AN MEL WHICH REQUIRED OPERATIONS WITHOUT EITHER FMS CREATES CONFLICTS AND MISUNDERSTANDING BETWEEN THE FLT CREW; DISPATCH AND ATC.

Narrative

I ACCIDENTALLY FLEW 2 LEGS WITH CLRNCS THAT WERE NOT WITHIN LEGAL VOR SVC VOLUME. UPON RPTING FOR THE NEXT FLT; THE CAPT DISCOVERED THAT WE WERE UNABLE TO COMPLY WITH AN MEL WHICH DEFERRED THE FMS AUTO-TUNE INHIBIT SWITCH. ACCORDING TO THE MEL; THE SWITCH MUST BE IN THE 'ON' (INHIBIT) POS; BUT THE SWITCH WAS SHEARED OFF AND WAS NOT FUNCTIONING IN ANY POS. THE CAPT CALLED MAINT AND AFTER DISCUSSING THE PROB WITH THE COMPANY DECIDED TO DEFER BOTH FMS SYS. ONE OF THE FIRST OF MANY PROBS WE RAN INTO WAS THAT WHEN BOTH FMS SYS ARE DEFERRED; THE MEL FLT CREW PROC CALLS FOR THE PLTS TO TURN BOTH FMS SYS OFF. UNLESS WE WERE MISTAKEN; THE COMPANY; CAPT AND MYSELF COULD NOT THINK OF ANY WAY OF TURNING OFF THE FMS SYS OTHER THAN SELECTING GREEN NEEDLES. IN MY OPINION; THE FLT CREW PROC IS MISLEADING AND VAGUE. I BELIEVE THE WORDING SHOULD BE CHANGED. AFTER MAKING THE DECISION THAT THE ACFT WAS NOW AIRWORTHY; THE CAPT AND I BEGAN MAKING THE NECESSARY CHANGES TO THE RELEASE. HE CALLED DISPATCH AT LEAST TWICE TO GET A NON RNAV ROUTING; PEN AND INKED THE NEW ROUTING; NEW BURN; NEW MINIMUMS; AND MADE AN ADDITIONAL CALL TO HAVE THE RNAV SUFFIX REMOVED FROM THE CLRNC. AFTER APPROX 1 HR OF PREPARATION; THE CAPT AND I MISTAKENLY AGREED THAT THE PAPERWORK WAS IN ORDER. OUR MISTAKE OCCURRED WHEN GOING OVER OUR ROUTING. WE HAD IN OUR MINDS THAT WE NEEDED OUR NAVAIDS NO MORE THAN 260 NM APART TO ENSURE PROPER SIGNAL COVERAGE. THIS IS THE CORRECT NUMBER WHILE ON AN AIRWAY; HOWEVER; WHEN GOING DIRECTLY FROM VOR TO VOR; THE SVC VOLUME WOULD HAVE BEEN A MAX OF 130 NM FOR A HIGH NAVAID AND 40 NM FOR A LOW NAVAID. UNFORTUNATELY; WE DID NOT REALIZE THE ERRORS UNTIL THE RETURN LEG. ALONG THE WAY; ATC KEPT TRYING TO GIVE US SHORTCUTS. WE INFORMED THEM THAT WE WERE NOT RNAV CAPABLE AND UNABLE TO ACCEPT MANY OF THOSE SHORTCUTS. THAT IS; HOWEVER; WHAT TRIGGERED THE THOUGHT FOR THE CAPT THAT OUR REASONING WAS INCORRECT. ONCE WE FIGURED OUT THE ERROR; WE INFORMED ATC AND THE COMPANY. THE CAPT GOT INTO A LENGTHY DISCUSSION WITH THE DISPATCHER VIA ACARS TRYING TO EXPLAIN THE PROB AND I AM NOT SURE DISPATCH EVER UNDERSTOOD. THE CAPT AND I MADE AN EXTRA EFFORT TO ENSURE ALL OUR DUCKS WERE IN A ROW AND WERE AWARE THAT PREVIOUSLY SIMILAR MISTAKES HAVE BEEN MADE. IT WAS FRUSTRATING THAT THE DISPATCHER WAS NOT AWARE OF THE POTENTIAL PROBS ASSOCIATED WITH SUCH A DEFERRAL. HOWEVER; WE REALIZE THE ULTIMATE RESPONSIBILITY IS OURS. IMPROVEMENTS IN TRAINING AND FLT PLANNING SOFTWARE MAY HELP IN THE FUTURE.