What happened
On November 8, 2018, a solo pilot operating a DR 400/140 B, registration HB-KLE, was flying a visual flight rules (VFR) excursion from Grenchen toward Lausanne. While navigating through low cloud layers near La Sarraz, the pilot initiated a right-hand turn to return to Grenchen. Simultaneously, an A109A helicopter, registration HB-HDG, was performing a private flight from Chavannes-de-Bogis to Bressaucourt.
As the two aircraft approached each other, the pilot of HB-KLE did not initially detect the helicopter, and the onboard Powerflarm collision warning system failed to provide an alert. The helicopter pilot reported seeing the light aircraft approximately 30 seconds before the encounter but perceived no immediate danger. The aircraft eventually crossed paths at an altitude difference of 200 ft and a horizontal separation of approximately 500 to 700 meters.
The investigation
SUST examined the flight paths and the technical performance of the onboard equipment. The investigation focused on why the collision warning system on HB-KLE failed to alert the pilot to the approaching A109A. Investigators analyzed the installation of the Powerflarm antenna, which was located immediately in front of a bulkhead. Data from the collision warning device's records were also reviewed.
Findings
- The investigation established that the reduced transmission range of the Powerflarm system on HB-KLE was caused by the antenna's installation position in front of the bulkhead.
- The pilot of the A109A likely lost sight of the light aircraft in the final seconds of the encounter due to the high instrument panel of the helicopter and the low cloud ceiling.
- The pilot of the A109A took a risk by not adjusting his flight path despite having spotted the aircraft earlier.
- The collision warning system on HB-KLE only displayed the helicopter on the screen after the aircraft had already crossed paths.