Incident Overview
On 6 May 2019 at 14:41 local time, a serious incident (Airprox) occurred at Birrfeld Airfield (LSZF) involving two single-engine training aircraft: a F172N "Skyhawk" registered as HB-CNY and a PA-28-161 "Cadet" registered as HB-PMI. The minimum separation was 200 m laterally and 30 m vertically. No prescribed separation minima existed in Class G airspace. The incident was classified as ICAO Category B.
Sequence of Events
At approximately 14:38, HB-CNY was returning from a local flight and entered the West entry sector at about 3400 ft AMSL. The crew, consisting of a flight instructor and a pilot performing her annual check flight (seated left), reported the sector overflight on the airfield frequency. At the same time, HB-PMI was north of the North entry sector, which it overflew at 14:39 at about 4000 ft AMSL. That crew, two flight instructors, reported the North sector entry. The HB-PMI crew stated they heard HB-CNY's position reports and saw the other aircraft relatively early.
At 14:40, HB-CNY descended to 2790 ft AMSL and entered the circuit over the airfield buildings, reporting its intention to remain in the circuit because another aircraft was below. HB-CNY flew a left-hand circuit whose center was about 400 m west of the published circuit center on the Visual Approach Chart (VAC). It passed the northern apex at 14:40:34 at 2720 ft AMSL.
HB-PMI, with a ground speed of about 110 kt, was then about 500 m north of HB-CNY at the same altitude. Shortly after, both Flarm devices issued warnings, which were perceived by both crews. At 14:40:41, HB-PMI crossed HB-CNY's prior flight path at 2480 ft AMSL. The HB-PMI crew had HB-CNY in sight slightly right ahead, at 2590 ft AMSL and heading south.
The HB-CNY pilot heard HB-PMI report entering the circuit at 2600 ft AMSL and looked for the other aircraft. The HB-PMI crew, perceiving HB-CNY to be higher and slightly southwest of the runway axis, initiated a left turn according to the VAC circuit, overtaking HB-CNY on the left. The HB-CNY crew did not have visual contact until HB-PMI was very close; they then shallowed their bank to avoid. The closest approach occurred at 14:40:52, with 200 m lateral and 30 m vertical separation. HB-CNY remained in the circuit and crossed behind HB-PMI.
HB-PMI then flew a half circuit to join the downwind for runway 26, performed a touch-and-go, and landed. HB-CNY completed two full circuits, then a touch-and-go, followed by four more circuits. Both aircraft landed without further incident.
Aircraft and Crew
HB-CNY (F172N "Skyhawk") was operated by Fliegerschule Birrfeld AG. The crew comprised a Swiss female pilot (born 1982) holding a PPL(A) with 248:17 total flight hours (116:28 on type) and a Swiss male flight instructor (born 1949) holding a CPL(A) with 6863:25 total hours (141:27 on type).
HB-PMI (PA-28-161 "Cadet") was also operated by the same school. The crew consisted of a Swiss female pilot (born 1953) with a CPL(A) and 7669:04 total hours (1800 on type) and a Swiss male flight instructor (born 1950) with a CPL(A) and 4662 total hours (1960 on type).
Both aircraft were equipped with Flarm collision warning systems that were switched on and generated alerts during the incident.
Observations
The sky was cloudy, with virga (rain streaks from evaporating rain) around the airfield. Visibility was 25 km, and a light wind from the north-northwest was present. The VAC for Birrfeld specifies that all approaches should be at 2500 ft AMSL over the entry circle, and downwind should be flown at 2000 ft AMSL to ensure separation from approaching traffic. The entry circle procedure is designed so that later aircraft join above those already in the circle, then descend after the lowest aircraft turns into the pattern.
The analysis noted that HB-CNY entered the circuit approximately 400 m west of the published center, which could have led to uncertainty about its intentions. However, its radio call clarified that it intended to remain in the circuit. The analysis also stated that the decision by the HB-PMI crew to overtake HB-CNY in the circuit and turn ahead into downwind was not safety-conscious, though they had the other aircraft in sight and could manage separation via see-and-avoid.
Flarm Warnings
According to the Flarm system behavior, warnings were generated in the phases where the aircraft were in close proximity (as indicated in Figure 3 of the original report). Some crew members noted that such warnings are common in the airfield area and can be continuous, leading some pilots to switch off the device to avoid distraction. In HB-CNY, the high-wing design restricted visibility toward the center of the turn, especially rearward, during banked flight.