What happened
On May 31, 2015, at approximately 14:42 local time, a mid-air collision occurred near Oensingen, Switzerland, involving a Discus bT glider, registration HB-2207, and a DV 20 powered aircraft, registration HB-SCS.
The glider pilot had departed from Langenthal to seek thermals near the Jura mountains. During the flight, the pilot engaged in several engine-off maneuvers to utilize windmilling for restarts. Simultaneously, the crew of the DV 20 was flying from Birrfeld toward Grenchen.
As the two aircraft were on a collision course for approximately 80 seconds at an altitude of roughly 1,260 meters, the collision took place. The crew of the HB-SCS reported seeing the glider approaching from their left at a near 90-degree angle. The glider pilot experienced a sudden impact and heard a loud bang, observing the underside of the powered aircraft immediately before the contact.
Following the impact, the HB-2207 sustained damage to its left winglet, which was torn off. The HB-SCS suffered paint damage to its left wing and the loss of its tail skid. Both aircraft were able to continue their flights and land safely at their respective airfields without injuries to the occupants.
The investigation
The SUST investigation utilized data from the glider's Flarm collision warning device, the powered aircraft's AirNavPro GPS system, radar records, and radio communications. The investigation focused on the flight paths and the effectiveness of the crews' lookout procedures.
Investigators examined the technical equipment on both aircraft, noting that while the glider was equipped with a Flarm system, the powered aircraft relied on a Mode-S transponder. The investigation also reviewed the meteorological conditions, which were sunny with clear visibility, concluding that weather played no role in the incident.
Findings
- The primary cause of the collision was insufficient active lookout of the airspace by both flight crews.
- A significant systemic factor was the lack of a collision warning system on the DV 20.
- There was a technical incompatibility between the aircraft's safety systems, as the glider's Flarm system was unable to detect the signals from the powered aircraft's transponder.
Safety action
The investigation highlighted a recurring safety deficit regarding the limitations of the "see and avoid" principle and the lack of compatible collision warning technologies in general aviation. The report references previous recommendations for the Federal Office of Civil Aviation (BAZL) to increase crew awareness regarding collision risks and to work toward implementing standardized, compatible collision warning systems.