Incident Overview
On February 14, 2005, a serious airprox occurred in German airspace near the RILAX intersection, approximately four nautical miles southeast of Zurich. The incident involved a Tupolev Tu-154M operating a ferry flight from Warsaw to Zurich and an Airbus A319 arriving from Cologne/Bonn. Both aircraft were under the control of Zurich Air Traffic Control (ATC). The investigation was delegated to the Swiss Bureau for Aircraft Accident Investigation (BFU) due to the ATC jurisdiction, though German legislation applied to the report's structure.
Sequence of Events
At 06:52 UTC, the Tu-154M contacted Zurich Area Control and was instructed to descend to Flight Level (FL) 170. Shortly after, at 06:53 UTC, the A319 made contact and was cleared to descend to FL 150, then subsequently to FL 130. The Tu-154M received a clearance to descend to FL 150 around 06:57 UTC. Both crews transferred to Zurich Approach East (APE) for vectoring onto the ILS approach for Runway 14.
The APE controller assigned the A319 a heading of 180 degrees and the Tu-154M a left turn to heading 150 degrees, with both instructed to reduce speed to 210 KIAS. The controller intended to manage both aircraft on a long approach. However, the Tu-154M continued descending beyond its cleared altitude of FL 150.
Conflict and Resolution
At 06:59:15 UTC, the APE controller received a Short Term Conflict Alert (STCA) warning. Radar data indicated the Tu-154M had descended to FL 138, while the A319 was level at FL 130. The vertical separation had reduced to approximately 300 feet with a horizontal distance of 1.5 nautical miles.
The controller immediately instructed the Tu-154M to stop descending and provided traffic information regarding the A319. The Tu-154M crew reported seeing the other aircraft. The controller then ordered the Tu-154M to climb back to FL 140, which it achieved by 07:01 UTC. The A319 crew received a TCAS Traffic Advisory (TA) and observed the Tu-154M descending before stopping and climbing away.
Analysis of Causes
The investigation highlighted that the Tu-154M crew believed they had been cleared to descend to FL 110, despite no such clearance being issued in the radio recordings. The controller's initial instruction included multiple instances of the number 'one' (heading 150, speed 210), which may have contributed to a memory error regarding the altitude clearance. The report noted that the Tu-154M crew's adherence to Crew Resource Management (CRM) principles was suboptimal, as they failed to verify the conflicting altitude instruction against their cleared limits. Weather conditions were stable with light snow and deicing operations in progress at Zurich Airport.