C560 AT 8000 FT WITH RDU TRACON EXPERIENCED DELAY VECTORS BECAUSE OF UNRPTED SPD REDUCTION.

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WE WERE FLYING A VECTOR AT 9000 FT AS CLRED. WE WERE NEVER ASSIGNED AN AIRSPD; BUT FLEW AT 250 KIAS. ABOUT 25 MI FROM RDU WE WERE GIVEN A CLRNC TO MAINTAIN 8000 FT. ABOUT THAT TIME WE ENCOUNTERED LIGHT RAIN AND MODERATE TURB ON AN INTERMITTENT BASIS. WE REDUCED OUR SPD TO ABOUT 220 KTS. AFTER A FEW MINS WE WERE EXPECTING A TURN S FOR DSCNT AND DOWNWIND FOR OUR APCH. THE APCH CTLR ASKED OUR SPD. I REPLIED '215 KTS.' HE GAVE US A HDG TO THE S AND TOLD US TO MAINTAIN OUR CURRENT SPD. I THEN CALLED THE CTLR SAND SAID THAT I REALIZED THAT I SHOULD HAVE TOLD HIM ABOUT OUR SPD CHANGE. HE REPLIED 'YES; ESPECIALLY SINCE WE HAD TFC 3 MI IN TRAIL.' THE REST OF THE FLT WAS UNEVENTFUL. AS FAR AS I AM AWARE; NO EVASIVE ACTION WAS TAKEN BY THE FOLLOWING AIRPLANE. IN HINDSIGHT; I REALIZE THAT AT THE TIME THAT WE REDUCED SPD WE WERE DISTR BY WX AND CHANGING PLANNED INST APCHS; SET UP BRIEFING; AND XFER OF CTLS. IN ADDITION; WHILE I REALIZE THAT IT IS MY RESPONSIBILITY TO INFORM ATC OF SPD CHANGES; IT WOULD HAVE BEEN HELPFUL TO KNOW THAT THERE WAS AN AIRPLANE CLOSE IN TRAIL IN IMC AND TO KEEP THE SPD UP. I KNOW THAT IT IS NOT AN ATC REQUIREMENT; BUT I HAVE HAD MOST CTLRS WARN OR HEADS UP A SIT LIKE THIS IN THE PAST. (IE; 'YOU HAVE AN XYZ 3 MI IN TRAIL; ADVISE OF SPD REDUCTION' OR 'KEEP THE SPD UP.') WE WERE ONLY ABOUT 12-15 NM FROM THE ARPT WHEN THIS SIT OCCURRED.