What happened
On April 25, 2013, at approximately 17:36 local time, a Boeing 737-800 operating an IFR flight from Alghero to Frankfurt-Hahn and an ASH 25 Mi motor glider operating under VFR rules experienced a close proximity event in Class E airspace, roughly 10 nautical miles east of Frankfurt-Hahn Airport.
The Boeing 737-800 was descending through 6,500 ft AMSL on an RNAV arrival when it passed the motor glider at a distance of only 0.19 NM and a vertical separation of 161 ft. The crew of the Boeing 737-800 reported the close encounter to Langen Radar shortly after the passing. The motor glider pilot, flying with the engine deactivated, had been alerted to a potential conflict by a collision warning device (TRX1090) and had initiated a left-hand turn to avoid the larger aircraft. The pilot of the motor glider maintained a parallel course to monitor the Boeing's flight path to ensure safety.
The investigation
The BFU examined flight data from the Boeing's Quick Access Recorder (QAR) and the motor glider's GPS. The investigation also reviewed radar records from both the air traffic control service and the German military. The investigation established that the motor glider was flying without an active transponder, as it was operating as a glider with the engine deactivated. Consequently, the aircraft did not appear on the Boeing's TCAS or the controller's radar as a distinct, identifiable target with altitude information.
Findings
- The Boeing 737-800 crew had no prior information regarding the presence of the motor glider and only visually identified the aircraft at the moment of passing, which was too late to execute an effective avoidance maneuver.
- The ASH 25 Mi was not visible on the Boeing's TCAS because it was not transmitting a transponder signal.
- The air traffic controller could not provide specific traffic information or avoidance recommendations because the motor glider lacked a transponder signal, making it impossible to distinguish the aircraft from other primary radar targets.
- The motor glider's small size, lack of lighting, and the high speed of the Boeing 737-800 significantly hindered visual detection.
- The lack of a transponder signal from the motor glider prevented both the flight crew and air traffic control from identifying the collision risk in time.