Incident
An air ambulance helicopter conducted an emergency landing on the parking lot of an elementary school after losing engine power. The event occurred during night visual meteorological conditions. The pilot stated that during cruise flight, he heard a loud bang and felt a low frequency vibration. The helicopter sustained minor damage as a result of internal engine components exiting through the engine collector and exhaust stack.
Examination
Post-incident examination of the engine focused on the third stage turbine wheel. Investigators found that one of the turbine blades had a fracture surface exhibiting features consistent with high cycle fatigue. The fatigue crack initiated at the trailing edge of the airfoil and propagated for 0.515 inch before a fast fracture separated the blade. A small area of porosity was observed in the microstructure near the fracture, but it did not intersect the fracture surface, and no porosity was visible on the fracture surface itself. The material's chemistry and hardness were within specified limits.
Findings
Given the presence of high cycle fatigue features, the blade's proper material properties, and the absence of a stress-concentrating defect at the crack origin, the fatigue may have been secondary to some other event that induced unusual loading or vibrations in the blade. The specific cause of the engine power loss was not definitively determined.