What aircraft was involved and where did it happen?

The accident on 2018-08-11 involved a SIAI MARCHETTI S.P.A. S.208, registration HB-EVP, at WIL VOR, CH.

What was the probable cause of the accident?

The near-collision was caused by the convergence of two aircraft on similar flight paths at the same altitude, exacerbated by the pilot of the Siai Marchetti being distracted by engine adjustments and the lack of timely traffic information from air traffic services.

Two aircraft narrowly avoid collision near Willisau

A near-miss between a training Tecnam P2008 and a private Siai Marchetti S208 occurred near the Willisau VOR due to converging flight paths and pilot distraction.

What happened

On August 11, 2018, at approximately 16:35 local time, a serious near-collision occurred approximately 4 nautical miles southwest of the Willisau (WIL) VOR. The incident involved two aircraft flying under Visual Flight Rules (VFR) at nearly the same altitude.

The first aircraft, a Tecnam P2008 with registration HB-KMG, was conducting a training flight from Sion to Speck-Fehraltdorf. During the flight, the flight instructor received an alert from the onboard collision warning system (Flarm), which prompted an intensive scan of the airspace. The instructor subsequently spotted a white aircraft approaching rapidly from the rear right and immediately executed a full left-hand turn to avoid a collision.

The second aircraft, a Siai Marchetti S208 with registration HB-EVP, was returning to Birrfeld following an Alpine tour. At the moment of the encounter, the pilot was preoccupied with adjusting the engine's mixture controls. The pilot noticed the other aircraft had already initiated an evasive maneuver and responded by turning right. The horizontal separation between the two aircraft was estimated to be between 50 and 100 meters.

The investigation

Because the incident was initially reported via an EU occurrence report rather than directly to the SUST, investigators were unable to recover data from the HB-KMG as the onboard logs had already been overwritten. However, investigators were able to analyze flight data from the collision warning system of the HB-EVP, as that aircraft had not flown since the event. Additionally, air traffic control records were secured for review.

Radar data and flight path recordings confirmed that both aircraft were on converging paths at a similar altitude. The analysis showed the HB-EVP performed a slight rightward course change, while the HB-KMG executed a full left-hand circle.

Findings

The Willisau VOR is a heavily used navigation point for VFR traffic, which inherently increases the risk of aircraft convergence.
The pilot of the HB-EVP was distracted by engine mixture adjustments, which prevented them from prioritizing airspace scanning and potentially missing the collision warning alert.
The flight information service (FIS) controller failed to provide timely traffic information to the HB-KMG crew, only inquiring if the other aircraft was visible rather than issuing a warning.
The collision warning system on the HB-KMG was instrumental in alerting the crew and preventing the collision.