What happened
On September 4, 2019, a Diamond DA 40 NG, registration HB-SGS, was conducting touch-and-go training maneuvers at Grenchen Airport (LSZG). The aircraft, occupied by a flight instructor and a student, was operating on the outer circuit. Simultaneously, a Turbine Legend, registration N3YZ, requested taxi clearance for a local flight.
To manage heavy traffic on the outer circuit, the air traffic controller (ATC) instructed the N3YZ to climb via the inner circuit. As the N3YZ departed, it did not continue a continuous climb as expected by local regulations. Instead, the pilot leveled off and accelerated to approximately 170 knots, significantly exceeding the 90 KIAS speed limit prescribed for the inner circuit.
As the HB-SGS transitioned from the downwind leg to the base leg, the two aircraft crossed paths at a right angle. The vertical separation was only about 400 ft, with a horizontal distance of roughly 40 m. Both crews performed immediate evasive maneuvers—the HB-SGS climbing and the N3YZ descending—to avoid a collision. The incident was only brought to the controller's attention after the HB-SGS crew contacted the tower to inquire about the encounter.
The investigation
The Swiss Transportation Safety Investigation Board (SUST) examined the flight paths, the controller's instructions, and the functionality of onboard collision avoidance systems. The investigation looked into why the N3YZ pilot accelerated to high speeds and why the ATC's instructions were interpreted as allowing a level flight path. Investigators also reviewed the performance of the HB-SGS's Avidyne TAS 610 and the N3YZ's PowerFlarm system, noting that neither system provided an audible warning to the crews during the approach.
Findings
- The primary cause was the pilot of the N3YZ failing to continue a continuous climb and instead accelerating to a high speed in the inner circuit.
- The ATC's instruction, "you may climb via the inner westbound," was ambiguous, leaving the pilot with the impression that they could choose to level off.
- The controller failed to provide necessary traffic information due to inadequate monitoring of the two aircraft.
- The pilot of the N3YZ incorrectly assumed that the 90 KIAS speed limit for the inner circuit did not apply following the specific takeoff clearance.
- Onboard traffic advisory systems failed to alert the crews, possibly due to specific configuration settings or the high closing speed of the encounter.