What happened
On May 6, 2019, two light aircraft were involved in a serious airprox incident near Birrfeld Airport (LSZF). The first aircraft, a Cessna 172N with registration HB-CNY, was conducting a training flight. The crew, consisting of a flight instructor and a student pilot, entered the arrival pattern at approximately 14:40 UTC. However, the aircraft was flying its circuit roughly 400 meters west of the standard pattern prescribed in the Visual Approach Chart (VAC).
Simultaneously, a Piper PA-28-161 with registration HB-PMI, also performing a training flight, was approaching the airport. The crew of the HB-PMI observed the HB-CNY early but chose to perform an overtaking maneuver. Instead of following the standard arrival procedure—which involves descending into the pattern behind existing traffic—the crew of the HB-PMI overtook the HB-CNY on its left side.
This maneuver resulted in a dangerous loss of separation. At the point of closest approach, the two aircraft were separated by only approximately 200 meters horizontally and 30 meters vertically.
The investigation
The Swiss Transportation Safety Investigation Board (SUST) examined flight data from the Flarm collision warning systems and crew statements. The investigation noted that while both aircraft were equipped with active Flarm units that generated warnings, the crews did not find these alerts particularly helpful for increasing situational awareness, as such frequent alerts in busy airport environments often lead pilots to ignore or disable them.
Findings
Several contributing factors led to the near-collision:
- The crew of the HB-CNY was flying a circuit that was significantly offset from the official Visual Approach Chart.
- The crew of the HB-PMI performed an overtaking maneuver that bypassed the established arrival procedures.
- There was a lack of communication between the two aircraft, as the crew of the HB-PMI did not coordinate their intention to overtake with the crew of the HB-CNY via radio.
- Visual limitations in the Cessna 172N, caused by the aircraft's high-wing configuration and the banking angle during the turn, made it difficult for the crew to detect the approaching HB-PMI until it was in very close proximity.