What happened
On November 24, 2000, a private Piper Cheyenne II (N67JG) was conducting an IFR flight from Regensburg to Grenchen. While navigating through Class C airspace near Wil, the pilot received clearance to descend to flight level 120. During the descent, the aircraft's vertical speed reached approximately 1,500 to 2,000 feet per minute.
Simultaneously, a Swissair Airbus A321, registration HB-IOJ, was climbing from 3,200 feet toward flight level 110. As the two aircraft approached each other, the N67JG descended below its authorized altitude of flight level 120. This deviation caused a separation breach with the HB-IOJ, which was crossing the path of the smaller aircraft at flight level 110. The minimum vertical separation between the two aircraft dropped to approximately 500 feet, with a horizontal distance of 0.9 NM.
The investigation
The SUST investigation examined the flight paths, cockpit communications, and radar data from the Zurich Radar West Sector. Investigators established that the pilot of the N67JG noticed an error on the Horizontal Situation Indicator (HSI), which was displaying an incorrect compass heading. This technical issue diverted the pilot's attention during a critical phase of flight.
On the Swissair flight, the crew experienced both a Traffic Collision Avoidance System (TCAS) Traffic Advisory (TA) and a subsequent Resolution Advisory (RA). The crew reported that the N67JG appeared to be descending toward them. Radar monitoring also identified the altitude deviation via the Short Term Conflict Alert (STCA) system.
Findings
- The primary cause of the incident was pilot inattention during the descent phase.
- The pilot of the N67JG was distracted by a malfunctioning HSI displaying an incorrect course.
- The autopilot's altitude preselection feature was not engaged, which contributed to the uncontrolled descent below the assigned flight level.
- High traffic density and complexity in the sector at the time of the event were noted factors.