What happened
On the night of the accident, a Boeing 757-200 was climbing to FL 360 under the direction of ACC Zürich. Simultaneously, a Tupolev Tu154M was operating in the same airspace. While the Boeing crew was managing a cockpit discussion regarding a TCAS display, the co-pilot briefly left his station to use the lavatory, leaving the captain in sole command.
At approximately 21:34 UTC, the TCAS system issued a "Traffic" alert to both aircraft. The air traffic controller at ACC Zürich instructed the Tupolev Tu154M to expedite a descent to FL 350 due to crossing traffic. Following this instruction, the Tupolev Tu154M initiated a descent. However, the TCAS system issued a Resolution Advisory (RA) commanding the Tupolev Tu154M to climb. The crew of the Tupolev Tu154M continued the descent as instructed by ATC, even as the aircraft's descent rate reached approximately 1,900 ft/min.
Concurrently, the Boeing 757-200 received a TCAS RA commanding a descent. The pilot in command initiated the descent, but the aircraft's separation from the Tupolev Tu15 .4M dropped below the required 7 NM radar minimum. The collision occurred at 21:35:32 UTC at an altitude of 34,890 feet.
The investigation
The BFU investigation examined flight recorder data, TCAS sequences, and radar tracks from both aircraft. The investigation also reviewed the operational procedures at ACC Zürich, specifically the Single Manned Operation Procedures (SMOP) in effect at the time. The investigation analyzed the sequence of TCAS alerts, the timing of ATC instructions, and the vertical profiles of both aircraft leading up to the impact.
Findings
- The primary cause of the collision was the failure to adhere to TCAS Resolution Advisories by both crews.
- The Tupolev Tu154M crew followed an ATC instruction to descend to FL 350, which directly contradicted the TCAS RA command to climb.
- The Boeing 757-200 crew followed a TCAS RA to descend, which placed them on a collision course with the descending Tupolev Tu154M.
- The air traffic controller provided information regarding the position of conflicting traffic that was inaccurate, stating traffic was at the 2 o'clock position when it was actually at the 10 o'clock position.
- The investigation noted systemic issues at the ACC Zürich control center, including the use of single-manned operations during periods where traffic demand and technical limitations (radar/telephone issues) increased operational risk.