What happened
On April 18, 2015, at approximately 14:31 local time, a Dassault-Aviation Falcon 2000 flying from Munich to Paderborn experienced a serious loss of separation with an unidentified glider. The aircraft, operated by a German company, was flying under Instrument Flight Rules (IFR) and carrying a crew of two and four passengers.
While descending through 5,000 ft AMSL, the crew observed a glider approaching on a head-on course at the same altitude. The crew reported that the glider became visible at a distance of approximately 1,000 meters, and the lateral separation between the two aircraft was estimated at roughly 500 meters.
During the same period, a second encounter occurred involving the Falcon 2000 and a different glider. This second aircraft was operating with an active transponder, allowing the air traffic controller to provide traffic information and a maneuver recommendation. The minimum separation during this second event was approximately 1.3 nautical miles horizontally and 100 feet vertically; however, the BFU did not classify this second encounter as a serious incident.
The investigation
The BFU investigation examined radar data from both the air traffic control organization and the German military (Bundeswehr). The investigation focused on the visibility of aircraft within the controlled airspace and the capability of surveillance systems to detect non-transponder-equipped targets.
Radar analysis revealed several primary targets without transponder information in the flight path of the Falcon 2000. However, at the specific moment of the described encounter, no active radar target was documented in the immediate vicinity of the Falcon 2000's flight path.
Findings
- The primary cause of the serious incident was the lack of transponder information from the glider, which prevented continuous radar tracking and altitude monitoring.
- Because the glider was not transmitting a transponder signal, the aircraft's Airborne Collision Avoidance System (ACAS) was unable to detect the threat or generate resolution advisories.
- Air traffic controllers were unable to provide timely traffic information or avoidance instructions because the glider was not visible as a continuous radar target with altitude data.
- The origin and destination of the glider could not be determined due to the absence of transponder data.