Airprox Between SWR519 and SWR3587 at RILAX Holding (HB-IOF)

Incident Overview and Aircraft Details On December 1, 2000, at 21:43 UTC, an airprox occurred at the RILAX holding point involving two Swissair aircraft. The first aircraft was SWR519, an Airbus A321 with registration HB-IOF, operating a scheduled flight from Düsseldorf to Zürich. The second aircraft was SWR3587, a Saab 2000 (SB20) with registration HB-IZI, operating a scheduled flight from Köln to Zürich. Both aircraft were flying under Instrument Flight Rules (IFR) in Class C controlled airspace. ## Sequence of Events SWR519 was in the RILAX holding pattern, having been granted permission to extend its straight-leg holding time to two minutes. At 21:42:03 UTC, the Radar Executive North (RE-N) issued a descent instruction to flight DLH5591, an Avro RJ85, to descend to flight level 120. However, the pilot of SWR519 misinterpreted this instruction as being addressed to their own flight and read it back accordingly. The controller did not notice the error, and the Radar Planning (RP) officer did not hear the readback due to high traffic density. SWR3587 was also in the RILAX holding pattern at flight level 150, flying standard one-minute holds. At 21:42:09 UTC, SWR519 began descending based on the misinterpreted instruction. SWR3587 was positioned approximately 0.3 NM behind and 1,000 ft below SWR519. The pilots of SWR3587 maintained visual contact with SWR519 and received a TCAS Resolution Advisory (RA) to descend. ## Contributing Factors and Outcome The incident was primarily caused by the pilot of SWR519 misunderstanding an instruction intended for DLH5591, compounded by the controllers failing to hear the incorrect readback. Contributing factors included high traffic density, which caused frequency congestion, and the absence of a functioning Direction Finder (D/F) system. The two aircraft approached to a horizontal distance of 0.4 NM and a vertical distance of 600 ft before TCAS advisories and pilot actions restored separation. The report was issued by the Swiss Federal Office for Accident Investigation on October 30, 2001.