AN ACR PILOT DEPARTED KNOWING DURING PREFLT THAT THE FMS DATABASE WAS MEL'D. MAINT REPORTED ONLY THE NAV DATABASE WAS AFFECTED.

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DURING COCKPIT PREFLIGHT FOR DEPARTURE; DISCOVERED THAT FMS DATABASE HAD EXPIRED EARLIER SAME DAY; AND THAT SECONDARY DATABASE WAS ALSO EXPIRED. NOTIFIED MAINT. APPARENTLY; TIME TO LOAD DATABASE WAS EXCESSIVE; SO ITEM WAS MEL'D. I WAS VERY UNCOMFORTABLE TAKING THE AIRCRAFT; PRIMARILY DUE TO UNCLEAR GUIDANCE ON USE OF THE FMS RNAV CAPABILITY. WHAT RNAV ROUTES OR CLEARANCES COULD I ACCEPT? WAS OUR EQUIPMENT CODE STILL '--Q' OR SHOULD IT BE CHANGED? AFTER PHONE DISCUSSIONS WITH MAINT AND MY DISPATCHER; AND REVIEW OF THE MEL BOOK; WE PROCEEDED AS FOLLOWS: WE REMAINED --Q WITH ENRTE AND TERMINAL RNAV STILL USEABLE AS LONG AS THE ROUTE/CLEARANCE WAS NOT AFFECTED BY THE UPDATE. I CHOSE NOT TO PERFORM ANY APPROACH RNAV PROCEDURES. WE ALSO MAINTAINED VOR/DME CROSS-REFERENCES AND REVIEWED ALL ROUTES AGAINST OUR CURRENT NAV CHARTS. THIS DISCUSSION (WITH MAINT AND OPS) AND REVIEW DID DELAY OUR DEPARTURE FROM ZZZ. THE FLIGHT WAS STRAIGHTFORWARD; SINCE NOTHING ENRTE WAS AFFECTED BY THE UPDATE. BUT FOR THE NEXT FLIGHT; THE PRESCRIBED DEPARTURE WAS THE RNAV DEPARTURE; NEWLY ISSUED; AND NOT IN OUR DATABASE. THIS DEPARTURE FEATURED AT LEAST TWO WAYPOINTS WHICH WERE NOT IN OUR DATABASE. WE ASKED CLEARANCE FOR NON-RNAV DEPARTURE. WE ALSO DISCUSSED THIS WITH OUR DISPATCHER. WHILE MY THREE FLIGHTS WERE COMPLETED SUCCESSFULLY; SAFELY; AND I BELIEVE LEGALLY; THERE WAS NO GUIDANCE ABOUT WHAT TO DO IN THE FOM OR AOM. THERE WAS SOME GENERAL AND AMBIGUOUS GUIDANCE IN THE MEL. I WAS ALSO UNABLE TO CONTACT THE CHIEF PILOT FOR GUIDANCE. FURTHER; MAINT DID NOT SEEM TO UNDERSTAND THE OPERATIONAL SIGNIFICANCE OF GOING WITHOUT A CURRENT DATABASE. THERE IS ALSO ALMOST NO LOCATION DATA IN OUR PUBLICATIONS TO ENABLE US TO ENTER WAYPOINT DATA IF THE POINTS ARE NOT IN OUR DATABASE. AIRCRAFT NAVIGATION DATABASE WAS NOT UPDATED PRIOR TO EXPIRATION. AN AIRWORTHINESS INSPECTION WAS PERFORMED ONE DAY PRIOR TO EXPIRATION; BUT THE IMPENDING EXPIRATION WAS APPARENTLY NOT NOTED. MAINT CHOSE TO MEL THE ITEM RATHER THAN REPAIR. AND BY THE WAY; THE DELAY CODE ASSIGNED WAS 'PILOT PRECAUTIONARY' -- THIS WAS A MAINT ERROR. NEXT TIME I WILL JUST SAY NO. AT THE LEAST; DATABASE CURRENCY (LOOKING AHEAD FIVE DAYS) SHOULD BE PART OF THE AIRWORTHINESS INSPECTION. THERE SHOULD BE A BULLET-PROOF METHOD TO ENSURE THIS DOESN'T HAPPEN AGAIN. AN AOM OR FOM BULLETIN SHOULD BE ISSUED DETAILING PROCEDURES TO BE USED FOR OPERATING WITH AN EXPIRED DATABASE; IF OPERATIONS ARE TO BE ALLOWED. PERSONALLY; BECAUSE OF THE HEAVILY RNAV-DEPENDENT NATURE OF OUR FLYING; I WOULD RECOMMEND THAT POLICY SHOULD BE TO ALLOW ONLY ONE FLIGHT TO A MAINT BASE; NOT TO OPERATE THE AIRCRAFT ON A NORMAL SCHEDULE WITH AN EXPIRED DATABASE. A REVIEW OF THE FDC NOTAMS FOR MANY DESTINATIONS SHOWS THAT A LARGE NUMBER OF OUR BREAD-AND-BUTTER ILS APPROACHES NOW REQUIRE SUITABLE RNAV BECAUSE OF VOR/DME/NDB OUTAGES. HOW LEGAL IS IT TO SHOOT THOSE APPROACHES WITH AN EXPIRED DATABASE? I AM VERY FAMILIAR WITH RNAV AND DATABASE ISSUES; BUT I DON'T THINK THAT'S TRUE OF ALL OF OUR PILOTS. IT WOULD BE VERY EASY TO GET INTO TROUBLE IN THIS SITUATION.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: REPORTER STATED THAT NOT A LOT OF MANAGEMENT GUIDANCE WAS AVAILABLE TO HIM FOR AN EARLY MORNING DEPARTURE. HE FELT THAT MAINT AND DISPATCH GUIDANCE WAS SUFFICIENT TO EDUCATE HIM THAT ONLY THE NAV PORTION OF THE DATABASE WAS AFFECTED. HE ALSO FELT THAT BECAUSE IT WAS A RELATIVELY SHORT FLT THE CROSS CHECKING PROCESS WOULD NOT BE COMPLEX OR FATIGUING FOR THE CREW.