AN SA227 METROLINER WAS OPERATED SEVERAL DAYS WITH A DIRECTIONAL HDG INDICATOR DEFERRED IN CONFLICT WITH THE MEL.

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ACFT HAS BEEN WRITTEN UP ON JUL/XA/98 'L SIDE HDG INDICATOR FAILED; AVIONICS BUS -- IDR CIRCUIT BREAKER POPPED -- CANNOT RESET.' CORRECTIVE ACTION FROM MAINT IN ZZZ WAS THAT THEY SWAPPED THE L FOR THE R INST SO THE ITEM COULD BE DEFERRED. HIS WRITE-UP WAS 'R SIDE HDG INDICATOR FAILS -- AVIONICS BUS -- IDR CIRCUIT BREAKER POPPED -- CANNOT RESET.' THE ITEM WAS DEFERRED BY XA MAINT AS PER MEL 34-18 (C-ITEM) THAT SAME DAY. SINCE THAT DAY; THE ACFT WAS FLOWN BY SEVERAL PLTS. ON JUL/XC/98 AN AVIONICS SHOP LOOKED AT THE INST; COULD NOT FIX IT; AND REFERRED TO THE SAME MEL 'R SIDE WILL NOT SLAVE IN ANY MODE -- DEFERRED BY MEL 34-18; EXPIRED JUL/XD/98.' ON JUL/XE/98 THE ACFT WAS FLOWN TO OUR ZZZ BASE (OUR OF XYZ) FOR MAINT AND SPENT ALL DAY. THE PLT WHO FLEW IT HAD CONCERNS AS TO THE WRITE-UP AND APPARENTLY THOUGHT IT SHOULD BE UNDER A DIFFERENT MEL. HE WAS ASSURED THE MEL WAS CORRECT BY MAINT AND THE PLT CONTINUED TO FLY THE AIRPLANE. ON JUL/XF/98 I TOOK THE AIRPLANE FOR THE FIRST TIME SINCE THE ITEM WAS DEFERRED TO FLY IT FROM PHX TO ZZZ. I CHKED THE BOOKS INCLUDING THE MEL AND WAS CONVINCED THE ACFT WAS AIRWORTHY. THE ACFT WAS FLOWN TO ZZZ IN THE MORNING AND SPENT ALL DAY AGAIN AT OUR MAINT BASE. DURING THAT DAY; MAINT DECIDED TO LOOK INTO CLRING THE MEL. WHILE LOOKING INTO IT; THEY DETERMINED THAT IN FACT THE MEL WAS WRITTEN UP INCORRECTLY -- IT SHOULD HAVE BEEN MEL 34-24 IN THEIR OPINION; WHICH IS A NO-GO ITEM. THAT DECISION GNDED THE ACFT UNTIL THE INST WAS FIXED. WHEN I TOOK THE ACFT ON THE MORNING ON JUL/XA/98 I WAS CONVINCED THAT MY PAPERWORK WAS IN ORDER AND THE WRITE- UPS WERE CORRECT. ALL PARTIES INVOLVED MISINTERPED THE MEL AND; LIKE MYSELF; ACTED IN GOOD FAITH. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE ACFT WAS AN SA277 METROLINER AND THE ACFT FLEW SEVERAL DAYS UNTIL MAINT HAD THE AIRPLANE TO WORK THE DEFERRED ITEM AND THE ITEM WAS FOUND TO BE A NO-GO ITEM. THE RPTR SAID THE MEL WAS CONFUSING AND NOT CLR.