SPD DEV FROM CLRNC WHEN THE FLT CREW OF A CE560 ALLOWS THE SPD TO DROP OFF WHILE USING THE VERT SPD MODE OF THEIR AUTOPLT ON FREQ WITH ZDC; DC.

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ON CLBOUT FROM IAD; I WAS GIVEN AN ALT ASSIGNMENT OF APPROX FL270. SOON AFTER; I WAS GIVEN '280 KTS OR BETTER.' AT ABOUT 16000 FT; THE WING/ENG ANTI-ICE SYS WERE ACTIVATED WHICH REDUCED MY AIRPLANE PERFORMANCE. IN ADDITION; WE WERE REQUIRED TO REDUCE PWR SLIGHTLY TO STAY WITHIN ENG LIMITATIONS WHEN THE ANTI-ICE SYS IS ACTIVE. AT ABOUT FL180; I REALIZED THAT OUR SPD HAD DECAYED TO 265 KTS. I REDUCED THE CLB RATE TO REGAIN 280 KTS. AT FL220; THE CTLR QUERIED US. HE ASKED IF WE WERE STILL KEEPING 280 KTS. WE RESPONDED THAT WE WERE BACK UP TO 270 KTS AND ACCELERATING; BUT WOULD HAVE A VERY LOW CLB RATE. THE CTLR REMINDED US THAT 280 KTS WAS OUR ASSIGNED SPD AND THAT HE NEEDED THAT SPD. HE ALSO REMINDED US THAT THERE WAS TFC BEHIND US. I QUICKLY REGAINED MY AIRSPD. WE EXITED ICING/IMC AT APPROX FL260. TO MY KNOWLEDGE; NO OTHER ACFT HAD TO MANEUVER OR BE VECTORED FOR MY ERROR. THIS WAS A FAIRLY BUSY DAY WHICH CONTRIBUTED TO MY BEING DISTR. THE MAIN ACTION THAT COULD HAVE PREVENTED THIS SIT ALTOGETHER WOULD HAVE BEEN TO CLB USING THE 'SPD MODE' ON THE AUTOPLT INSTEAD OF THE 'VERT SPD' MODE. THIS IS A COMMON PRACTICE AMONG PLTS OF THIS TYPE OF AIRPLANE AS 'VERT SPD MODE' SEEMS TO BE SMOOTHER AND 'SPD' OF 'INDICATED SPD MODE' TENDS TO 'HUNT' FOR THE SPD WHILE OSCILLATING SLIGHTLY IN PITCH. I SHOULD HAVE ALSO ANTICIPATED THE REDUCED AIRPLANE PERFORMANCE FROM ACTIVATING THE ANTI-ICE SYS.