What happened
On September 18, 2024, a Cessna C152, registration HB-TSN, was operating under Visual Flight Rules (VFR) after departing St. Gallen-Altenrhein. The aircraft was transiting the Friedrichshafen Terminal Control Area (TMA) at an altitude slightly above 4500 ft AMSL. Simultaneously, a Cirrus SR22, registration D-ECZZ, was conducting an Instrument Flight Rules (IFR) approach to runway 06 at Friedrichshoden, descending from 5000 ft AMSL.
As the two aircraft crossed paths, they came within a horizontal distance of only 0.1 NM and zero vertical separation. The pilot of the Cessna C152 only spotted the approaching Cirrus via collision warning systems just moments before the encounter. The pilot of the Cirrus SR22 observed the traffic 17 seconds before the near-miss, following several traffic advisories from the Friedrichshafen Tower.
The investigation
The Swiss Transportation Safety Investigation Board (SUST) examined radar tracks and radio communications to reconstruct the event. The investigation focused on the communication status of both aircraft within the Transponder Mandatory Zone (TMZ). While the Cirrus SR22 was in active contact with Friedrichshafen Tower, the Cessna C152 was not in contact with the Alps Radar controller. Although the controller's Short Term Conflict Alert (STCA) triggered 1 minute and 44 seconds before the event, attempts to reach the pilot of the Cessna C152 were unsuccessful.
Findings
- The primary cause of the near-collision was that the pilots failed to visually detect the other aircraft in a timely manner.
- A systemic contributing factor was that the Cessna C152 was not in contact with air traffic services because the pilot had not opted into the voluntary "listening squawk" procedure while flying over Swiss airspace within the TMZ.
- In this specific region, while a listening squawk is mandatory over German airspace, it remains voluntary for aircraft operating over the Swiss portion of the TMZ.
Safety action
SUST has issued Safety Recommendation No. 604 to the Federal Office of Civil Aviation (BAZL). The recommendation suggests that the voluntary listening squawk requirement in the Swiss portion of the TMZ should be made mandatory, aligning it with German regulations. This change aims to ensure that VFR aircraft are readily reachable by controllers to prevent future conflicts, accompanied by appropriate pilot and controller training.