B737-300 CAPT ELECTS TO DIVERT TO LAS WHEN HIS FUEL CALCULATIONS INDICATE LESS THAN SATISFACTORY RESERVE FUEL AT LAX. THE COMPUTER FLT PLAN IN CMH INDICATED A TOTAL RESERVE OF 6500 LBS LNDG LAX. AN UNEXPLAINED ERROR OR LOSS OF 3500 LBS OF FUEL.

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ENRTE FROM CMH TO LAX. PREFLT; THE FMC WAS LOADED WITH ALL THE APPROPRIATE DATA FROM THE FLT RELEASE; INCLUDING RTE; WINDS ALOFT FOR EACH NAVAID; AND ARR INFO FOR LAX. THE FUEL LOAD WAS 28700 LBS WITH A BURN OF 21900 LBS. I MENTALLY ESTIMATED 20000 LBS TO XCHK THE RELEASE FUEL. I HAD 4000 LBS RESERVE AND 2500 LBS EXTRA FUEL. ENRTE ABEAM MKC; AFTER CHKING TIME/FUEL AND ENTERING DOWNLINE WAYPOINTS; A MESSAGE ON THE FMC SAID WE WOULD LAND IN LAX WITH LESS THAN OUR RESERVE OF 4000 LBS. I VERIFIED ALL FLT AND RTE DATA. WENT TO LONG RANGE CRUISE AND LOWERED OUR FLT INDEX. THE FUEL STILL SHOWED AN ARR FUEL OF 3000 LBS IN LAX. I CONTACTED DISPATCHER AND INFORMED HIM OF OUR ARR FUEL IN LAX. WE AGREED TO TAKE ANOTHER FIX ABEAM COS. THE FMC STILL INDICATED 3000 LBS ARR FUEL IN LAX. I TOLD THE DISPATCHER I WAS UNCOMFORTABLE WITH THIS ARR FUEL AND WOULD DIVERT TO LAS FOR FUEL. HE AGREED. WE REFUELED IN LAS AND CONTINUED TO LAX WITHOUT ANY FURTHER DELAY. SUPPLEMENTAL INFO FROM 380273: CREW CALLED 10 MINS PRIOR TO DEP AND ASKED FOR CLARIFICATION OF FLT RELEASE SHOWING A TAILWIND COMPONENT OF 59 KTS. I WENT THROUGH THE RELEASE WITH THE CAPT AND SINCE I PLANNED EXTRA FUEL OF 3900 LBS WE BOTH FELT COMFORTABLE TO INITIATE THE FLT. AFTER CONVERSING WITH A FELLOW DISPATCHER; THE SAME SCENARIO OCCURRED WITH A CMH-LAX FLT THAT LANDED SHORT IN LAS. AFTER RE-EXECUTING THE FLT PLAN 2 AND 3 TIMES IN THE SYS; I USED THE BACKUP; CADIS FLT PLANNING SYS; WHICH SHOWED A DRAMATIC CHANGE IN BURN. I SENT THE FLC A NEW CADIS RELEASE SO THAT THE FLT WOULD BE OPERATING WITH ACCURATE WINDS AND BURN. THE COMPANY CONTINUES TO OPERATE THIS SYS KNOWING THIS INFO IS INCORRECT AND STILL OPERATES THE LONG HAUL PORTION OF THE SYS WITH THE RISK OF STOPPING SHORT FOR FUEL. DUE TO THE FACT THAT THE FLT PLANNING SYS IS CALCULATING INACCURATE BURNS THE DISPATCHER IS AT RISK OF VIOLATION AND RISKING THE SAFETY OF FLT.