Introduction
On 9 March 2021 at approximately 11:25 local time, two aircraft experienced a near midair collision near Ursenbach, about 7 km south of Langenthal, Switzerland. The aircraft were a Reims Aviation F152 registered as HB-CYP and a Cessna C182 registered as SP-WTF. The incident occurred in class G airspace at an altitude of 3500 ft AMSL. The minimum separation was 0.1 NM horizontally and less than 100 ft vertically. The event was classified as an ICAO Airprox Category A, indicating a serious risk of collision.
Sequence of Events
HB-CYP was operated by the Flugsportgruppe Zürcher Oberland and used for training under visual flight rules (VFR). On board were a flight instructor and a student pilot. The flight originated from Langenthal Airport (LSPL) at 11:22, climbing southbound for a return to Speck-Fehraltorf (LSZK). At the time of the incident, the aircraft had just departed and was still on the Langenthal airport frequency.
SP-WTF was a private flight under VFR, piloted by a Polish national with three passengers on board. It had departed Samedan (LSZS) en route to Chambéry Aix-les-Bains (LFLB) in France. At about 11:25, while in cruise near Nebikon (LU) at 3500 ft AMSL on a west-southwesterly heading, a passenger in the back seat spotted HB-CYP on the right side and alerted the pilot that they were crossing ahead of it.
Seconds later, the flight instructor in HB-CYP briefly glimpsed SP-WTF below through the right side window after the crossing had already occurred. The student pilot was occupied with reporting on the airport frequency and beginning navigation toward the Napf region.
Weather
Meteorological conditions were sunny with light wind, good VFR conditions, and slight haze over the Swiss Plateau. Cloud base was around 5800 ft AMSL with 3/8 to 4/8 coverage. The investigation concluded that weather did not contribute to the incident.
Equipment
HB-CYP was equipped with a Mode S transponder and a PowerFlarm collision avoidance system, but the PowerFlarm was inoperative on the day of the incident due to a defective aircraft-side power supply. This deficiency had been recorded in the deferred defects report for four months.
SP-WTF had a Mode S transponder but no collision warning device. Its G1000 avionics could have integrated collision warnings, but such functionality was not activated or installed.
Analysis
The investigation noted that in class G and E airspace, the "see and avoid" principle may not always work effectively. The Swiss Federal Office of Civil Aviation (BAZL) has published information on the limitations of visual perception and recommendations for visual scanning. Factors such as structural blind spots, sun glare, low contrast, and constant bearing (the other aircraft appearing stationary relative to the pilot) can hinder detection. Additionally, tasks like reading checklists or changing radio frequencies increase "head-down time" during which pilots are not scanning the airspace.
Collision warning devices or "sense and avoid" systems can provide valuable assistance. However, they require proper installation, regular updates, and activation. In this case, HB-CYP's inoperative PowerFlarm eliminated the possibility of an early warning that might have allowed evasive action.
Conclusions
The near collision resulted from late mutual visual detection. Neither aircraft had operational collision warning technology that could have alerted the crew to the impending danger.