B737 CREW DEPENDS ON FMS FOR COMPUTATIONS AND MISS XING RESTR.

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FLT FROM ISP-BWI. SPD AND ALT CTLED FROM TKOF BECAUSE OF FLOW IN WASHINGTON AIRSPACE. DURING DSCNT; AIRSPD WAS CTLED FROM 250 KTS TO 280 KTS. THIS WAS CHANGED BACK AND FORTH A COUPLE OF TIMES. DSCNT ALTS WERE GIVEN IN 1000 AND 2000 FT BLOCKS UNTIL THE CTR TOLD ME TO BE AT ZIZZI AT 14000 FT. I WAS ALREADY IN A 1000-1200 FPM DSCNT WHEN GIVEN THIS CLRNC. I ENTERED THE ALT INTO THE FMC SO THE COMPUTER COULD HELP FIGURE THE DSCNT REQUIRED. ALTHOUGH THE FMS TOOK THE CHANGE; THE DSCNT PAGE SHOWED THE DSCNT BASED ON THE ALT OVER THE OM WHICH WAS STILL 40 NM AWAY. IT SHOWED A DSCNT RATE OF 1100 FPM; WHICH IS WHAT I WAS DOING. (MY PARTNER WAS GETTING ATIS FROM BWI DURING THIS PERIOD.) APPROX 7 NM FROM ZIZZI; I NOTICED THE DISCREPANCY. WE WERE VERY HIGH; ASSIGNED 280 KTS FOR SPACING. I DEPLOYED THE SPOILERS AND WENT TO IDLE. PASSED OVER ZIZZI 1000-1100 FT HIGH. REACHED 14000 FT; 3 - 3 1/2 NM PAST ZIZZI. NO TFC CONFLICT WAS NOTED. THE CTLR DID NOT SAY ANYTHING. THE FMC IS A GREAT TOOL; BUT DO NOT DEPEND ON IT TO KEEP YOU LEGAL.