What happened
On June 22, 2018, a Bombardier CL-600-2B16 (registration HB-JWA), operated by Schweizerische Luft-Ambulanz AG, was on an instrument approach to Zurich Airport. The aircraft was being vectored by Zurich Approach West at an altitude of 6,000 ft AMSL. Simultaneously, a JS-MD.1C glider, registration D-KAHZ, was performing a cross-country flight in Class E airspace near the Leibstadt nuclear power plant.
The glider pilot, while turning north, observed the medical jet approaching from the west at an estimated altitude 500 ft higher. During the encounter, the two aircraft reached a minimum horizontal separation of 0.57 NM and a vertical separation of approximately 690 ft. The crew of the HB-WJA did not receive any alerts from their Traffic Alert and Collision Avoidance System (TCAS) and continued their approach, subsequently notifying air traffic control of the glider's position.
The investigation
The investigation focused on why the collision avoidance systems failed to provide warning. It was established that the D-KAHZ was not equipped with a transponder. While the glider was equipped with a PowerFlarm module for collision awareness, this system only warns pilots when other aircraft also carry compatible equipment.
Furthermore, because the glider lacked a transponder, it was invisible to the air traffic controller's radar. This prevented the use of the Short Term Conflict Alert (STCA) system and meant controllers could not provide traffic information to other aircraft in the area. The investigation also noted that the glider reached a maximum altitude of 5,400 ft AMSL, placing it just below the minimum vectoring altitude (MVA) of 5,500 ft.
Findings
- The primary cause of the near-collision was the lack of a transponder on the glider, which prevented the HB-JWA's TCAS from generating a Traffic Advisory (TA) or Resolution Advisory (RA).
- The absence of a transponder rendered the glider invisible to air traffic control, eliminating the possibility of radar-based conflict alerts.
- The glider was operating near the boundary of controlled airspace, and the lack of vertical or horizontal buffer zones at this airspace limit contributed to the proximity of the two aircraft.
- Visual separation was only possible due to favorable weather conditions with high visibility.