THE FORK CONNECTING THE ROTORCRAFT'S BLUE BLADE TO THE MAIN ROTOR HUB SEPARATED AFTER A LONG TERM FATIGUE CRACK HAD SPREAD THROUGH MORE THAN 60% OF THE FORK CIRCUMFERENCE. THE OPERATOR FAILED TO CARRY OUT THE APPROPRIATE INSPECTION OF THE FORK UNIT AS DELINEATED IN THE FAA AIRWORTHINESS DIRECTIVE DUE IN PART TO CONFUSION OVER THE WORDING OF THE AD COUPLED WITH THE MISPERCEPTION THAT THE FORK REQUIRED ONLY A VISUAL INSPECTION RATHER THAN A DYE PENETRANT CHECK. THE MISPERCEPTION OF THE TYPE OF INSPECTION WAS BORN OUT OF A BELIEF THAT THE ORIGINAL BLADE FORK WITH ITS TEN THOUSAND SERIES SERIAL NUMBER REQUIRED ONLY A VISUAL CHECK. THE INSTALLED BLADE FORK SERIAL NUMBER WAS NO LONGER EVIDENT ON THE PART AND MANUFACTURER RECORDS HAD BEEN LOST THUS PREVENTING THE OPERATOR FROM VERIFYING THE SERIAL NUMBER AND INSTITUTING THE NECESSARY INSPECTION. STRONG GUSTY WINDS EXISTED AT THE ACCIDENT SITE ASSOCIATED WITH FRONTAL PASSAGE AT THE TIME OF THE ACCIDENT.