B747-400 FLT CREW DURING PREFLT PROGRAMS THE FMC WITH THE WRONG TEMP FOR THE REDUCED THRUST TKOF SETTINGS.

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WHEN OUR FINAL WTS CAME OUT OVER ACARS WE HAD LOST ABOUT 12000 LBS AND WERE WELL BELOW OUR MAX ATOG AND RELATIVELY LIGHT FOR THE ORD-HKG FLT. ATC ASKED IF WE COULD ACCEPT RWY 32R TO AVOID DELAYS BUT ACCORDING TO THE PERFORMANCE DATA WE COULD NOT. WE WERE CLRED TO TAXI TO RWY 32L FULL-LENGTH AND EXPECT EXTENSIVE DELAYS DUE TO THE RWY CONFIGN IN USE. WE NOTICED THAT OUR FLT (ANOTHER B747) WAS WAITING FOR DEP ON RWY 9R. WHEN I HEARD THE WINDS RPTED FROM 040 HDG DEG; I ASKED THE RELIEF FO TO CHK ON THE NUMBERS FOR RWY 9R TO SEE IF WE COULD USE IT. BY THEN WE WERE OUT TO THE END OF RWY 32L BUT THE PERFORMANCE DATA SAID WE COULD USE RWY 9R WITH FLAPS 20 DEGS AND EVEN A SLIGHT REDUCTION FROM MAX EPR. WE THEN REQUESTED TO TAXI TO RWY 9R AND DID SO WITHOUT INCIDENCE. THE RELIEF FO HAD STATED THAT THE PWR REDUCTION FROM MAX TO REDUCED WAS NEGLIGIBLE BUT IT WOULD STILL SHOW AS REDUCED EPR TKOF. WHEN THE FLYING FO FIRST TYPED IN THE TEMP FOR THE REDUCED EPR; IT LOWERED THE EPR NUMBERS BY A SUBSTANTIAL NUMBER. I STARTED LOOKING AT THE PERFORMANCE NUMBERS MYSELF TO FIGURE OUT WHY THAT WAS SHOWING AND WHILE I WAS DOING THAT THE FO REALIZED THAT SHE HAD ENTERED 48 DEGS F INSTEAD OF THE 8 DEGS C WHICH BROUGHT THE EPR BACK UP NEAR MAX. THIS WAS A VERY EASY MISTAKE TO MAKE AS ON THE LIGHTER FLTS WE ARE USED TO SEEING 48 DEGS C FOR THE REDUCED EPR TKOF. WHY DO WE EVEN STILL GET THE PERFORMANCE NUMBERS WITH A FAHRENHEIT NUMBER IN THEM WHEN WE ALWAYS USE CELSIUS? THIS COULD HAVE BEEN A VERY SERIOUS TKOF INCIDENT IF WE HADN'T CAUGHT IT AND I WONDER HOW MANY OTHER CREWS HAVE MADE THE SAME MISTAKE WITHOUT CATCHING IT. I STRONGLY SUGGEST WE RE-PROGRAM PERFORMANCE DATA TO ELIMINATE THE FAHRENHEIT TEMPS.