A BEECHCRAFT 1900 TURBOPROP ACFT WITH PT6 ENGINES WAS FLOWN WITH INCORRECT PROP ATTACH BOLTS.

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THIS RPT WILL BE FROM MY PERSPECTIVE AND ONLY INCLUDES THE ITEMS THAT I KNOW OR HEARD FROM MY COWORKERS. I AM A CAPT FOR A 121 ACR. I STARTED MY DAY YESTERDAY LIKE ALWAYS. IT INCLUDED A FLT IN A SMALL TURBOPROP ACFT FROM ZZZ1 TO ZZZ2. THIS FLT WAS WITHOUT INCIDENT AND THE ONLY THING THAT WAS DIFFERENT WAS THAT MY SCHEDULE HAD BEEN MODIFIED TO SHOW A PART 91 REPOSITIONING FLT LATER THAT DAY FROM ZZZ3 TO ZZZ. ONCE ON THE GROUND IN ZZZ2 MY COMPANY CALLED THE OTHER CAPT I WAS FLYING WITH AND INFORMED HIM OF THIS CHANGE TO BOTH OF OUR SCHEDULES. THIS IS STILL NOT THAT OUT OF THE ORDINARY; SO WE LOADED OUR NEXT PAX AND HEADED FROM ZZZ2 BACK TO ZZZ1 AGAIN WITHOUT INCIDENT. ONCE THERE WE AGAIN EXCHANGED PAX AND LEFT ZZZ1 FOR ZZZ3. AGAIN THIS FLT WAS WITHOUT INCIDENT. ONCE ON THE GROUND IN ZZZ3 THE OPS PERSONNEL SAID THEY THOUGHT THIS PART 91 FLT GOING TO ZZZ WAS MEANT TO PICK UP A MANAGEMENT PLT OR SOME OTHER ITEM. THIS SOUNDED STRANGE TO FLY AN ACFT JUST TO PICK SOMEONE UP; HOWEVER BOTH CREWMEMBERS BOARDED THE PLANE AND COMPLETED THE FLIGHT TO ZZZ UNDER PART 91 FLT RULES ALTHOUGH NOT UNDER A FERRY PERMIT OR ANY SPECIAL AUTHORIZATION TO OUR KNOWLEDGE. ONCE THE ACFT WAS PARKED THE CREW DISEMBARKED AND HEADED INTO THE OPS BUILDING. WHILE ENTERING; WE PASSED SOME MAINT PERSONNEL DRIVING A TUG TO THE ACFT THAT WE JUST FLEW IN TO ZZZ. SINCE WE WERE NOT TOLD ABOUT ANY MAINT BOTH OF THE CREW QUESTIONED THE PERSONNEL ABOUT WHY THEY WERE GOING TO BRING THE PLANE INTO THE HANGAR. ONE OF THE PERSONNEL SAID SOMETHING ABOUT THE BOLTS ON THE PROPS POSSIBLY BEING INCORRECT. THIS IMMEDIATELY STRUCK ME AS A SERIOUS SITUATION AND I ASKED FOR FURTHER DETAILS BUT THE PERSONNEL DIDN'T PROVIDE ANY FURTHER INFO. SO THE CREW ENTERED OPS AND ASKED THEM WHY THIS ACFT WAS ALLOWED TO BE FLOWN WHILE POSSIBLY HAVING THE INCORRECT PARTS INSTALLED. WE WERE TOLD THAT THIS WAS NOT A SERIOUS SITUATION AND THAT IT WAS OK TO FLY BECAUSE THEY HAD AUTHORIZATION FROM THE FAA. ALTHOUGH THEY MAY HAVE HAD THIS AUTHORIZATION; BOTH CREWMEMBERS FELT THAT THEY SHOULD HAVE BEEN TOLD ABOUT THIS PROB AS THE INCORRECT BOLTS ON A POWER PLANT COULD POSSIBLY BE A SEVERE PROB AND SHOULDN'T BE ALLOWED TO CONDUCT 121 OPS AND AT THE VERY LEAST THE CREW SHOULD BE MADE AWARE OF THE DEFICIENCY. I WAS ALSO TOLD NOT TO ASK ABOUT THE SITUATION SINCE THEY DID HAVE AN AUTHORIZATION AND EVERYTHING WAS TAKEN CARE OF. HOWEVER THE DISPATCHER SEEMED CONCERNED AND SAID HE WOULD NOT HAVE DISPATCHED THE ACFT IF HE HAD KNOWN ABOUT THE PROB. THIS AGAIN MADE ME WORRY. I WAS THEN TOLD BY A COWORKER THE NEXT MORNING THAT THEY DIDN'T HAVE AN AUTHORIZATION TO FLY ACFT LIKE THIS AND THAT THERE WAS SOME CONCERN IN OPS THAT THERE MIGHT HAVE BEEN MORE ACFT WITH THE INCORRECT BOLTS INSTALLED. I FOUND THIS ALSO TO BE TROUBLING AND AN UNSAFE CONDITION IN MY OPINION AND THAT IS WHY I AM WRITING THIS RPT.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THE BEECHCRAFT 1900 HAS PT6 ENGINES. RPTR BELIEVES MANUFACTURER MAY HAVE SHIPPED THE INCORRECT BOLTS; WHICH WERE TOO SHORT. MAINT WAS AWARE OF INCORRECT PROP ATTACH BOLTS (4 EACH) BEING INSTALLED. ACFT CONTINUED TO BE OPERATED UNDER FAR 121 WITHOUT DOCUMENTATION LIMITING REVENUE OPS. RPTR BELIEVES THREE SIMILAR COMPANY ACFT HAVE BEEN FLYING UNDER SAME INCORRECT PROP ATTACH BOLTS. SITUATION APPEARS TO BE ASSOCIATED WITH PROP REPLACEMENT. RPTR VERY CONCERNED ABOUT HIS COMPANY NOT INFORMING FLT CREWS.