Incident Overview
On 13 September 2016, a Schweizer 269C-1 helicopter, registration HB-ZLB, operated by Heli-Lausanne SA, was conducting a training flight for a private pilot licence (helicopter) examination. The aircraft departed Lausanne-La Blécherette at 1:51 p.m. local time with a planned landing at Gruyère Aerodrome. The flight was uneventful until the examiner noticed a power deficiency.
Flight Sequence
The navigation flight and exercises, including hovering, autorotation, and square patterns, proceeded without issues. Approximately halfway through the flight, the examiner noted that the helicopter's performance had not improved despite half of the fuel being consumed. Instrument and magneto checks revealed no anomalies. The flight continued toward Gruyère, but during overflight, the examiner confirmed a noticeable power loss and photographed the flight parameters for later discussion with the operator. As the helicopter approached Botterens, engine noise increased without alarm lamps activating. The examiner decided to terminate the flight with a precautionary landing in a field near Botterens. On short final, the engine's remaining power allowed only a running landing; the helicopter came to a stop without damage.
Engine Examination
After the incident, the engine was removed and inspected at a Swiss Transportation Safety Investigation Board facility. The engine had accumulated 869 hours since overhaul. Visual inspection revealed a long crack in the head of cylinder 4, originating from the lower spark-plug hole. This crack caused a loss of compression, explaining the partial power loss. Metallurgical examination determined the crack's origin as fatigue, with surfaces showing signs of overheating.
Conclusions
The examiner's decision to perform a precautionary landing short of the destination was considered appropriate. No compression measurements from recent periodic inspections were available to determine when the crack began affecting engine power.