What happened
On the afternoon of September 11, 2018, a FR172K Reims Hawk XP, registration HB-CCA, was conducting a solo flight from Raron to Speck Fehraltdorf. While cruising at approximately 12,500 ft AMSL near the Jungfraujoch, the pilot observed an oncoming aircraft roughly 80 meters to the right. This second aircraft, a PA-28-181 Archer II with registration HB-PEW, was performing an Alpine tour with two passengers.
The HB-PEW had been flying a route along the Chielouwen Glacier and was turning right toward the Aletsch Glacier at about 12,600 ft AMSle. The crew and passengers of the HB-PEW did not perceive the HB-CCA, and no evasive maneuvers were performed by either aircraft.
The investigation
An investigation by the SUST established that the two aircraft reached a minimum separation of 0.1 NM horizontally and 100 ft vertically. Radar data analysis revealed that the terrain prevented the pilots from seeing each other until approximately 15:52:11 UTC.
For the pilot of the HB-CCA, the HB-PEW appeared at a 35-degree angle to the right. Due to the cockpit layout of the PA-28-181, the pilot in the left seat could not see the approaching aircraft. Furthermore, the investigation identified a "constant bearing" phenomenon, where the relative movement of the aircraft made the HB-PEW appear to change size very slowly, making detection difficult. Shortly after, the HB-PEW passed behind the instrument panel of the HB-CCA, momentarily becoming invisible to the pilot.
Findings
- The close proximity was a result of converging flight paths in a mountainous region where terrain significantly restricted visibility.
- The phenomenon of a constant bearing (standing bearing) hindered the pilot's ability to recognize the approaching threat.
- Both aircraft were equipped with Mode S transponders, but neither featured collision warning systems.
- The cockpit structures and instrument panels of both aircraft created blind spots that prevented early detection of the other aircraft.