What happened
On December 18, 2020, a Bombardier CL-600-2B19 (registration 9H-ILB) was performing an instrument approach to Sion Airport. Simultaneously, a Diamond DA-40 (registration HB-SDV) was flying under visual flight rules (VFR) through the same area, traveling southbound.
While air traffic controllers provided traffic information to both aircraft, the pilots were unable to visually acquire the other aircraft in time to maintain separation using the "see and avoid" principle. At approximately 12:22 UTC, the 9H-ILB received a TCAS Resolution Advisory (RA) commanding a climb. The crew of the 9H-ILB followed the instruction, while the pilot of the HB-SDV also initiated a climb to avoid the jet. Despite these maneuvers, the aircraft reached a minimum vertical separation of only 100 ft and a horizontal distance of 1.1 NM.
The investigation
The Swiss Transportation Safety Investigation Board (SUST) examined the airspace structure and operational procedures in the Sion region. The investigation focused on the status of the Terminal Control Area (TMA) Sion at the time of the incident. At the time of the encounter, the TMA was not active, meaning the aircraft were operating in Class E airspace.
Investigators also reviewed the history of similar near-misses in the area, noting a pattern of close encounters between IFR and VFR traffic near Sion. The investigation looked into the specific activation procedures for the Sion TMA, which, unlike other Swiss TMAs, cannot be activated instantly via radio but requires prior publication through NOTAM and DABS.
Findings
- The primary cause of the near-miss was that both flight crews identified the opposing aircraft too late to implement effective avoidance maneuvers using visual scanning.
- A contributing factor was the inability to activate the Sion TMA on short notice due to outdated procedural requirements, which left the area in Class E airspace where VFR traffic is expected but not necessarily monitored by radar or radio.
- The use of the TCAS RA on the 9H-IILB was the critical factor that prevented a collision.