What happened
On July 11, 2020, a mid-air collision occurred approximately 2 km northwest of the Borkenberge airfield near Dülmen, Germany. Two gliders, a Rolladen-Schneider LS4-b and a DG Flugzeugbau DG-300, were both maneuvering to utilize a developing cumulus cloud for thermal lift.
Radar data and flight logs indicate that the LS4-b was performing left-hand circles at approximately 1,280 m before proceeding westward. The DG-300 was flying roughly 80 m below and to the right of the LS4-b. As the LS4-b overtook the DG-300, the latter pilot reduced speed and began a straight-line climb. At approximately 13:42:30 local time, the two aircraft collided at an altitude of roughly 1,100 m.
The impact caused the destruction of both aircraft. The pilot of the DG-300 was found fatally injured in the cockpit. The pilot of the LS4-b managed to exit the aircraft using a rescue parachute, but was found deceased outside the wreckage.
The investigation
The BFU examined radar data from the Bundeswehr and air traffic services, as well as FLARM data from the aircraft. The investigation focused on the visibility of the aircraft to one another and the functionality of onboard collision avoidance technology.
Investigators analyzed the wreckage of both gliders, noting significant structural damage and impact marks that corresponded between the two aircraft. The investigation also looked into the status of the FLARM collision warning system in the DG-300 and the condition of the rescue parachute harness in the LS4-b.
Findings
- The primary cause of the collision was that neither pilot was able to see the other aircraft in time to initiate an evasive maneuver, effectively causing the failure of the "see and avoid" principle.
- The DG-300's FLARM collision warning system was non-functional because a required firmware update had not been performed, meaning the pilot received no electronic warning of the approaching LS4-b.
- The pilot of the LS4-b was likely distracted, as the rescue parachute harness was found with the leg straps unfastened; investigators noted the pilot may have opened them to urinate during the flight.
- The visibility of the DG-300 from the LS4-b was severely limited because the DG-300 was flying significantly lower and positioned in a way that obscured it from the higher aircraft's view during the overtake.
- The pilot of the DG-300 was unable to successfully execute the canopy jettison procedure, and investigators believe the canopy may have been torn away by aerodynamic forces due to the impact rather than being manually released.