What happened
On June 29, 2019, an H269 (Schweizer 300) helicopter, registration HB-ZHN, was conducting a training flight near Schindellegi when the crew experienced a sudden engine malfunction. While returning to the home base, the aircraft was climbing toward 4,000 ft AMSL between Siebnen and Galgenen when an unusual noise was heard. The helicopter began to yaw to the left, and the rotor RPM decreased.
Upon recognizing the issue, the flight instructor immediately took control and initiated an autorotation. To ensure safety, the instructor directed the aircraft toward Wangen-Lachen (LSPV), a location chosen for its flat, unpopulated terrain. During the descent, the instructor issued a Mayday call to alert emergency services. Although the engine continued to produce some power, the instructor maintained the autorotation due to persistent vibrations and abnormal engine noise. During the final flare, the instructor noted that engine power was still available and managed to perform a controlled touchdown. The two crew members sustained no injuries during the landing.
The investigation
Investigators examined the engine of the HB-ZHN following the incident. A visual inspection revealed that the upper spark plug of cylinder 1 had loosened from its threads and was no longer attached to the harness cable. Additionally, the copper spark plug gasket was missing.
Maintenance records indicated that the last technical inspection had been completed approximately 11 flight hours prior to the event. The maintenance facility stated that the spark plug had been professionally installed according to manufacturer specifications and that the cylinder threads showed no signs of damage or wear.