What happened
On July 30, 2000, an air traffic incident occurred in Class C airspace during the transition to ATS Route UN871. A Swissair Boeing MD11, registration HB-IWN, was climbing through various flight levels following a departure from Zurich. Due to heavy thunderstorm activity, the aircraft had deviated from its standard route and requested a heading change to avoid weather cells.
Simultaneously, a Crossair Avro RJ-100, registration HB-IXX, was descending toward Zurich. Like the MD11, the RJ-100 was also flying off-track to avoid convective activity. As the two aircraft approached each other on opposing headings, a conflict developed regarding their vertical separation. The aircraft eventually came within 2 NM of each other with a vertical separation of only 800 FT. The collision was averted only when both crews executed TCAS RA (Resolution Advisory) maneuvers.
The investigation
The investigation focused on the coordination between the Zurich and Munich Air Traffic Control (ATC) centers. Investigators examined radar data and transcripts of communications between the radar planners. It was established that the Munich controller intended to use lateral separation by directing the HB-IXX to a more southerly course, while the Zurich controller was operating under the assumption that vertical separation would be maintained.
Crucially, the investigation found that a significant misunderstanding occurred during a coordination call. The Zurich radar planner failed to perceive the essential content of a message regarding a change in flight path for the HB-IWN. This failure was attributed to the fact that the controllers were using non-standard phraseology and were engaged in simultaneous, overlapping conversations. Furthermore, the heavy weather conditions and the limited climbing performance of the heavy MD11 made the pre-planned separation strategy highly risky.
Findings
- The primary cause was the failure of the Zurich radar planner to perceive critical information during a coordination exchange.
- This lack of perception was driven by the use of non-standard phraseology during coordination, which led to a breakdown in communication.
- The controllers employed a risky separation concept that was inappropriate for the prevailing weather conditions and the aircraft's performance limitations.
- The use of overlapping, non-standard communications prevented the Zurich controller from analyzing new data and proposing alternative clearances.