The first aircraft, a Swiss International Air Lines Embraer ERJ145LU, registration HB-JAY, was performing a scheduled flight from Zurich to Luxembourg. To avoid an active thunderstorm cell, the crew requested and received clearance to deviate from their assigned departure route.
Simultaneously, an Easyjet Switzerland Airbus A319-111, registration HB-JZJ, was operating a flight from Basle to Rome-Ciampino. This crew also requested a deviation from their standard departure route to avoid similar convective weather. As both aircraft maneuvered to bypass the storm, they entered a conflict course.
At approximately 15:27 UTC, the two aircraft converged within 1.8 NM of each other with an altitude separation of only 500 feet. Both crews experienced Traffic Advisory (TA) alerts on their TCAS units as the aircraft passed each other on opposing headings.
The investigation
The investigation examined the coordination between the Zurich and Basle air traffic control (ATC) units. It was established that both aircraft were deviating from standard routes due to unstable weather conditions in the region. The investigation focused on the communication between the Basle Approach and Zurich Departure controllers, specifically regarding the intended flight levels and the coordination of the aircraft's climb.
Investigators found that while both ATC units could see the other aircraft on their radar monitors, there was a lack of clear information regarding the specific flight levels being maintained. The investigation also reviewed the telephone coordination between the Basle coordinator and the Zurich controllers, noting that the identity of the Zurich controller was not always clearly identifiable to the Basle unit.
Findings
- The primary cause of the incident was a failure in coordination regarding the flight levels of both aircraft during weather-related deviations.
- The Basle ATC coordinator mistakenly assumed that the Zurich controller was speaking from the Sector West unit, which led to misunderstandings regarding the aircraft's position and altitude.
- The Zurich Departure controller instructed the Embraer ERJ145LU to stop its climb at FL 090 because the Airbus A319-111 was not climbing to its assigned FL 110 as previously communicated.
- The crew of the Airbus A319-111 remained at FL 090 to avoid thunderstorms, a fact that was not clearly understood by all involved controllers during the coordination process.
- The presence of significant thunderstorm activity in the Olten-Liestal region necessitated the deviations that placed the aircraft on conflicting paths.