REQUIRED INSTRUMENT WAS INOPERATIVE AND ACFT WAS FLOWN ILLEGALLY.

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ARRIVED AT ACFT FOR A MEM-BOS FLT ABOUT DUSK. LOGBOOK INDICATED STANDBY HORIZON INOP. MEL STATES STANDBY HORIZON MAY BE INOP FOR DAY VMC. ALSO; THAT ACFT MAY CONTINUE FLTS OR SERIES OF FLTS UNTIL REPAIR OR REPLACEMENT MAY BE MADE. THE CREW ADVISED MEM MAINT THAT ITEM SHOULD BE REPAIRED THERE. THEY REPLIED THAT THERE WERE NO SPARE PARTS; SO REPAIRS COULD NOT BE MADE AND THAT WE WERE OK TO DEPART. DOUBLECHKED WITH MINNEAPOLIS MAINT AND WERE AGAIN TOLD WE WERE LEGAL TO CONTINUE FLT. WE DEPARTED MEM AT XA00 LCL. AMBIGUITY OF MEL AND PRESSURE FROM MAINT PERSONNEL AND PERCEPTION THAT THEY KNEW OF WHAT THEY SPOKE. NO WHERE ELSE TO TURN SO A DECISION WAS MADE TO CONTINUE. WE CERTAINLY WOULD NOT HAVE CONTINUED IF WE HAD UNDERSTOOD THE INTENT OF THE MEL. THE AMBIGUOUS NATURE OF THE MEL SHOULD BE CHANGED TO BE MUCH MORE SPECIFIC SO THAT MORE INTELLIGENT DECISIONS MAY BE MADE. ALSO; IF MAINT CTL IS GOING TO PRESSURE A FLT CREW TO PROCEED DUE TO AN UNCLR SITUATION OR FOR WHATEVER REASON; SOMEONE IN MAINT SHOULD ALSO BE HELD RESPONSIBLE; NOT JUST THE FLT CREW. IF ONE IS RESPONSIBLE FOR THEIR ACTIONS OR STATEMENTS THEY WILL BE MUCH MORE CAREFUL OF ANY ADVICE. IT IS MY UNDERSTANDING NOW THAT MAINT CAN GIVE ALL THE BAD ADVICE THEY WANT AND ARE STILL NOT CONSIDERED RESPONSIBLE. I UNDERSTAND THE FINAL AUTHORITY IS THE PIC AND FLT CREW; BUT FLYING SAFELY IS A TEAM EFFORT; SO LET'S SHARE TEAM RESPONSIBILITY. CBC CONVERSATION WITH RPTR REVEALED THE FOLLOWING: RPTR SAID THAT AIRLINE X HAD BOUGHT OUT AIRLINE Y. HE SAID THAT THE MEL FOR THE NOW NONEXISTENT AIRLINE Y PERMITTED FLT WITH THE STANDBY HORIZON INDICATOR INOP; BUT THAT THE MEL FOR THE SURVIVING CARRIER DID NOT. HE SAID THAT THE MAINT DEPT WENT TO THE OLD AND APPARENTLY OBSOLETE MEL TO DETERMINE THAT THE FLT COULD OPERATE. DURING CALLBACK; ANALYST ASKED REPORTER TO SEND A COPY OF THEIR PERTINENT MEL WHICH HE DID. THE MEL DOES NOT SEEM TO PERMIT FLYING AT NIGHT WITH THE STANDBY HORIZON INDICATOR INOP.