What happened
On July 4, 2022, a training flight involving a glider and a military turboprop aircraft resulted in a near-collision near Fruthwilen, Switzerland. The glider, an ASK 21 with registration HB-1630, departed from Amlikon airfield carrying a student and an instructor. Flying under Visual Flight Rules (VFR) at approximately 1,250 meters, the glider was traveling east at roughly 60 knots.
Simultaneously, a Swiss Air Force PC-9/F, registration C-408, was transiting the area at a much higher speed of approximately 275 knots. The military aircraft was conducting training maneuvers near the Tägerschen fuel depot and had recently passed over the Lommis airfield area.
At 11:12 AM, the student pilot in the HB-1630 spotted the approaching PC-9/F on the right. The instructor immediately initiated an aggressive evasive maneuver, first banking the glider steeply to increase its visibility and then performing a steep dive, descending about 160 meters in just seven seconds. The pilot of the PC-9/F observed the glider performing what appeared to be an aerobatic maneuver rather than an avoidance tactic. The two aircraft reached a minimum separation of approximately 250 meters horizontally and less than 50 meters vertically.
The investigation
An investigation by the SUST established that the pilot of the PC-9/F was tuned to the Lommis airfield frequency, while the glider crew was monitoring Amlikon. Because of this discrepancy, the military pilot was unaware of the glider's position and was not in contact with a military mission control center that could have provided traffic advisories. Furthermore, the investigator found that the PC-9/F lacked a collision warning system, a known safety deficiency previously identified in other investigations.
While the glider was equipped with a Flarm device, it failed to provide a warning prior to the encounter. The investigation also noted that the high speed of the military aircraft significantly reduced the window of time available for effective avoidance once the aircraft were sighted.
Findings
- The primary cause of the near-collision was the failure of the crews to sight each other until a point where only limited evasive action was possible.
- The pilot of the PC-9/F operated without monitoring the Amlikon airfield frequency, preventing the receipt of vital traffic information.
- The high velocity of the PC-9/F (275 knots) minimized the time available for a safe maneuver once the aircraft were detected.
- The lack of a collision warning system on the PC-9/F represented a continuing safety deficit.