What happened
On March 27, 2017, a formation of two Pilatus PC-21 turbo-trainers belonging to the Swiss Air Force was conducting low-level flight maneuvers near Sitterdorf. Due to weather conditions in western Switzerland, the flight leader redirected the formation toward eastern Switzerland. Simultaneously, a private Pilatus PC-6/B2-H4 (registration HB-FKL) departed Sitterdorf for a camera calibration flight.
As the HB-FKL climbed, its Power-Flarm system triggered an audible and visual collision warning. The pilot of the HB-FKL observed an aircraft at the 11 o'clock position, approximately 500 meters away, but due to the aircraft's nose-up attitude, visibility was restricted. Meanwhile, the flight leader of the A-108 performed a series of turns to avoid the Sitterdorf airfield. During these maneuvers, the leader momentarily looked right to check on his wingman. Upon returning his gaze forward, the leader realized the HB-FKL was crossing his path at a distance of only about 30 meters. The A-108 was traveling at approximately 270 knots at the time of the encounter.
The investigation
The investigation established that the flight leader miscalculated the turns intended to bypass the airfield, resulting in the formation flying directly into the path of the ascending HB-FKL. The investigation also noted that the wingman in the military formation detected the civilian aircraft too late to provide a warning to the leader. The small silhouette of the Pilatus PC-6/B2-H4 against a low-contrast background made it nearly impossible to identify in time for an evasive maneuver.
Findings
- The primary cause of the near-collision was the miscalculation of the flight path by the military flight leader during turns intended to avoid the Sitterdorf airfield.
- The pilot of the HB-FKL misidentified the source of the collision warning due to restricted forward visibility caused by the aircraft's climb angle.
- The lack of compatible collision warning equipment on the military aircraft prevented early detection of the approaching civilian aircraft.
Safety action
This incident highlights a recurring safety deficit previously identified in 2011 and 2015. The SUST noted that a consistent implementation of previous recommendations—specifically ensuring that Swiss Air Force aircraft operating in civilian airspace are equipped with collision warning systems compatible with civilian standards—would likely have prevented this occurrence.