What happened
On June 19, 2018, a Carat A motor glider, registration D-KBBZ, departed from the Bohlhof glider airfield for a sightseeing flight around Lake Constance. After climbing to approximately 900 meters (2,950 ft) AMSL, the pilot continued the flight over various towns toward the Rhine Valley. Upon returning to the airfield roughly three hours later, the pilot was notified of a potential airspace violation.
The investigation
An investigation by the Swiss Transportation Safety Investigation Board (SUST) revealed that the aircraft had not actually entered the Zurich Terminal Control Area (TMA) LSZH 1, which begins at 3,000 ft AMSL. Instead, the aircraft remained within the lower-altitude temporary reserved area (TRA) LS-T 72 Bohlhof.
Post-flight inspections determined that the aircraft's transponder was transmitting an incorrect altitude. Due to a defective altitude encoder, the transponder was reporting a flight level of FL37, which was approximately 2,000 feet higher than the aircraft's actual altitude.