DASH 8 FLC FAILED TO NOTICE THAT; WITH ENG CTL UNIT MELED THAT WITH BOTH YAW DAMPERS INOP; FLT WAS PROHIBITED.

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ORIGINATED IN MHT. ACFT HAD #2 ENG CTL UNIT MELED. WE READ THE MEL AND WERE IN COMPLIANCE. ENRTE TO LGA; #1 DC GENERATOR FAILED. BOTH YAW DAMPER CHANNELS FAILED ALSO AFTER TKOF. BOTH ITEMS WERE WRITTEN UP IN LGA. MAINT REPLACED #1 DC GENERATOR. THE YAW DAMPER WAS MELED. WE READ THE MEL FOR YAW DAMPER AND NOTED THAT AUTOPLT CANNOT BE USED. AFTER FLYING 3 LEGS; WE WERE INFORMED BY MAINT IN PHL THAT WE COULDN'T FLY THE AIRPLANE WITH BOTH YAW DAMPERS AND AN ENG CTL UNIT INOP. WE REMEMBERED THEN; THAT THE ENG CTL UNIT MEL STATED THIS; BUT WE BOTH FORGOT BECAUSE AT THE TIME WE READ IT; THE YAW DAMPER WAS WORKING. THE YAW DAMPER MEL DIDN'T MENTION BOTH ENG CTL UNITS MUST BE OPERATING TO BE IN COMPLIANCE. I BELIEVE THE PROB WAS CAUSED BY US MENTALLY THINKING ENG CTL UNIT MEL WAS IN COMPLIANCE BECAUSE IT WAS WHEN WE READ IT. WITH ALL THAT HAPPENED ENRTE TO LGA; THE MEL FOR THE ENG CTL UNIT WAS IN THE BACK OF OUR HEADS. WE DIDN'T REREAD THE ENG CTL UNIT MEL TO SEE IF THE YAW DAMPER AFFECTED IT. THIS WAS A MISTAKE ON OUR PART. AS A RULE; WE DON'T READ THE MEL ITEMS OVER BEFORE EACH FLT UNLESS IT IS A NEW MEL; WHICH IS WHAT WE DID WITH THE YAW DAMPER. ONE PROB THAT HAPPENED WAS THE ENG CTL UNIT MEL STATED BOTH YAW DAMPER CANNOT BE INOP; HOWEVER THE YAW DAMPER MEL DIDN'T MENTION THE ENG CTL UNIT. IT WOULD HELP IF MELS INCLUDED EVERY POSSIBLE ITEM AND SYS AFFECTED BY IT IF INOP; (EX; REFER TO ENG CTL UNIT INOP MEL) OR COMPANY POLICY HAVING US READ MELS PRIOR TO EACH FLT.