What happened
On the afternoon of September 25, 2024, a Piper PA-28RT-201 Arrow IV, registration HB-PEL, was performing an instrument approach to Grenchen Airport (LSZG) under Instrument Flight Rules (IFR). During a missed approach (go-around) following the arrival from Les Éplatures, the aircraft unintentionally entered the active glider restricted area LS-R 83.
Simultaneously, a Schleicher ASK 21 glider, registration HB-3026, was performing aerobatic maneuvers within the same restricted area. The pilot of the glider identified the approaching motor aircraft and alerted the tower. The two aircraft reached a minimum separation of approximately 80 meters horizontally and 190 meters vertically, classifying the event as a Category B2 airprox.
The investigation
The Swiss Transportation Safety Investigation Board (SUST) examined flight track recordings, radio communications, and pilot statements. The investigation focused on the proximity of the IFR missed approach procedure to the boundaries of the glider restricted area. Investigators also reviewed the technical equipment of both aircraft, noting that the HB-PEL lacked ADS-B-in capabilities and the HB-3026 had its ADS-B-out transponder turned off, which is common practice in that specific restricted area to avoid cockpit distractions for motor pilots.
Findings
- The primary cause of the near-collision was that the HB-PEL entered the LS-R 83 restricted area during its missed approach, and neither pilot detected the other in time via visual means.
- Systemic cause: The lateral distance between the IFR missed approach path and the LS-R 83 boundary is only 120 meters. This narrow margin is difficult to detect using standard cockpit navigation instruments (HSI) during an IFR procedure.
- The aircraft were not equipped with compatible collision warning systems that could have provided mutual alerts.
- The pilot of the HB-PEL experienced a course deviation during the missed approach, which may have been due to task saturation or distraction.
Safety action
SUST issued Safety Recommendation No. 592 to the Federal Office of Civil Aviation (BAZL), urging them to work with stakeholders to ensure that regular airspace protection values are used for IFR approaches at Grenchen, or that other measures are implemented to minimize the safety deficit. Additionally, a safety notice was issued to pilots and controllers regarding the tight separation between IFR traffic and glider operations in this area.