MULTIPLE ACFT EQUIP PROBS CONFUSES FLC GND CREW AS TO THE LEGALITY OF THE MEL REQUIREMENTS VERSUS THE LOGBOOK WRITE-UP AND DISPATCH RELEASE. ACFT OPERATED UNDER INCORRECT ASSUMPTIONS.

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PART 121 DISPATCH RELEASE SHOWED 2 MEL ITEMS: APU INOP AND ENG #2 N2 TACHOMETER GAUGE INOP. APU WAS MEL'ED ON SATURDAY; #2 N2 GAUGE MEL'ED ON MONDAY. WE ACCEPTED THE AIRPLANE ON JAN/TUE/97; AND COMPLIED WITH ALL PROCS TO OPERATE THE AIRPLANE SAFELY. WHILE ENRTE ON OUR FIRST LEG WITH THE AIRPLANE WE REVIEWED ALL PROCS AGAIN AND DISCOVERED THAT THE 2 MELS CONFLICT BECAUSE: APU INOP REQUIRES ENG #1 START AT GATE FROM EXTERNAL AIR; ENG #2 START XBLEED AFTER PUSHBACK. #2 N2 INOP SPECIAL PROC REQUIRES BOTH ENGS BE STARTED FROM SAME AIR SOURCE. WE INADVERTENTLY OPERATED THE ACFT WITH THIS CONFLICT DUE TO THE LACK OF ANY XREFED 'CONFLICT ALERT' AND A PREPONDERANCE OF OPERATIONAL STEPS LISTED IN THE SPECIAL PROC FOR OPERATING WITH AN N2 GAUGE INOP. THE INBOUND CREW ADVISED US THAT THE N2 INDICATOR THAT WAS MEL'ED OPERATED NORMALLY AND WE CONFIRMED ITS NORMAL OP DURING OUR START AND THROUGHOUT OUR ENTIRE FIRST LEG. WE RESOLVED THE CONFLICT AT OUR FIRST ENRTE STOP BY ALERTING MAINT OF THE CONFLICT AND HAVING THEM CLR THE #2 N2 GAUGE MEL ITEM. THE MEL BOOK IS FULL OF THESE CONFLICTS. PLTS SHOULD NOT HAVE TO PROOFREAD APPROVED DOCUMENTS FOR OBVIOUS CONFLICTS AND MAINT AND DISPATCH SHOULD NOT HAVE RELEASED THE AIRPLANE ON JAN/MON/97; WHEN THE SECOND MEL DEVELOPED. OF COURSE PLTS MUST ALWAYS CONSIDER INTERRELATIONSHIPS BEFORE ACCEPTING MULTIPLE MELS. IN THIS CASE PLTS CAUGHT THE CONFLICT AFTER ABOUT 1 DAY OF OP AND BEFORE MAINT OR DISPATCH! CONTRIBUTING FACTOR FOR THIS SIT IS THAT WITH AN APU INOP THE B737-400 FLT HANDBOOK PRECLUDES STARTING BOTH ENGS AT THE GATE UNLESS THE RAMP/TXWY IS TOO SLIPPERY FOR A XBLEED START. THIS IS BECAUSE AN AIR START AT GATE OF BOTH ENGS SUBJECTS THE GND CREW TO UNREASONABLE ENG INLET RISK AND IS ONLY A LAST RESORT. THIS INFO IS IN THE B737-400 FLT HANDBOOK; SOMETHING THAT MAINT OR DISPATCH WOULD NOT REFER TO NORMALLY.