MD88 FLC FAILS TO MAKE A XING RESTR EVEN THOUGH THEY WERE MONITORING THEIR DSCNT PROFILE AS SET UP BY THE FMS. TOO LATE THEY REALIZED THAT THEY WERE TOO HIGH TO MAKE IT. CTLR THEN ASKED FOR AN EXPEDITED DSCNT.

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ENRTE FROM ATL TO DAY; ATC CLRNC WAS TO DSND OUT OF CRUISE (FL350) TO FL330 WITH PLT'S DISCRETION TO CROSS 20 NM S OF FLM VORTAC AT FL180. THE FMS COMPUTER WAS PROGRAMMED AND VNAV SELECTED. INITIALLY; FMS SHOWED US WELL BELOW GLIDE PATH AND FO MANUALLY VERIFIED THAT WE WERE. FO CONTINUED THROUGH FL330 AND WE WERE WELL BELOW GLIDE PATH AND MANUALLY DERIVED GLIDE PATH. FURTHER INTO DSCNT; THE FO AND I GOT DISTR WITH ATIS AND IN-RANGE RPT TO COMPANY. FO KEPT EYE ON VNAV GLIDE PATH WHICH STILL INDICATED WE WERE LOW FOR XING RESTR. ONCE AGAIN; FO MANUALLY CHKED DSCNT PROGRESS AND HE/WE REALIZED WE WERE ACTUALLY WAY HIGHER -- ALMOST 6000 FT ABOVE FL180 WITH ONLY 10.5 NM TO GET DOWN! FO DID HIS BEST TO SMOOTHLY GET ACFT DOWN BUT WE ACTUALLY CROSSED THE FIX ABOUT 1000 FT HIGH. NO TFC CONFLICTS WERE NOTED (ATC DID ASK US TO EXPEDITE OUR DSCNT JUST AFTER ERROR WAS NOTED). SUBSEQUENT EXAMINATION REVEALED THE FMS HAD BEEN PROGRAMMED CORRECTLY; BUT HAD MALFUNCTIONED ON GLIDE PATH COMPUTATIONS. MORAL TO STORY: PLTS MUST NEVER TOTALLY RELY ON AUTOMATION.