Near Collision Between Two General Aviation Aircraft in Swiss Alps (HB-KLZ)

Incident Overview On 8 February 2020 at 10:43:33 local time, a near collision occurred between two general aviation aircraft in Class E airspace approximately 2 km east of the Clariden-Hüfifirn mountain landing site (LSVD) in the municipality of Glarus Süd, Switzerland. The minimum separation was 12 m horizontally and 21 m vertically, classified as ICAO Airprox Category A. ## Aircraft and Crews The first aircraft was a Robin DR 400 registered HB-KLZ, operated privately by Fluggruppe Mollis. It was on a round trip from Flugplatz Mollis (LSMF) around the Matterhorn, with a Swiss pilot born in 1984 and a passenger holding a private pilot licence. The second aircraft was a Piper PA-18 registered HB-PPJ, operated for training by Cub-Freunde Zürcher Oberland. It was on a mountain flying training flight from Flugplatz Speck-Fehraltorf (LSZK) to Clariden-Hüfifirn, with a Swiss flight instructor born in 1976 and a student pilot born in 1991. ## Sequence of Events The Robin DR 400 departed Mollis at 09:26 and was returning over the Hüfipass toward Mollis. The Piper PA-18 departed Speck-Fehraltorf at 10:09 and was approaching Clariden-Hüfifirn from the east. At 10:42, one minute before the near collision, the Piper was at 10,240 ft AMSL on a reconnaissance pass over its intended landing site, while the Robin was at 10,430 ft AMSL over the lower part of the Hüfifirn, heading toward the pass at approximately 150 kt ground speed. After crossing the Hüfipass, the Robin descended and approached the Piper, which was in a crosswind approach at 78 kt. The near collision occurred at 10,433 at 10,100 ft AMSL, 450 ft above the Hüfipass and 2,850 ft above ground. Both crews continued their flights; the Piper landed at Clariden-Hüfifirn at 10:50 and the Robin at Mollis at 11:01. ## Contributing Factors Both aircraft were equipped with Mode S transponders, Flarm collision warning systems, and collision warning lights. Both crews were monitoring the mountain frequency 130.355 MHz and had visual and collision warning indications of other aircraft in the area. The investigation noted that the presence of multiple aircraft made it difficult to correctly associate radio and collision warning information with visually observed traffic. The Robin's flight path over the Hüfipass at approximately 150 m above ground and 150 kt was considered risky given the landing site activity. The Piper's reconnaissance pass was a standard procedure for mountain landing preparation. ## Weather Conditions Weather was partly sunny with 5/8 to 7/8 cirrus clouds at approximately 34,000 ft AMSL. Visibility exceeded 70 km. Wind was 210 degrees at 8 kt at 3,100 m MSL. Temperature and dew point were -4 °C and -33 °C respectively at 3,100 m MSL. QNH was 1026 hPa.