B757-200 FLT CREW FAILS TO TURN ON CENTER TANK FUEL PUMPS PRIOR TO ENGINE START. OUT OF LIMIT FUEL STATUS FOR LANDING RESULTS.

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ACFT DISPATCHED WITH INOP APU GENERATOR. FIRST GPU AT ZZZ WOULD NOT HOLD THE LOAD. MAINT BROUGHT A SECOND UNIT THAT WOULD ONLY STAY ONLINE ONLY AT A REDUCED LOAD. WE COULD RUN 1 PACK/FAN; TURNING ON THE SECOND PACK/FAN WOULD CAUSE THE UNIT TO BOG DOWN AND FAIL. WHEN IT CAME TIME TO START THE ENGS; I THOUGHT; I DON'T NEED ALL 6 FUEL PUMPS ON TO START THE #1 ENG FOR FEAR THE LOAD WOULD BRING DOWN THE GPU. THEN I THOUGHT; HECK WITH IT; STAY WITH THE CHKLIST AND IF THE GPU FAILS THEN WE'LL DEAL WITH THAT. AT THIS POINT; I FAILED TO TURN ON THE CTR TANK PUMPS. THE NEXT OPPORTUNITY TO CATCH THIS WOULD HAVE BEEN ON THE AFTER TKOF CHKLIST; WHICH I PERFORMED. THE ITEM IS FUEL TANK PUMPS -- AS REQUIRED. I NOTED THAT THE MAIN TANKS SHOWED 8600 LBS. THEY SHOULD HAVE SHOWN FULL AT 9800 LBS; BUT IT IS NOT UNUSUAL FOR IT TO SHOW LESS THAN FULL AT HIGH DECK ANGLES. I DID THE FIRST POS RPT; AND THE TOTAL FUEL ON BOARD CHKED WITH THE FLT PLAN. I FOUND MY ERROR BY ROUTINE SCANNING AND GOT THE CTR TANK FUEL PUMPS ON WHEN THE MAINS SHOWED 2700 LBS AND THE CTR TANK SHOWED 15000 LBS. WE ARE SUPPOSED TO CONSUME ALL THE FUEL IN THE CTR TANK BEFORE WE START BURNING FROM THE MAINS. AT THIS POINT WE WERE TOO FAR INTO THE FLT AND WOULD STILL HAVE FUEL IN THE CTR TANK AT LNDG. THERE IS NO WAY TO XFER FUEL FROM THE CTR TANK TO THE MAIN TANKS IN FLT. WE LANDED WITH 6400 LBS IN THE CTR TANK AND 3000 LBS IN THE MAINS. MOTIVE FLOW HAD RETURNED SOME FUEL TO THE MAINS. IF SOMETHING DOESN'T LOOK RIGHT; FIND OUT WHY!