What happened
On June 16, 2014, a serious near-collision occurred in the airspace near St. Moritz, Switzerland, involving two gliders. The first aircraft, a single-seat DG-808 C with registration HB-2461, was flying northeast near Piz Surlej. At approximately the same time, a two-seat DG-500/22 ELAN, registered as HB-3097, was traveling in the opposite direction across the north flank of Piz da Staz.
The pilot of HB-246 and had already performed a slight course correction to avoid a third glider (a Duo Discus) after receiving a warning from an onboard collision avoidance system. As the pilot of HB-2461 looked up after passing the first glider, they suddenly spotted the HB-3097 approaching at high speed on a conflicting path. Simultaneously, the crew of HB-3097 was focused on observing cables at a nearby cable car station. The pilot of HB-3097 only noticed the oncoming aircraft during a routine forward scan. Both crews initiated instinctive left-hand turns to avoid impact, resulting in the wingtips passing within a few meters of each other. All aircraft involved landed safely at Samedan Airport.
The investigation
SUST examined flight data from navigation computers and collision warning systems, as well as statements from the crews. The investigation focused on why the aircraft were not detected earlier. The investigation looked into the performance of the Flarm collision avoidance technology used in both aircraft. For HB-3097, investigators found that the system's transmission and reception range was insufficient. While the crew had previously attempted to improve the range by installing a new antenna, the effectiveness was limited. The investigation also analyzed environmental factors, noting that the background of bright clouds and snow-covered terrain made the slender silhouettes of the gliders extremely difficult to spot.
Findings
- The primary cause of the near-collision was the late visual detection of the opposing aircraft by both crews.
- The collision avoidance system on the DG-500/22 ELAN (HB-3097) was functioning with insufficient range.
- The crew of HB-3097 was distracted by a ground-based obstacle (cables at the Corvatsch station).
- The pilot of HB-2461 was focused on avoiding a different glider that had recently crossed their path.
- Environmental conditions, specifically a background of bright clouds and snow, significantly hindered mutual visibility.
Safety action
- Safety Recommendation 500: The Federal Office of Civil Aviation (FOCA) should work toward developing a technical procedure to verify the operational range of Flarm collision avoidance systems while on the ground.
- Safety Notice 1: Due to the difficulty of spotting gliders due to their slim profiles, the use of high-visibility color markings on gliders should be encouraged.
- Following the incident, the Flarm device in HB-3097 was replaced with a newer model, and the glider group involved replaced similar older units in their fleet.