What aircraft was involved and where did it happen?

The accident on 2021-01-24 involved a DIAMOND AIRCRAFT INDUSTRIES GMBH DA 42, registration HB-SDM, at 1 km nordöstlich von Sursee, LU, CH.

What was the probable cause of the accident?

The near-collision was caused by an evasive maneuver in the Diamond DA 42 that, while establishing vertical separation, unintentionally reduced lateral distance between the two aircraft, combined with a failure to maintain visual contact with the approaching traffic.

Near-collision of two light aircraft near Sursee

A Diamond DA 42 and a Piper Archer II narrowly avoided a collision near Sursee, Switzerland, after an evasive maneuver unintentionally reduced lateral separation.

What happened

On January 24, 2021, at approximately 14:22 local time, a Diamond DA 42, registered HB-SDM, was performing a descent toward Birrfeld, Switzerland. The aircraft was traveling northwest at a ground speed of roughly 165 knots. Simultaneously, a Piper Archer II PA-28-181, registered HB-PRM, was flying southeast toward Birrfeld at a ground speed of 133 knots, having departed from Basel Mulhouse.

As the two aircraft approached each other near Sursee, the pilot of the HB-SDM received a warning from an onboard Traffic Awareness System (TAS) indicating traffic at eleven o'clock, low, and less than one mile away. Approximately 30 seconds before the closest point of approach, the pilot of the HB-SDM initiated an evasive maneuver by stopping the descent, entering a right turn with increased bank angle, and transitioning into a slight climb. Due to the bank angle and the change in flight path, the pilot and passenger did not visually identify the other aircraft.

On the HB-PRM, the passenger spotted the HB-SDM just before the point of closest approach. The pilot of the HB-PRM subsequently identified the aircraft and executed a short, steep descent. At the moment of closest proximity, the aircraft were separated by only about 270 meters horizontally and 300 feet vertically.

The investigation

The investigation examined the effectiveness of the onboard collision avoidance technologies and the flight maneuvers performed by both crews. Investigators looked into why the pilot of the HB-SDM failed to visually acquire the other aircraft despite the early TAS warning. It was determined that the specific nature of the maneuver—a right turn combined with a climb—likely obstructed the pilot's forward-left field of view.

Furthermore, the investigation analyzed the performance of the PowerFlarm device on the HB-PRM. The device provided a warning only about five seconds before the encounter, which was deemed too late to be helpful. This delay was attributed to the fact that the HB-SDM was not equipped with a Flarm-compatible device, meaning the HB-PRM could only receive unoriented transponder signals.

Findings

The pilot of the HB-SDM initiated a lateral maneuver (a right turn) that unintentionally decreased the horizontal distance between the two aircraft.
The vertical separation of 300 feet was maintained only because both pilots performed vertical corrections (a climb by the HB-SDM and a descent by the HB-PRM).
The TAS warning on the HB-SDM served as a 'sense and avoid' tool, but the lack of visual contact prevented a more effective 'see and avoid' response.
The flight information center (FIC) Zurich Information did not issue any traffic advisories during the encounter.
The PowerFlarm warning on the HB-PRM was delayed due to system limitations regarding transponder signal compatibility.