MD80 ARR TO SFO; WITH NCT; ACCEPTED LDA/PRM CLRNC TO RWY 28R; DISCOVERING LATER THAT THE ACFT/FLT CREW WAS NOT AUTHORIZED FOR PROC.

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CHKING IN WITH NORCAL APCH; WE WERE CLRED TO A FIX ON THE LDA 28R APCH. TRIED TO ENTER IT INTO GPS; WE DID NOT HAVE THE FIX. AFTER BOTH PLTS TRIED TO PUT IT IN; I ADVISED ATC THAT WE NEEDED A VECTOR TO THIS FIX. WE WERE THEN TOLD TO EXPECT AN LDA PRM 28R APCH. QUICK SETUP; AS WE WERE ANTICIPATING OUR USUAL QUIET BRIDGE VISUAL TO RWY 28R; ESPECIALLY SINCE WX WAS PRETTY GOOD AND WE HAD ALL THE VISUAL CHKPOINTS IN SIGHT. VECTORED ONTO THE LOC; WE HAD THE ARPT AND THE B737 IN SIGHT FOR RWY 28L. TRACKING THE LOC; I ASKED THE CAPT (A CHK AIRMAN) 'ARE WE AUTH TO FLY AN LDA PRM APCH?' HE WAS VERY CERTAIN WE WERE. I WAS NOT CERTAIN; HOWEVER; IN VFR CONDITIONS WITH TFC IN SIGHT AND A CHK AIRMAN ASSERTING WE WERE; I PUT IT OUT OF MY MIND AND PERFORMED MY PLT MONITORING DUTIES. WE MADE AN ORDINARY LNDG IN SFO; AND GAVE IT NO FURTHER THOUGHT UNTIL 4 DAYS LATER WHEN I PULLED OUT OUR MANUAL. THE MANUAL STATES WE ARE APPROVED FOR LDA APCHS; ILS; AND ILS/PRM; BUT NO LDA/PRM APCHS. AT THE FIRST HINT OF SOMETHING OUT OF THE ORDINARY; I SHOULD HAVE BEEN MORE FORCEFUL IN MY QUESTIONING; HOWEVER; WITH THE AIRLINE INDUSTRY NOW FLYING EVERYWHERE WITH MINIMUM FUEL; VOLUNTARILY TAKEN YOURSELF OFF AN APCH IS KIND OF UNCOMFORTABLE.