What happened
On 13 September 2016, a Schweizer 269C-1 helicopter, registration HB-ZLB, was conducting a flight examination for a private pilot license. The flight originated from Lausanne, La-Blécherette, with the intention of completing a navigation exercise and landing at Gruyère Aerodrome. After completing various flight maneuvers, including hovering and autorotation exercises, the examiner noted that the aircraft's performance had not improved despite a significant reduction in fuel weight.
As the flight continued toward the destination, a noticeable loss of power became apparent. While approaching the village of Botterens, the engine noise increased, though no cockpit alarm lamps were activated. To ensure safety, the examiner decided to terminate the flight early. The crew performed a precautionary landing in a field near Botterens. The engine's remaining power was only sufficient for a running landing, but the aircraft came to a stop without sustaining any damage to the airframe or the ground.
The investigation
Following the incident, the engine was removed for detailed inspection. The engine had accumulated 869 hours of operation since its last overhaul. Investigators discovered a long crack in the cylinder 4 head, originating at the lower spark-plug hole. This defect caused a loss of compression in that cylinder, which directly led to the observed reduction in engine power. Examination of the crack indicated that it was caused by fatigue and that the surrounding surfaces showed evidence of overheating.
Findings
- A fatigue crack in the cylinder 4 head caused a loss of compression.
- The resulting loss of compression led to a partial loss of engine power during the flight.
- The engine exhibited signs of overheating associated with the crack formation.