Incident Overview
On 23 March 2017 at 10:54 local time, a serious incident occurred approximately 2 kilometers east of Sitterdorf Airfield (LSZV) in Switzerland. A Pilatus PC-21 (registration A-108) operated by the Swiss Air Force and a Pilatus PC-6/B2-H4 (registration HB-FKL) operated by Weibel Aviation AG came into close proximity. The minimum separation was less than 50 meters horizontally and less than 50 feet vertically, categorized as an ICAO Airprox Category A event. No collision occurred, and both aircraft continued to their destinations without further incident.
Aircraft and Crew
The PC-21, a single-engine turboprop trainer, was on a visual flight rules (VFR) formation flight originating from and returning to Emmen Air Base. The pilot, a Swiss national born in 1961, held a military license and had accumulated 5,817 total flight hours, including 1,611 hours on the PC-21. The PC-6, a single-engine high-wing utility aircraft, was conducting a camera calibration flight from Sitterdorf. Its pilot, a Swiss national born in 1943, held a commercial pilot license and had 6,237 total flight hours, with 5,300 hours on the PC-6 type.
Sequence of Events
On the morning of the incident, the PC-21 leader led a two-ship formation on a low-level training sortie. Due to weather in western Switzerland, the flight was diverted to the east. Meanwhile, the PC-6 pilot departed Runway 07 at Sitterdorf and initiated a right turn to a heading of approximately 150 degrees to avoid the control zone of St. Gallen-Altenrhein Airport.
After overflying Appenzell in a northwesterly direction, the PC-21 leader ordered his wingman to take a formation spaced several hundred meters apart. As they approached the town of Gossau, the leader anticipated reaching Sitterdorf and commanded a right turn, followed after about 3 nautical miles by a left turn back to a northwesterly course, intending to pass east of the airfield.
Shortly thereafter, the PC-6 pilot, in the process of retracting flaps, received an audio and visual alert from his Power-Flarm collision avoidance device. He scanned for traffic and spotted an aircraft at his 11 o'clock position, estimated 500 meters away, which he believed to be the source of the alert.
The PC-21 leader, looking to his right to check on his wingman, turned his gaze forward and saw a high-wing fixed-gear aircraft approximately 30 meters ahead, crossing his path. The PC-21 was traveling at about 270 knots. The wingman also detected the PC-6 at the last moment, too late to warn the leader. Neither pilot had time to execute an evasive maneuver. The PC-6 pilot later realized he had seen the wingman, not the leader; he was informed of the near-collision only after landing.
Analysis and Findings
At the time of the encounter, the PC-6 was in a climb, which limited forward visibility due to the aircraft's nose-up attitude. This likely caused the PC-6 pilot to mistake the wingman for the alerting aircraft. The PC-21 leader misjudged his position relative to Sitterdorf, resulting in his formation flying horizontally at low level about 2 kilometers east of the airfield, on a converging course with the departing PC-6. Against a low-contrast background, the small silhouette of the PC-6 was not visible in time for either military pilot to react. The investigation concluded that a compatible collision warning system aboard the PC-21s could have provided earlier warning, potentially preventing the incident.
This deficiency had been previously identified in 2011 and 2015 serious incidents, leading to Safety Recommendation No. 474, which urged equipping Swiss Air Force aircraft operating in civil airspace with collision warning systems compatible with civil standards. The SUST noted that consistent implementation of this recommendation would likely have prevented the current incident. As no new safety deficit was identified, the investigation was closed with a summary report.
Probable Cause
No explicit probable cause was stated in the report. The analysis indicates the PC-21 pilot's misjudgment of the turn and the lack of timely visual detection due to the aircraft's silhouette against the background contributed to the close encounter.