What happened
On August 25, 2016, an Airbus A330-343, registration HB-JHR, operated by Edelweiss Air, was on an instrument approach to runway 14 at Zurich Airport. Simultaneously, a Scheibe SF25 motor glider, registration D-KOWC, was conducting a VFR flight from Bremgarten to Biberach.
During the flight, the pilot of the D-KOWC experienced a generator fuse failure, which caused the radio to fail. To conserve battery power, the pilot turned off the transponder. Additionally, a navigation tablet failed, leaving the pilot to rely solely on a compass and paper charts. Due to a visual error, the pilot mistook Lake Zurich for Lake Constance and inadvertently steered the aircraft into the Zurich airport approach sector.
At approximately 15:34 UTC, air traffic controllers noticed a primary radar echo and alerted the crew of the Airbus A330-343. The airliner crew established visual contact with the motor glider, describing it as a red, blue, and white propeller aircraft. The two aircraft passed within approximately 300 meters horizontally at the same altitude, roughly 13 NM northwest of Zurich.
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 technical failures in the motor glider and the subsequent navigation error that placed the aircraft in the path of the commercial airliner.
Findings
- The primary cause of the incident was a navigative error by the pilot of the D-KOWC, who entered the Zurich approach sector without radio communication and with a deactivated transponder.
- The lack of a transponder signal prevented secondary safety nets from alerting controllers to the aircraft's presence.
- The pilot's limited flight experience was a contributing factor to the error.
- The pilot failed to establish contact with air traffic services via emergency frequencies after realizing the error and did not report the incident to authorities following the flight.