What happened
On February 14, 2005, at approximately 07:00 UTC, a Tupolev 154M operating a ferry flight from Warsaw to Zurich and an Airbus 319 arriving from Cologne/Bonn entered a dangerous proximity situation in German airspace under the control of Zurich Approach.
While both aircraft were being vectored for an ILS approach to runway 14, the air traffic controller (ATC) assigned the Airbus 319 to level 130 and the Tupolev 154M to level 150. However, the crew of the Tupolev 154M continued their descent well below the authorized altitude. The aircraft eventually closed to a vertical separation of only 300 ft and a horizontal distance of 1.5 NM. The crew of the Airbus 319 received a TCAS Traffic Advisory (TA) and observed the other aircraft descending toward them before the conflict was resolved.
The investigation
The investigation, conducted by the Swiss Transportation Safety Investigation Board (in coordination with German authorities), examined radar data, radio communications, and TCAS logic. Investigators analyzed the ATC instructions provided to both crews and reviewed the cockpit procedures of the Tupolev 154M.
Radio recordings confirmed that the ATC had explicitly cleared the Tupolev 154M to descend only to FL 150. The investigation also looked into why the crew believed they had authorization to descend to FL 110. The controller's Short Term Conflict Alert (STCA) system eventually triggered, prompting the controller to intervene and instruct the Tup0lev 154M to stop its descent and climb back to FL 140.
Findings
- The primary cause of the incident was that the Tupolev 154M crew failed to comply with the ATC instruction to maintain FL 150, continuing their descent for reasons that remain unconfirmed.
- The crew of the Tupolev 154M mistakenly believed they had been cleared to descend to FL 110. This error may have been triggered by a complex instruction from ATC that contained multiple repetitions of the number 'one' (e.g., heading 150, level 110, speed 210), leading to a potential misinterpretation during execution.
- The investigation identified a lack of effective Crew Resource Management (CRM) within the Tupolev 154M cockpit, as the crew failed to utilize 'closed-loop' communication or cross-check the descent against the actual ATC clearance.
- The TCAS on the Airbus 319 functioned correctly, issuing a Traffic Advisory based on the predicted flight paths of the two aircraft.
- High traffic volume at the Zurich approach sector contributed to the complexity of the environment at the time of the event.