What happened
On May 8, 2020, a Pilatus PC-24, registration HB-VZZ, was conducting a production flight test near Buochs, Switzerland. The aircraft was flying under Visual Flight Rules (VFR) in Class E airspace. Simultaneously, an ASG 29E/18m glider, registration D-KAVE, was performing a cross-country flight, climbing through thermals near Neuheim.
At approximately 15:28 local time, the flight test engineer aboard the PC-2AX visually identified the glider directly ahead on the same altitude. The engineer immediately alerted the pilot via the onboard intercom. The pilot responded by disabling the autopilot and executing a rapid, controlled maneuver, banking the aircraft 60 degrees to the right and pitching the nose down. This maneuver prevented a collision, as the minimum separation between the two aircraft was approximately 320 meters horizontally and 60 meters vertically.
The investigation
An investigation by the Swiss Transportation Safety Investigation Board (SUST) examined flight data, cockpit video from HB-VZZ, and communications from Emmen Approach. The investigation established that the air traffic controller had no knowledge of the glider, as it did not appear on radar. Furthermore, the PC-24's Traffic Alert and Collision Avoidance System (TCAS) provided no warning because the glider's transponder was not transmitting a signal.
Findings
- The primary cause of the near-collision was the inherent limitations of the "see and avoid" principle, as the glider pilot did not perceive the approaching business jet.
- A significant contributing factor was that the transponder on the D-KAVE was switched off. Although not mandatory in this airspace, the pilot had disabled it to conserve battery life.
- The glider's PowerFlarm device, a collision warning system, failed to provide an acoustic alert because the specific version installed was unable to process transponder signals from faster-moving aircraft.
- The visibility of the glider was highly dependent on sunlight reflecting off its surfaces during its turns, making it difficult to detect from the side.