What happened
On October 24, 2016, a serious airprox occurred approximately 3.3 NM northwest of St. Gallen-Altenrhein Airport (LSZR). The incident involved an AA-5B Tiger, registration HB-UCM, operating under visual flight rules (VFR), and a Cessna C56X (Citation XLS), registration CS-D XQ, operating under instrument flight rules (IFR) for NetJets.
The pilot of the HB-UCM had requested and received permission to transit the St. Gallen-Altenrhein control zone. Simultaneously, the CS-DXQ departed St. Gallen-Altenrhein on a standard instrument departure toward Basel, cleared to climb to 5,000 ft. As the business jet climbed, the air traffic controller observed a high rate of climb and prepared to transfer the aircraft to the next control sector.
At approximately 14:13 UTC, the Short Term Conflict Alert (STCA) was triggered. The controller in the approach sector identified the impending conflict and instructed the CS-DXQ to climb to Flight Level 100, while also providing traffic information regarding the HB-UHD. Immediately following this, the crew of the CS-DXQ reported a Resolution Advisory (RA) from their onboard TCAS, which they promptly followed. At the point of closest approach, the two aircraft crossed in opposite directions with a vertical separation of 525 ft and a horizontal separation of only 0.2 NM.
The investigation
The Swiss Transportation Safety Investigation Board (SUST) examined radar data, radio communications, and statements from the flight crews and air traffic controllers. The investigation focused on the decision-making processes of the tower controller and the accuracy of the information provided by the aircraft involved.
Findings
The primary cause of the incident was the tower air traffic controller's decision to simultaneously authorize the departure of a business jet and the transit of a light aircraft, based on an incorrect assessment of the developing situation.
Several contributing factors were identified:
- Misleading communication led the HB-UCM to fly a path that deviated from the pilot's intended route.
- The tower controller failed to coordinate a higher altitude with the next sector, which would have allowed the CS-DXQ to climb beyond its initial 5,000 ft limit safely.
- The Tower Air Situation Display (TASD) at the regional airport provided only a visual conflict alert without an audible alarm, making the warning easy to miss.
- The position report provided by the pilot of the HB-UCM was imprecise.
- There was a misconception by the VFR pilot regarding the level of separation services provided by air traffic control in Class D airspace.
Safety action
SUST issued a safety recommendation (No. 543) urging the Federal Office of Civil Aviation and Skyguide to implement audible alarms for the Short Term Conflict Alert (STCA) at regional airports to improve controller situational awareness. Additionally, safety notices were issued to remind pilots of the necessity for precise position reporting and to clarify that pilots operating under VFR are responsible for maintaining separation from IFR traffic in Class D airspace.