What happened
On 23 May 2019, a Bombardier BD100 1A10 Challenger 300, operated by Premium Jet Ag, was performing a positioning flight from Zurich to Basel-Mulhouse airport. While descending through 4,600 ft in a left turn as part of an ILS approach to runway 15, the crew observed a glider in close proximity. The glider, a Schempp Hirth HS-5 registered D-KPHJ, was performing left-hand spirals to regain altitude.
Upon spotting the glider, the pilot flying of the Challenger 300 disconnected the autopilot and executed an evasive maneuver, banking the aircraft to 45° and increasing the rate of descent. The crew estimated the lateral separation was less than 30 meters and the vertical separation was approximately 1 meter. No contact or damage occurred, and the aircraft landed safely.
The investigation
The investigation analyzed Flight Data Recorder (FDR) data from the business jet, radio communications, and Open Glider Net (OGN) data. Investigators established that the glider had entered the Basel TMA 3 (Class D airspace) at approximately 5,200 ft and remained within the controlled airspace for roughly 20 minutes without making radio contact with air traffic controllers.
While the glider pilot believed he was flying below the controlled airspace, FDR data indicated the glider was actually at a higher altitude, entering the TMA at 5,200 ft and exiting at 3,900 ft. Furthermore, the investigation found that the glider's transponder had not been activated, which prevented the aircraft's TCAS from detecting the threat and left controllers unaware of the glider's presence.