What happened
During a post-maintenance test flight, a pilot and an aircraft maintenance technician were operating the helicopter when the aircraft experienced a vibration. This was immediately followed by a total loss of engine power. The pilot performed a forced landing in a muddy field. The impact during the landing resulted in structural damage to the aircraft. No fatalities or injuries were reported.
The investigation
An examination of the engine following the accident revealed that an exhaust valve in the number 3 cylinder had failed. Investigators found that two exhaust spring seats had been installed on top of one another. This improper installation caused excessive pressure on the spring and valve. Records indicated that the engine had undergone an overhaul approximately 1.5 service hours before the loss of power occurred.