EMB145 CREW DEPARTED WITH 2 CONFLICTING MEL ITEMS WHICH PROHIBITED BOTH BEING DEFERRED.

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ON JUN/TUE/02; WE RPTED AT LGA RPT FOR OUR ACR TRIP PAIRING OF 2 ROUND TRIPS TO BUFFALO; NY. WE BOARDED OUR EMB145 TO BEGIN FLT XXXX. ON ACCEPTING THE PLANE; I NOTED 2 DEFERRALS FOR THE PLANE. GENERATOR #3 WAS DEFERRED; AS WAS THE 'APU FAIL' CAPTION THAT IS DISPLAYED ON OUR EICAS IF THE APU MALFUNCTIONS. THE 'MESSAGE' WAS DEFERRED BECAUSE THE APU WAS FUNCTIONING NORMALLY; AS WERE ALL ACCESSORIES ON THE APU (GENERATOR; BLEED VALVE; ETC) BUT THE CAPTION WAS BEING DISPLAYED ON OUR EICAS. AS PROC AND GOOD JUDGEMENT DICTATE; I REFERRED TO THE MEL TO CHK FOR ANY RESTRS THAT MAY BE IN PLACE DUE TO THESE DEFERRALS. THE MEL STATES 1 OF THE 4 ENG DRIVEN GENERATORS MAY BE INOP; PROVIDED THE APU IS FUNCTIONING AND ITS GENERATOR IS USED TO SUPPLEMENT THE 3 ENG DRIVEN GENERATORS. THE MEL STATES THAT THE 'APU FAIL' MESSAGE IS DEFERRABLE PROVIDED THE APU IS OPERATED ON THE GND ONLY. THIS IS IN DISAGREEMENT TO THE REQUIRED PROCS IF AN ENG DRIVEN GENERATOR IS DEFERRED. I DOUBLECHKED THE LOGBOOK; AND VERIFIED THAT BOTH ITEMS HAD BEEN DEFERRED BY MAINT; AND PLACARDED. THEY WERE. MAINT WAS CALLED TO THE ACFT; AND WE CHALLENGED THE DEFERRALS; NOTING THE MEL NOTES; AND WE WERE TOLD THAT BOTH DEFERRALS WERE ACCURATE AND THE ACFT WAS 'GOOD TO GO.' I RESTATED THE MEL NOTES AND AGAIN WAS TOLD THE ACFT WAS LEGAL. WITH THIS; WE BOARDED AND FLEW TO BUFFALO UNEVENTFULLY. WHEN WE LANDED IN BUFFALO; WE WERE TOLD WE HAD A DELAY FOR MAINT. UNAWARE OF ANY NEW MAINT ITEMS; I PHONED OUR MAINT CTL AND WAS TOLD THE DEFERRALS ON THE ACFT WERE; IN FACT; INCORRECT. CONTRACT MAINT CAME AND FIXED THE 'APU FAIL' CAPTION DEFERRAL; BUT BY THAT TIME THE INCORRECT DEFERRAL MATTER WAS A LEGALITY ISSUE. I STATED TO OUR CHIEF PLT THAT WE WERE TOLD IN LGA BY MAINT THAT THE ACFT WAS LEGAL TO FLY WITH THE 2 ITEMS DEFERRED. THE COMPANY IS NOW HANDLING THE OCCURRENCE THROUGH A 'SELF DISCLOSURE' POLICY WITH THE FAA. I FEEL THERE IS A MIX OF ITEMS THAT LED TO THIS PROB THAT LED TO 3 LEGS BEING FLOWN WITH ULTIMATELY AN INCORRECT DEFERRAL. (A SECOND CREW HAD DONE AN RDU TRIP PRIOR TO OUR BUF LEG.) A MIX OF MAINT MISHANDLING THE DEFERRALS; DISPATCH RELEASING THE TRIP; A 'POOR' MEL PROC FOR THE 'APU FAIL' CAPTION; AND MISINFO GIVEN TO MY CREW AND I ALL LED TO THE OCCURRENCE. REVISING THE MEL PROC FOR THE 'APU FAIL' CAPTION WOULD EASILY AVOID AN EVENT LIKE THIS RECURRING; APU OP SHOULD BE ALLOWED IN CONJUNCTION WITH A DEFERRED ENG DRIVEN GENERATOR. THERE ARE OTHER INDICATIONS OF PROPER APU OP ON THE EICAS; INCLUDING RPM AND EGT. THERE ARE SEVERAL OTHER EICAS WARNINGS AND CAUTIONS THAT WOULD ALERT THE CREW TO IMPROPER APU FUNCTION (LOW OIL PRESSURE; HIGH OIL TEMP; GENERATOR OVERLOAD; APU FIRE ARE A FEW). ALSO; THE APU IS CTLED BY AN ELECTRONIC SEQUENCE UNIT THAT AUTOMATICALLY SHUTS THE APU DOWN FOR CERTAIN MALFUNCTIONS.