What happened
On December 1, 2000, at approximately 21:43 UTC, an airprox incident occurred within the controlled Class C airspace near Zurich. An Airbus A321, operating as SWR519, was flying a scheduled service from Düsseldorf to Zürich. While positioned in the RILAX holding pattern, the pilot of SWR519 mistakenly intercepted a descent instruction intended for a different aircraft, a Lufthansa flight (DLH5591).
Simultaneously, a Saab 2000, operating as SWR3587, was also in the RILAX holding pattern on a flight from Cologne to Zürich. Because the pilot of SWR519 believed they had been cleared to descend to flight level 120, the aircraft began a descent that placed it on a collision course with SWR3587, which was positioned approximately 1,000 feet below. The separation between the two aircraft decreased to a horizontal distance of 0.4 NM and a vertical distance of 600ft.
The investigation
The Swiss Transportation Safety Investigation Board (SUST) examined the communications between the Zurich Radar North Sector and the involved aircraft. The investigation established that the air traffic controller issued a descent clearance to DLH5591, but the pilot of SWR519 incorrectly applied the instruction to their own flight. The controller did not notice the incorrect readback, and the adjacent controller was unable to hear the error due to high traffic density and coordination tasks.
Findings
- The primary cause was a misunderstanding by the pilot of SWR519, who mistakenly followed an instruction meant for DLH5591.
- The error was compounded because the air traffic controllers did not identify the incorrect readback.
- High traffic density and frequency congestion contributed to the lack of situational awareness.
- The absence of a fully functional Direction Finder (D/F) prevented the controller from identifying that the radio transmission originated from a different position than the intended recipient.
- The pilots of SWR3587 were able to maintain visual contact with the aircraft above them, which significantly mitigated the danger of the encounter.
- The Traffic Collision Avoidance System (TCAS) provided Resolution Advisories (RA) to both aircraft, prompting necessary altitude changes to restore separation.