FLT CREW DISCOVERED THAT FLT PLANNED FUEL WAS INSUFFICIENT TO COMPLETE A NORMAL APCH INTO IAD.

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DURING MY PREFLT ANALYSIS OF THE DISPATCH RELEASE I FOUND AN ERROR IN THE CALCULATION OF OUR TOTAL FUEL BURN TO IAD. AFTER DISCOVERY OF THIS ERROR; DISPATCH WAS CONTACTED. OUR TOTAL PLANNED FUEL BURN TO IAD WAS 3828. BY EXAMINING THE DETAILED FLT PLAN; I DETERMINED THAT THE COMPUTER INACCURATELY HAD US BURNING 28 LBS OF FUEL FROM PASSING OVER THE ELSIN INTXN ON THE SHANNON 2 ARR INTO IAD AND LNDG AT IAD. 28 LBS OF FUEL REPRESENTS ABOUT 2 MINS OF FUEL BURN AT IDLE THRUST. ELSIN IS NOT THE LAST POINT ON THIS ARR. THERE IS A POINT TO THE N OR S THAT YOU WOULD CONTINUE TO FLY TO ON THE ARR DEPENDING ON RWY CONFIGN. NONE OF THIS BURN WAS ACCOUNTED FOR; NOR THE FUEL REQUIRED TO FLY AN INST APCH. DISPATCH STATED THAT THEY WERE UNABLE TO ADD POINTS TO THIS ARR INTO IAD TO ACCOUNT FOR THIS ADDITIONAL BURN. BECAUSE OF THIS APPARENT COMPUTER LIMITATION WE CAME TO AN AGREEMENT THAT WE WOULD ADD 400 LBS TO OUR BURN TO ACCOUNT FOR AN APCH INTO IAD. THIS 400 LBS IS THE STANDARD NUMBER WE ARE TRAINED TO ADD WHEN DOING A LONG HAND FUEL PROB FOR THE EMB -- BOTH DISPATCHERS AND CAPTS ARE TRAINED TO ACCOUNT FOR THIS DURING THEIR TRAINING. THIS INCREASED OUR FUEL BURN TO 4228 AND A NEW MINIMUM OF 6534; NOT 6128. A CONSISTENT INACCURATELY CALCULATED BURN TO IAD THAT IS APPARENTLY THE RESULT OF A SOFTWARE LIMITATION DID NOT ACCOUNT FOR REAL LIFE FUEL BURN SCENARIOS ON THE ARR AND APCH PHASES INTO IAD. ACTUAL BURN ON THIS FLT TO IAD WAS 4160 LBS. A COMPREHENSIVE FIX NEEDS TO RESOLVE THIS SOFTWARE LIMITATION TO GIVE OUR CREWS ACCURATE FUEL BURN INFO INTO IAD FROM ALL ARRS. WITHOUT THIS FIX; CREWS WILL CONSISTENTLY OVERBURN RIGHT THROUGH THEIR USUAL 500 LBS OF CONTINGENCY FUEL AND LAND AT RESERVE FUEL LEVELS. ANY OPERATIONAL ABNORMALITY SUCH AS HOLDING OR DIVERSION DUE TO WX WILL ONLY AMPLIFY THIS INACCURACY AND RESULT IN POSSIBLE DIVERSIONS; MINIMUM FUEL AND FUEL EMER DECLARATIONS.