AN ACR LTT WAS DISPATCHED INTO WX THAT REQUIRED AN ILS DME APCH. THE DME WAS PLACARDED INOP. MAINT INTERPRETS DEFERRAL AS APPLYING ONLY BEYOND 20 NM. FLC QUESTIONS LEGALITY OF MAINT DEFERRAL INTERP.

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ACFT HAD DME DEFERRED. ORIGINAL ENTRY OF DISCREPANCY INDICATED DME OPERATIONAL WITHIN 20 NM OF STATION; ALTHOUGH WEAK. DEFERRAL COUPON REFLECTED SAME. FIRST LEG OF TRIP WAS INTO SFO; A COMPANY LINE MAINT STATION. I REQUESTED MAINT PERSONNEL TO EITHER CLR THE DISCREPANCY OR RESOLVE THE AMBIGUITY OF THE DEFERRAL: WAS THE WHOLE SYS DEFERRED OR WAS ITS USE DEFERRED ONLY BEYOND 20 NM? ALTHOUGH CURRENT CONDITIONS AND FORECAST INDICATED DME WOULD NOT BE NECESSARY FOR OPS; THERE WAS ALWAYS THE POSSIBILITY THAT DME WOULD BE REQUIRED SHOULD WX CHANGE IN ORDER TO EXECUTE AN INST APCH AT CRESCENT CITY. SFO MAINT ELECTED TO INTERPRET DEFERRAL AS APPLYING ONLY BEYOND 20 NM. GIVEN CURRENT FORECASTS AND CONDITIONS SHOWING VFR CONDITIONS ALONG OUR RTE; I ELECTED TO CONTINUE WITH DEFERRAL IN TACT. WX IN CEC WAS INDICATING A CONSIDERABLE DETERIORATION. AN INST APCH PROC WOULD BE LIKELY; AND DME WOULD BE REQUIRED. FROM ACV; I CALLED MAINT CTL AND AGAIN EXPLAINED MY DOUBTS AND QUESTIONS ABOUT THE DEFERRAL; AS WELL AS PRIOR SFO MAINT INTERP. I ALSO PASSED ON MY OBSERVATIONS THAT THE DME WAS PERFORMING NORMALLY AND RELIABLY WITHIN 20 NM OF STATION. MAINT CTL REVIEWED THE WRITE-UP OF THE DEFERRAL; AND MY INPUT; AND CONCLUDED THAT DME WAS AVAILABLE FOR USE WITHIN 20 NM AND THEREFORE WE COULD EXECUTE DME PREDICATED APCHS. WE CONTINUED TO CRESCENT CITY. AN ILS DME WAS REQUIRED. THE DME FUNCTIONED PERFECTLY WITHIN 20 NM. WE SUCCESSFULLY EXECUTED THE APCH. I AM STILL UNCOMFORTABLE WITH THE LEGALITY OF AN APCH PREDICATED ON DME RECEPTION. I DEFERRED TO THEIR INTERP OF THE DEFERRAL. I AM NOW CONVINCED THAT THE DME WAS DEFERRED COMPLETELY AND THEREFORE COMPLETELY UNUSABLE.