A BEECHCRAFT 90C WAS OPERATED FOR A PERIOD OF 5 YRS IN NON COMPLIANCE WITH AN AIRWORTHINESS DIRECTIVE PERTAINING TO BOTH ENG TURBINE SECTIONS.

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I AM A NEWLY HIRED CHIEF PLT OPERATING A KING AIR C90 UNDER PART 91. THIS IS MY FIRST POS IN MGMNT AS A PLT. BEING NEW TO THE MGMNT SIDE OF AVIATION; I HAD PLACED OUR KING AIR ON XYZ SYS TO KEEP TRACK OF ALL AIRWORTHINESS DIRECTIVES; SVC BULLETINS; AND TIME-LIMITED ITEMS SINCE WE DO NOT HAVE A COMPANY OWNED MAINT DEPT. WE ARE UNDERGOING A PHASE 4 INSPECTION WITH A DIFFERENT MAINT FACILITY THAN OUR COMPANY HAD ORIGINALLY USED FOR PRIOR PHASE 2 AND PHASE 3 INSPECTIONS. THE PHASE 4 INSPECTION UNVEILED AIRWORTHINESS DIRECTIVE 76-06-1 HAD NOT BEEN COMPLIED WITH. THIS AIRWORTHINESS DIRECTIVE IS A MAJOR OVERHAUL TO PWR TURBINE MODIFICATION OF THE PT6A-20'S. MY QUESTION IS; HOW COULD SOMETHING LIKE THIS GO UNNOTICED DURING A PRE-BUY AND SEVERAL PHASE INSPECTIONS SINCE AUG/94? ONCE HIRED WITH THIS COMPANY I TRIED TO FIND ALL AIRWORTHINESS DIRECTIVES AND SVC BULLETINS THAT PERTAIN TO OUR ACFT. UNFORTUNATELY; I WAS UNABLE TO FIND ANY INFO READILY AVAILABLE FOR MY PARTICULAR ACFT THROUGH THE FAA; DOT; RAYTHEON; ETC. OUR COMPANY HIRED XYZ TO DO THE 'LEG WORK' AND PULL ALL RECORDS INVOLVING OUR TYPE OF ACFT. ONLY AFTER SEVERAL MONTHS OF PROFESSIONALS DOING THE RESEARCH COULD WE FIND ALL PERTAINING INFO. IF THERE WERE A WAY TO SEARCH OR MAKE THIS INFO MORE READILY AND EASILY ACCESSIBLE; IT MAY HAVE BEEN FOUND SOONER. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED HE TOOK OVER THE POS OF MAINT MANAGER AND CHIEF PLT AND WAS NEW TO ACFT MAINT. THE RPTR SAID EVERY EFFORT WAS MADE TO RESEARCH THE ACFT HISTORY TO ENSURE ALL AIRWORTHINESS DIRECTIVES NOTES WERE ACCOMPLISHED. THE RPTR STATED IT WAS NECESSARY TO BRING IN AN OUTSIDE CONTRACTOR WHO RESEARCHED THE RECORDS AND DISCOVERED AN AIRWORTHINESS DIRECTIVE NOT ACCOMPLISHED. THE RPTR STATED THE AIRWORTHINESS DIRECTIVE NOTE REQUIRED MODIFICATION TO THE ENG TURBINE SECTION TO PREVENT UNCONTAINED FAILURE.