Incident Overview
On 15 June 1998 at 13:43 UTC, an airprox incident occurred near the FIR boundary between Zurich and Geneva involving two aircraft: CRX985, a SAAB340 (registration HB-AKB) operated by Crossair, and SWR3779, a SAAB2000 (registration HB-IZL) operated by Swissair.
Flight Paths and Altitude Instructions
SWR3779 was en route from Marseille to Zurich at FL260. The flight plan included waypoints EBOTO, FRI, WIL, and EKRON. The aircraft was later instructed by Zurich to fly heading 040° or direct to EKRON. Prior to switching frequencies, Geneva air traffic control instructed SWR3779 to descend to FL220, which the pilot acknowledged correctly. The aircraft then changed to Zurich Radar West Sector frequency 135.675 MHz.
CRX985 was flying from Geneva to Lugano/Agno at FL200 along the route FRI-WIL. It was later cleared by Zurich Radar West to heading 065° or direct to ALBIX.
Communication and Altitude Discrepancy
At 13:41 UTC, both aircraft contacted Zurich West Sector. During the initial call, SWR3779 reported that it was descending to FL200, contrary to the earlier instruction from Geneva to descend to FL220. This transmission was not noticed by either the radar executive controller (RE-West) or the planning controller (RP-West). Approximately one minute later, the RE-West observed that SWR3779 was approaching FL200, the same altitude as CRX985.
SWR3779, approaching from the south, was rapidly converging with CRX985, which was coming from the southwest. The RE-West immediately instructed SWR3779 to continue descending to FL190 and issued traffic information to CRX985, which reported visual contact with SWR3779. Despite these actions, a significant loss of separation occurred, with a minimum horizontal distance of 0.6 nautical miles and a vertical separation of 400 feet.
Findings
- Both aircraft were in radio contact with Zurich Radar West Sector on 135.670 MHz.
- SWR3779 was approaching Zurich, while CRX985 was overflying the Zurich region.
- CRX985 was cruising at FL200.
- The Short Term Conflict Alert (STCA) system at Zurich was under testing and not operationally available.
- The altitude preselect (APA) system on the SAAB2000 had previously been reported as unstable, with occasional "APA jumps". The pilot of SWR3779 had filed an occurrence report on 13 July 1998 regarding this issue. However, subsequent investigation revealed that the APA on SWR3779 had been changed directly from FL260 to FL200.
- The Multi Radar Tracking (MRT) system for SWR3779 at Zurich showed significant deviations compared to the individual radar tracks from La Dôle and Lägern radars.
- At the time, TCAS was not mandatory in Europe.
Assessment
Traffic on Zurich frequency was moderate, representing a standard traffic situation with routine tasks. The West Sector controllers missed the initial call from SWR3779 because the RE-West was engaged in a coordination call with the North Sector, and the planning controller did not hear it. The data passed from Geneva to the West Sector indicated "SWR3779 descending FL220". Despite the missed call, the controllers would normally rely on the received data being correct.
Shortly after, the RE-West noticed the developing conflict and asked SWR3779 to confirm its cleared altitude. SWR3779 confirmed FL200 and stated, "we are just reaching now". The controller's response was prompt and appropriate: SWR3779 was instructed to descend to FL190 immediately, and traffic information was given to CRX985. SWR3779 was subsequently cleared to FL140, resolving the conflict. The situation was further de-escalated by CRX985 reporting visual contact with the SAAB2000; no evasive maneuver was necessary.
The incident involved a significant loss of separation, resulting in a hazardous encounter between the two aircraft. Radar plot analysis revealed that the flight path of SWR3779 was depicted differently depending on the radar source. The MRT of ACC Zurich deviated substantially from the individual radar tracks of La Dôle and Lägern, which were in general agreement. This distortion was likely due to a faulty track presentation from Approach Radar Zurich, which influenced the ACC MRT, albeit in a weaker form, but still noticeably compared to the other radars.
These deviations were assessed as safety-relevant issues for aircraft separation. This was communicated to the directors of the Federal Office for Civil Aviation (BAZL) and swisscontrol in an interim recommendation on 20 August 1998, requesting a review of the matter.
Cause
The incident occurred because the pilot of SWR3779 inadvertently set the altitude preselect to FL200 instead of FL220. A contributing factor was that the West Sector air traffic controllers did not realize that the pilot of SWR3779 had stated FL200 during the first call.
Safety Recommendation
The optimization of the two MRT systems in Zurich and Geneva (review of MRT algorithms to improve coherence between the two systems) must be implemented urgently, as potential track distortions can lead to substantial safety problems.