What happened
On March 3, 2021, at approximately 11:27 AM, a Diamond DA 40 NG, registration HB-SGD, was conducting a flight from Zurich to Lugano. The flight was planned using a combination of instrument and visual flight rules. While cruising at 10,000 ft AMSL near Bellinzona, the pilot of HB-SGD received an alert from the onboard Traffic Advisory System (TAS) indicating another aircraft at a 1 o'clock position.
Simultaneously, a Socata TB21, registration N705TB, was climbing through the same altitude near the same location. The pilot of N70/TB was operating under visual flight rules (VFR) in Class E airspace, heading toward a navigation point.
Despite the warning from the TAS on HB-SGD, the crew of that aircraft struggled to visually identify the approaching plane. Approximately 20 seconds after the initial warning, the crew of HB-SGD spotted the Socata TB21 crossing their path at the same altitude. Radar analysis confirmed a critical separation breakdown, with the aircraft passing just 100 ft vertically and 0.1 NM horizontally apart.
The investigation
The Swiss Transportation Safety Investigation Board (SUST) examined the flight plans, radar data, and communication logs. The investigation established that the pilot of N705TB was operating in Class E airspace, where the responsibility for separation rests entirely on the "see and avoid" principle. The investigation also noted that the pilot of N705TB had recently been assigned a new transponder code, which likely caused a distraction in the cockpit during the climb.
Furthermore, the investigation found that air traffic services in Locarno and Zurich Information lacked the capacity to provide traffic advisories to the crews at that time. The two aircraft were also communicating on different radio frequencies, preventing direct coordination.
Findings
- The Socata TB21 was not equipped with a collision warning system (TAS).
- The crew of HB-SGD failed to identify the approaching aircraft in a timely manner due to the difficult viewing geometry from the left seat, combined with diffuse lighting and a flight path toward the sun.
- The pilot of N705TB relied on the expectation of receiving traffic information from air traffic services, which were unable to provide such warnings due to capacity constraints.
- The pilot of N705TB was distracted by the task of updating the transponder code during the climb.