EMB145 FLT CREW IS CONCERNED WITH FUEL BURN RESERVES DURING HOLDING.

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AFTER GETTING SIGNIFICANT REROUTING INTO LGA; WE WERE INSTRUCTED TO HOLD AS PUBLISHED AT THE HOLEY INTXN (KORRY3 ARR) WITH AN EFC OF 30 MINS AFTER THE INSTRUCTIONS WERE GIVEN. AS WE ENTERED THE HOLD; WE ATTEMPTED TO CONTACT DISPATCH; BUT SEVERAL OTHER PATCHES WERE GOING ON THROUGH COMPANY RADIO AND WE WEREN'T IMMEDIATELY ABLE TO GET THROUGH. WHEN WE FINALLY DID GET THROUGH AT APPROX XA50Z; I TOLD DISPATCH THAT WE WERE HOLDING AT HOLEY AND THOUGHT IT WAS POSSIBLE THAT THE HOLDING WOULD BE EXTENDED DUE TO POOR WX APCH LGA AND WANTED TO DISCUSS OUR OPTIONS SHOULD WE BE UNABLE TO COMPLETE OUR ARR INTO LGA (AT THE TIME OF DEP; THE WX WE WERE GIVEN DID NOT INDICATE THE NEED FOR AN ALTERNATE AND I THOUGHT IT WAS LIKELY WE WOULDN'T BE ABLE TO CONTACT DISPATCH IN THE EVENT OF A DIVERSION SINCE COMPANY RADIO WAS SO BUSY). OUR DISPATCHER ASKED US HOW MUCH FUEL WE HAD ON BOARD; WHICH; TO MY MEMORY; WAS ABOUT 3600 LBS. HE TOLD US THAT FROM THE HOLEY INTXN; OUR FUEL BURN TO LAND IN LGA WAS 223 LBS; GIVING US A 'BINGO FUEL' OF 2123 LBS. I INSTRUCTED HIM THAT OUR FMS COMPUTER GAVE US A FUEL BURN FAR HIGHER THAN HIS ESTIMATE; AND THAT WE EXPECTED TO FLY WELL PAST LGA FOR THE SEQUENCE FOR THE ARR TO RWY 22 (GENERALLY; ARRS FLY WELL PAST THE OM ON BUSY EVENINGS ON THE APCH TO RWY 22; SOMETIMES PAST HPN). I TOLD OUR DISPATCHER THAT OUR ESTIMATE WAS CLOSER TO 800 LBS AND THAT WE WOULD NEED A MORE CONSERVATIVE ESTIMATE THAN WHAT HE'S GIVEN US. HE DID NOT GIVE US ANY OTHER FIGURES BUT TOLD US THAT ALLENTOWN WOULD BE A VIABLE ALTERNATE IF NECESSARY AND THAT WE WOULD HAVE A CLEAR PATH TO IT IF WE WERE UNABLE TO GET TO LGA (WHICH WAS QUESTIONABLE; SINCE THERE WAS A LINE OF SEVERE TSTMS W OF OUR APCH PATH TO LGA AND IT WOULD HAVE BEEN MORE PRUDENT TO FLY E; BUT WITHOUT DIRECTING OUR RADAR AT ALLENTOWN. IT WAS DIFFICULT TO SAY IF THERE WAS AN AMPLE HOLE WITH WHICH TO FLY THROUGH). SHORTLY THEREAFTER; WE WERE CLRED TO EXIT THE HOLD FROM HOLEY; AND I RECORDED OUR FUEL LOAD AT THAT TIME AS 3320 LBS; SINCE I DOUBTED THE FIGURE THAT DISPATCH HAD GIVEN ME. WE FLEW ROUGHLY ALONG THE KORRY 3 ARR OR DIRECT LGA UNTIL REACHING MANHATTAN; AT WHICH POINT WE WERE VECTORED TO ROUGHLY A 15 MI FINAL TO RWY 22. WE TOUCHED DOWN IN LGA WITH 2560 LBS; A FUEL BURN OF 760 LBS; WHICH WAS NEAR THE ESTIMATE WHICH WE HAD CALCULATED AND GIVEN DISPATCH. THOUGH IN THIS INSTANCE THERE WAS AMPLE FUEL TO MAKE UP FOR THE SITUATION; IT DEMONSTRATES TO ME THAT THERE IS A MALFUNCTION IN THE LOGIC THAT IS BEING UTILIZED IN DISPATCH TO CALCULATE THESE FUEL BURN ESTIMATES AND THAT WE ARE NOT BEING GIVEN ADEQUATE CUSHIONS IN THE EVENT THAT WE ARE UNABLE TO LAND AT OUR ORIGINAL DEST. WHILE I HAD NO INTENTION OF WAITING UNTIL WE WERE AT 2123 LBS OF FUEL BEFORE DECIDING TO GO ELSEWHERE; HAD I WAITED AND THEN BEEN RELEASED TO LGA; WE WOULD HAVE LANDED AT 1470 LBS; WHICH IS BARELY OVER 30 MINS OF FUEL; AND COULD HAVE BEEN SIGNIFICANTLY WORSE HAD CONDITIONS DETERIORATED MORE QUICKLY.