What aircraft was involved and where did it happen?

The accident on 2021-03-09 involved a REIMS AVIATION S.A. F152, registration HB-CYP, at Ursenbach, BE, CH.

What was the probable cause of the accident?

The near-collision was caused by the failure of the flight crews to detect each other in time, compounded by the fact that the collision avoidance equipment on the training aircraft was out of service.

Near-collision of two light aircraft near Ursenbach

A near-collision occurred between a training flight and a private aircraft in Swiss airspace due to delayed visual detection and non-functional collision avoidance equipment.

What happened

On March 9, 2021, at approximately 11:25 AM, a near-collision occurred near Ursenbach, Switzerland, involving two aircraft operating under Visual Flight Rules (VFR). The first aircraft, a Reims Aviation F152 with registration HB-CYP, was conducting a training flight from Speck-Fehraltorf to Langenthal. On board were a flight instructor and a student pilot.

The second aircraft, a Cessna C182 with registration SP-WTF, was performing a private flight from Samedan to Chambéry with three passengers. The two aircraft crossed paths at an altitude of approximately 3,500 ft AMSL. The separation at the moment of closest approach was extremely narrow, with a horizontal distance of only 0.1 NM and a vertical separation of less than 100 ft.

The encounter was only noticed after the fact. A passenger in the Cessna C182 observed the other aircraft on the right side and alerted the pilot. Shortly after the crossing had already occurred, the flight instructor in the HB-CYP spotted the Cessna C182 below them through the right window.

The investigation

The Swiss Transportation Safety Investigation Board (SUST) examined the circumstances leading to the encounter. The investigation focused on the visibility of the aircraft and the availability of collision avoidance technology.

Investigators found that while the HB-CYment F152 was equipped with a PowerFlarm 6 system, the unit was non-functional due to a power supply failure. This defect had been documented in the aircraft's deferred defects report for four months, with instructions to address it at the next inspection. The Cessna C182 was equipped with a Mode-S transponder but lacked a dedicated collision warning system, despite its modern G1000 avionics being capable of such integration.

At the time of the incident, the student pilot in the HB-CYP was focused on radio communications, specifically signing off from the Langenthal airport frequency, which contributed to a period of reduced external scanning.

Findings

The primary cause of the near-collision was that mutual visual detection occurred too late to allow for effective avoidance.
The non-functional PowerFlarm on the HB-CYP prevented an early electronic warning of the approaching aircraft.
The aircraft were flying on a constant bearing (standing bearing), a geometry that makes detecting an approaching object significantly more difficult for pilots.
Internal cockpit tasks, such as frequency changes and navigation adjustments, increased "head-down time," reducing the crew's ability to monitor the surrounding airspace.