A B737 FLC RECOGNIZED THE FMS WOULD BE UNABLE TO MEET A XING RESTR AND ADVISED APCH CTL. THE RESTR WAS REMOVED.

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ON DSCNT INTO FLL; WE WERE GIVEN CLRNC TO CROSS MRLIN AT 7000 FT. APPROX 15 MI FROM THE RESTR; THE FMC BEGAN TO SHOW A TREND THAT THE ACFT WAS GETTING BEHIND IN THE DSCNT TO MAKE THE XING RESTR. AT THIS POINT; I (THE PNF) MADE A QUICK STANDARD '3 TO 1' CALCULATION AND CONFIRMED THAT WE WERE GETTING BEHIND AND INSTRUCTED THE FO TO STEEPEN OUR DSCNT. APPROX 10 MI FROM THE FIX; IT BECAME CLR TO ME THAT MAKING THE RESTR AT MRLIN WAS QUESTIONABLE AND I INFORMED ATC THAT WE WOULD BE UNABLE TO MAKE THE RESTR AND IF THEY WOULD LIKE US TO TURN. ATC (MIA APCH) RESPONDED 'NO PROB; MAINTAIN 7000 FT.' THIS NEW CLRNC EFFECTIVELY RELIEVED US OF THE ALT RESTR. FORTUNATELY; THERE WAS NO CONFLICT WITH OTHER TFC. AS IT TURNED OUT; WE WOULD HAVE MISSED THE RESTR BY APPROX 2 MI. WHILE THE FMC DOES AN EXCELLENT JOB MOST OF THE TIME IN MAKING PLANNED DSCNTS; OCCASIONALLY IT CAN LAG BEHIND DUE TO WIND OR OTHER VARIABLES. USING THE '3 TO 1' COMPUTATION AS A BACKUP IS A MUST IN THESE RARE CIRCUMSTANCES. FORTUNATELY WE WERE ABLE TO DETERMINE HOW FAR BEHIND IN THE PLANNED DSCNT WE WERE AND COULD ADVISE ATC AND THEY COULD ISSUE ANOTHER CLRNC TO ACCOMMODATE OUR SIT.