What happened
On March 12, 2019, a Cessna Citation XLS+, registration D-CBEN, was performing an instrument approach to runway 10 at St. Gallen-Altenrhein airport. Simultaneously, a Robinson R44 II helicopter, registration HB-ZWN, departed the same airfield under visual flight rules.
Prior to departure, the air traffic controller instructed the helicopter pilot to remain south of the runway axis to avoid the arriving business jet. Although the instruction was repeated in German to ensure understanding, the helicopter pilot followed a route along the Lake Constance shoreline that crossed the approach path of the arriving aircraft.
As the aircraft approached each other, the crew of the D-CBEN received a TCAS traffic advisory, followed by a resolution advisory (RA) commanding a descent. The pilot immediately initiated a descent, which was later increased in rate by the system. The two aircraft crossed paths at a horizontal distance of only 0.1 NM and a vertical separation of 225 ft. The intensity of the maneuver triggered an enhanced ground proximity warning (EGPWS) as the pilot intercepted the glidepath from below.
The investigation
The SUST investigation focused on the flight paths and the adherence to air traffic control instructions. Investigators found that the helicopter pilot's flight preparation was inadequate, as they were unaware that the chosen route to waypoint Victor would intersect the runway axis. The investigation also examined the effectiveness of the TCAS and the role of the air traffic controller, who monitored the helicopter visually and via radar but did not issue a corrective instruction as the conflict developed.
Findings
- The primary cause was the helicopter pilot's failure to follow the air traffic controller's instruction to stay south of the runway axis.
- The pilot's limited flight experience and insufficient flight planning contributed to the deviation.
- The air traffic controller failed to recognize the developing conflict in time to issue a corrective warning.
- The operational procedures at St. Gallen-Altenrhein, which allow simultaneous IFR arrivals on runway 10 and VFR departures from runway 28, present an inherent safety risk.
- The helicopter's Flarm system was unable to detect the D-CBEN because the business jet was not equipped with compatible Flarm technology.