FLT CREW AND DISPATCHER QUESTION MEL LIMITATION ON Q400 WITH BOTH GPS SYSTEMS DEFERRED INOP.

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JUST BEFORE DEP TIME; CREW CALLS WITH QUESTION REGARDING CROWN ONE RNAV DEP OUT OF PORTLAND. CROWN ONE RNAV DEP IS THE DEFAULT DEP OUT OF PORTLAND AND WAS THE FILED DEP ON THEIR FLT PLAN. ACFT HAD BOTH CAPT AND FO GPS SENSORS INOP RENDERING THE GPS INOP. THE SENSORS WERE DEFERRED UNDER MEL; WHICH STATES ONLY THAT FMS BASED APCHES AND TERMINAL NAVAID SUBSTITUTIONS PROHIBITED. THE CREW'S CONCERN WAS THAT THERE'S NO MENTION OF RNAV DEPS LISTED IN THE MEL. THE DEP STATES THAT 'GPS REQUIRED' AND MEL STATES 'GPS DESELECTED ON AFFECTED FMS' SO CAPT AND DISPATCHER CONCURRED THAT RNAV DEPS ARE PROHIBITED AS WELL. BOTH CREW AND DISPATCHER WOULD LIKE TO SEE MEL REVISED TO INCLUDE RNAV DEPS LISTED UNDER OPERATING PROCS AS PROHIBITING ACTIONS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE COMPANY ADVISED THE RPTR THEY ARE LOOKING INTO THIS ISSUE. IN THE INSTANT CASE THE ULTIMATE DECISION WAS TO SIMPLY REFILE VIA A CONVENTIONAL DEP AND; AS A /A EQUIP TYPE; FLY THE ENTIRE SEGMENT BY APPROPRIATE /A NAVAIDS.