B737 FLT CREW IS CURIOUS WHY DME DEFERRAL PROHIBITS ACFT FROM RNAV APPROACHES AND Q ROUTES BUT ALLOWS LNAV USE FOR AIRWAYS NAVIGATION.

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ACFT HAD FO DME INOP DEFERRAL. THE RESTR WAS DO NOT RNAV DEPS; RNAV RTES; RNAV ARRS; OR Q RTES. THE FLT WAS FILED AS B733/W. I CALLED DISPATCH AND GOT A MAINT 3-WAY PATCH TO DISCUSS THE RESTRS AND TO INQUIRE AS TO IF LNAV WAS STILL AVAILABLE FOR USE SINCE THE RESTR DID NOT FORBID ITS USE. ON THE B737 THE NAV COMPUTERS ONLY REQUIRE 1 OPERABLE DME TO FUNCTION AND I WAS CURIOUS ABOUT NOT BEING ABLE TO USE RNAV RTES BUT NOT BEING RESTR FROM USING LNAV FOR JET RTES AND NON RNAV DEPS/ARRS. MAINT SAID LNAV WAS INDEED AVAILABLE FOR USE AND COULD NOT EXPLAIN THE REASON FOR THE RESTR EITHER; SINCE WE USE LNAV TO FLY THE RESTR PROFILES ALSO. MAINT CLRED US TO FLY BACK ON THE JET RTE FILED FLT PLAN USING LNAV BUT SUGGESTED WE FILE A SAFETY RPT TO HAVE THE ISSUE LOOKED INTO. WE FLEW THE RTE FILED AND DID NOT ACCEPT ANY DIRECT DOWN RTE CLRNCS; DUE TO BEING A /W; AND FILED THIS RPT AS SUGGESTED. AN ANSWER AS TO WHY WE WERE RESTR FROM RNAV ROUTING BUT NOT LNAV WHEN THE SAME SYS CTLS BOTH WOULD BE APPRECIATED.