What happened
On May 16, 2001, at approximately 13:35 UTC, an air traffic incident occurred 7 NM N/NE of MOLUS. A private Pilatus PC12, registration HB-FOT, was operating a flight from Zurich to Valence. The aircraft had been cleared to maintain flight level 230. Simultaneously, a Crossair Europe Saab 340B, registration ECC757, was flying a commercial service from Marseille to Basel Mulhouse at flight level 240.
During the flight, the pilot of HB-FOT requested to maintain flight level 230 for a period of time. Meanwhile, the crew of ECC757 requested a slight heading change to avoid thunderstorm activity. Shortly after, the pilot of ECC757 reported a TCAS/ACAS Resolution Advisory (RA), indicating that the aircraft had encountered a conflict with opposing traffic. The investigation revealed that HB-FOT had climbed to flight level 234, encroaching on the altitude of the oncoming Saab 340B.
The investigation
Investigators examined the radar data and radio communications from the Geneva Area Control Centre (sectors INE and INS). The investigation established that the two aircraft passed with a minimum separation of 600 feet at a distance of 1.1 NM.
Air traffic controllers utilized the Short Term Conflict Alert (STCA) system, which alerted the radar controller to the proximity of the two aircraft. The investigation also reviewed the flight logs and the pilot's explanation regarding the aircraft's deviation from its assigned altitude. While the controllers and the crew of ECC757 did not file formal air traffic incident reports, the radar controller filed an official ATIR (Air Traffic Incident Report).