What happened
On June 30, 2018, a Mooney M20K, registration N15YB, was conducting an instrument approach to Grenchen Airport (LSZG) under Instrument Flight Rules (IFR). While established on the final approach course at approximately 6,000 feet, the pilot reported having just flown over a glider. The encounter involved a Duo Discus glider, registration HB-3-438, which was operating under Visual Flight Rules (VFR) near Roggwil.
The two aircraft passed within a horizontal distance of approximately 115 meters and a vertical separation of 175 meters. At the time of the encounter, the pilot of the N15YB was focused on configuring the aircraft for landing, which included reducing speed and managing cockpit checklists. The pilot of the N15YB noted that while the glider was visible due to good weather, no traffic information regarding the glider had been provided by air traffic control.
The investigation
The investigation by the Swiss Transportation Safety Investigation Board (SUST) established that the glider's transponder was incorrectly set to "ON" instead of "ALT" (Altitude), meaning the aircraft's altitude was not being transmitted to radar. This prevented air traffic controllers from providing accurate traffic information and rendered ground-based conflict alert systems ineffective.
Furthermore, the investigation found that the glider pilots were unaware that the area near Langenthal served as an approach point for IFR traffic to Grenchen. This information, which was previously available on official VFR charts, is no longer published, creating a gap in situational awareness for glider pilots.
Findings
- The Mooney M20K pilot was experiencing high workload during the approach phase, leading to increased "head-down" time in the cockpit.
- The Duo Discus transponder failed to transmit altitude data because it was incorrectly configured to mode S without altitude reporting.
- There was a lack of interoperability between collision avoidance systems; the glider's Flarm system could not detect the N15YB, and the N15YB lacked a collision warning system to detect the glider.
- The removal of IFR approach information from official VFR charts represents a systemic risk to airspace users.