Bell 206L-3, Abilene, Texas, March 29, 2009–The NTSB determined the fatigue crack in the trailing edge of a main rotor blade was caused by interconnected porosity and resulting corrosion resulting from an undetected manufacturing defect. During a post-flight inspection following a flight in turbulence, the pilot noted the crack in the blade. Examination of the damage revealed a path of interconnected porosity between the lower trailing-edge skin, trailing-edge strip and epoxy that allowed moisture penetration and eventual corrosion. According to Bell Helicopter, while trailing-edge blade cracks are rare, they can usually be detected as a one-per-revolution vibration in flight, and are identified during post-flight inspection while attempting to diagnose a rotor imbalance.