What happened
On June 15, 1998, at 13:43 UTC, a serious airprox incident occurred near the boundary between the Zurich and Geneva Flight Information Regions (FIR). The incident involved two aircraft: a SAAB2000, registration HB-IZL, operated by Swissair, and a SAAB340, registration HB-AKB, operated by Crossair.
Flight SWR3779 (SAAB2000) was en route from Marseille to Zurich. Prior to entering the Zurich West Sector, the aircraft had been instructed by Geneva controllers to descend to FL220. However, the pilot incorrectly set the Altitude Preselect Alert (APA) to FL200. Simultaneously, the SAAB340 was cruising at FL200 on a flight from Geneva to Lugano.
Upon contacting Zurich Radar West, the pilot of SWR3779 reported they were descending to FL200. This specific detail was not recognized by the controllers. Shortly thereafter, the controller noticed that SWR3779 was approaching the same altitude as the SAAB340. The two aircraft quickly closed in on each other, resulting in a significant breach of minimum separation standards, which require at least 0.6NM horizontally and 400 feet vertically.
The investigation
The investigation examined the communication between the aircraft and Zurich Radar West, as well as the technical performance of the radar tracking systems. Investigators looked into whether the aircraft's altitude preselect system had malfunctioned, as similar "jumps" in altitude settings had been reported in the SAAB2000 previously. However, the investigation confirmed that the pilot had manually and directly set the altitude to FL200.
Furthermore, the investigation scrutinized the Multi Radar Tracking (MRT) systems used by the Zurich and Geneva sectors. It was discovered that the flight path displayed by the Zurich MRT deviated significantly from the data provided by the La Dôle and Lägern radars. This discrepancy was attributed to errors in the Zurich Approach Radar's processing.
Findings
- The primary cause of the incident was that the pilot of SWR3779 incorrectly set the altitude preselect to FL200 instead of the instructed FL220.
- The incident was exacerbated because the Zurich West Sector controllers failed to note the pilot's initial report regarding the FL200 descent during their first contact.
- The Zurich Short Term Conflict Alert (STCA) was not available for operational use at the time, as it was still in a testing phase.
- Significant discrepancies existed between the flight path representations of the different radar systems, which could lead to serious safety risks.
Safety action
Following the investigation, a safety recommendation was issued calling for the urgent optimization of the MRT systems in both Zurich and Geneva. The goal was to review the tracking algorithms to ensure better coherence between the two systems and prevent track distortions that could lead to future separation failures.