A WDB COMMERCIAL FIXED WING ACFT WAS DISPATCHED WITH THE #2 FMS DEFERRED AS INOP PER THE MEL BUT IN CONFLICT WITH AN AIRWORTHINESS DIRECTIVE.

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UPON ARR AT THE ACFT; MAINT TOLD US FMC #2 HAD BEEN CHANGED AND WAS BEING TESTED. IT FAILED THE TEST SO MAINT THEN DEFERRED THE ITEM AS PER OUR MEL. WE QUESTIONED THAT BUT REVIEWED THE MEL; AND PERFORMED THE CREW ITEMS. WE QUESTIONED THE STATUS OF CAT III AND WAS SHOWN ONLY ONE FMC WAS REQUIRED THERE AS WELL. WE REVIEWED THE ABNORMAL PROCS FOR SINGLE AND DUAL FMC FAILURES. NO CTL NUMBER FOR THE MEL WAS ISSUED BECAUSE THE MAINT COMPUTER WAS DOWN. WE DEPARTED AND; FOLLOWING THE CREW PROCS; HAD AN UNEVENTFUL FLT. 3 DAYS LATER; I WAS ADVISED THERE IS AN AIRWORTHINESS DIRECTIVE THAT REQUIRES DISPATCH WITH BOTH FMC'S OPERATIONAL. THE CREW FORCE WAS TO BE NOTIFIED BY OUR READ. I CERTAINLY DIDN'T REMEMBER SEEING IT. (IT HAS BEEN REISSUED; AND IT WAS NOT FAMILIAR.) THIS IMPORTANT INFO SHOULD BE COPIED IN THE APPLICABLE SECTION OF THE MEL OR INCLUDED IN THE MAINT LOG AS OTHER SHORT TERM REQUIREMENTS HAVE BEEN. THIS IS TOO SPECIFIC AN ITEM TO BE PLACED IN A VERY LARGE READ FILE -- ONE THAT IS NOT AVAILABLE TO US EASILY DURING OUR INTL OPS. IT IS SOMETHING THAT THE MAINT COORDINATOR SHOULD HAVE AT HIS FINGERTIPS IN AN OFFICE SIT. CREWS ARE NOT WHERE AIRWORTHINESS DIRECTIVES ARE TARGETED; THOUGH WE ARE AT GND ZERO. IT IS RARE WE ARE EVEN NOTIFIED OF THEM.