B747 CAPTAIN IS CONCERNED THAT THE NEW ONBOARD PERFORMANCE COMPUTER POWER SETTING MAY NOT HAVE COMPLIED WITH JFK NOISE ABATEMENT PROCEDURES.

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DEPARTING RWY 31L AT INTXN KK AT JFK WITH APPROX 11000 FT OF RWY AVAILABLE. WE PLANNED ON A REDUCED TEMP TKOF USING THE NOISE SENSITIVE CLB PROFILE ON DEP WITH FLAPS 20 DEGS. PREVIOUSLY; OUR COMPANY ALLOWED US TO USE A MAX DERATE OF 20% FROM MAX PWR SETTING. NOW WITH OUR ONBOARD PERFORMANCE SYS WE CAN FURTHER DERATE PWR UP TO 25% USING RWY SPECIFIC DATA AND INPUTTING CURRENT WX CONDITIONS. AT JFK CONDITIONS WERE: WIND 240 DEGS AT 7 KTS; TEMP 26 DEGS C; AND BAROMETRIC PRESSURE 30.03. OUR OLD PROC ALLOWING A 20% REDUCTION PROVIDED A MAX ALLOWABLE TKOF GROSS WT OF 356300 KG; OUR ACTUAL WT WAS 346000 KG; THUS WE HAD A 10300 KG PAD. BY USING THE ONBOARD PERFORMANCE SYS WE FURTHER REDUCED OUR PWR SETTING 5% MORE TO A DERATED TEMP OF 52 DEGS C; PROVIDING A MAX ALLOWABLE TKOF GROSS WT OF 347946 KG AT INTXN KK; AND AN APPROX PAD OF 1800 KG. THE INTENT OF THIS LOWER PWR TKOF IS TO SIGNIFICANTLY REDUCE ENG WEAR. THE TKOF ITSELF SEEMED SOMEWHAT SLOWER IN ACCELERATING SUCH THAT WE ROTATED THE ACFT WITH 2000 FT OF RWY REMAINING; AND THE INITIAL CLBOUT WAS SHALLOWER SUCH THAT WE MAY HAVE SET OFF A NOISE MONITOR ON THE CLBOUT. CONCLUSION: I WILL MORE CLOSELY EVALUATE WHEN TO USE THE MAX DERATE THRUST SETTING. IF IT IS A NOISE SENSITIVE ARPT I WOULD NOT OPT FOR THIS LOW OF A THRUST SETTING; NOR IF THERE IS AN ALT CONSTRAINT ON THE CLBOUT.