Incident Overview
On 29 September 2018 at 12:23 local time, a serious incident involving a near collision occurred over the former military airfield Interlaken (BE), Switzerland. The two aircraft involved were a rescue helicopter, an Agusta AW109SP registered as HB-ZRT, operated by Swiss Air Rescue (REGA), and a training aircraft, a Reims Aviation F172P registered as HB-CIE, operated by Motorfluggruppe Thun. The closest horizontal distance between the aircraft was approximately 100 m at the same altitude of about 600 m above ground.
Sequence of Events
The helicopter, on a rescue flight from Interlaken Heliport to Jungfraujoch, had broadcast its departure on the published frequency 120.525 MHz. After takeoff at 12:23, while approaching the former military airfield runway from the northeast at about 30 m above ground, the helicopter pilot unexpectedly saw a high-wing aircraft in landing configuration heading toward the runway from the opposite direction at roughly the same altitude. He immediately initiated a right evasive maneuver.
The training aircraft had departed from Reichenbach for the second part of an annual check flight. The flight instructor and pilot were practicing emergency landing procedures following an engine failure, which included a gliding phase toward Ringgenberg at about 4500 ft AMSL. The pilots of the training aircraft later stated they had set the frequency of the adjacent control zone of Meiringen (130.150 MHz) and had sighted the helicopter early.
Both aircraft were operating under Visual Flight Rules (VFR) in Class G airspace. No Air Traffic Services were involved.
Aircraft and Crew
The helicopter was crewed by a pilot (Swiss, born 1964, holding an EASA CPL(H) with 7793 total hours, 1780 on type), a paramedic, and an emergency physician. The training aircraft carried a flight instructor (Swiss, born 1954, CPL(A) with 1997 total hours, about 600 on type) and a pilot undergoing training (Swiss, born 1961, PPL(A) with 259 total hours, 226 on type).
Findings
The helicopter was equipped with a Traffic Advisory System (TAS) and a Floice collision warning device. According to the helicopter pilot, no warning was issued by either system, and no transponder signal of the approaching aircraft was visible on the navigation display. The training aircraft had a Mode S transponder without ADS-B capability and a Flarm collision warning device. The pilots of the training aircraft reported that no Flarm warning was triggered.
The dangerous approach was reported to the Swiss Transportation Safety Investigation Board (STSB) on 4 October 2018, six days after the incident. As a result, certain recordings, including those of the collision warning devices, had been overwritten and were unavailable for investigation.
Radar data analysis showed no transponder signal from HB-CIE in the region between 11:30 and 13:00. The first radar contact of HB-ZRT was registered at 12:28:39 at over 7000 ft AMSL, indicating that the incident occurred below radar coverage. The report notes that most Traffic Advisory Systems, by design, only display surrounding traffic if their transponder mode S responds to radar interrogation; outside radar coverage, they rely on ADS-B out, which the training aircraft lacked.
Meteorological Conditions
The weather was sunny and hazy with a weak easterly wind (060°, 8 kt). Cloud cover was 1/8 to 2/8 at 2400 ft above aerodrome elevation. Visibility was 8 km. QNH was 1021 hPa.
Probable Cause
The STSB did not state a probable cause for this serious incident.