AbstractA military helicopter crash was investigated. The aircraft was flying at cruise speed in clear sky at 1000 feet over a flat area. The crew suddenly lost the control of the helicopter that crashed in the immediately causing fatal injuries to the whole crew. Four main rotor blades were found close to the impact point, while the fifth blade was found about 900 m before the wreck. Therefore efforts were directed to the failure of this blade that had apparently separated in the air. The rotor blade comprised a long hollow 6061-T6 aluminum alloy extrusion and 25 thin metallic pockets that provided the trailing edge airfoil shape. Visual examination of the fracture surface of the aluminum extrusion indicated fatigue crack growth followed by ductile overload separation. Examination by optical and electronic microscopy of the fatigue fracture revealed an abnormal incision that appeared to be the fracture origin site. The incision was about 2,3cm long and 190μm deep. Fatigue failure growth time was determined using fracture mechanics. Electronic microscopy equipped with X-EDS analyzer revealed the presence of iron in the incision. This evidence allowed to ascertain that the incision at the crack origin resulted from the use of an inappropriate tool used to remove pockets during maintenance activities. In addition to IBIS and in order to improve the safety of flights, NDTs were developed and then established every 200 flight hours.