Incident
On 29 June 2019, a Schweizer 300 helicopter (registration HB-ZHN) operated by Robert Fuchs AG was conducting a training flight. The aircraft departed from Schindellegi heliport (LSXS) and after completing planned exercises was returning to base. While climbing to cruise altitude near Siebnen and Galgenen, the crew heard an unusual noise. The helicopter yawed left and rotor RPM decreased. The instructor immediately took control, initiated an autorotation, and decided to head toward Wangen-Lachen airfield (LSPV) due to its flat, unpopulated terrain. He broadcast a Mayday call on the airfield frequency to alert rescue services. During the autorotation, the instructor noted that some engine power remained but continued the maneuver due to persistent vibrations and abnormal engine sounds. After flaring, he used the remaining power to make a controlled landing. The engine was then shut down per the checklist.
Post-Landing Inspection
Visual inspection revealed that the upper spark plug of cylinder 1 had become completely detached from its thread and was missing along with its copper gasket. The spark plug cable nut was also loose. The maintenance organization reported having recently serviced the engine and stated they had installed the spark plug correctly. The last technical inspection had occurred approximately 11 flight hours before the incident. Manufacturer guidelines require spark plug inspection every 50 flight hours and replacement if worn. Installation involves screwing the plug about five turns into the cylinder and then torquing it to specification, followed by tightening the spark plug cable nut.
Outcome
Both crew members—the instructor and student—were uninjured. The helicopter sustained no damage from the landing. The missing spark plug and gasket were the likely cause of the rough engine run, vibrations, and partial power loss experienced during the flight.