Narrative:

Descending into bos and level at 21000', controller gave us an immediate descent through 20000' to level at 19000'. We responded and he then repeated the immediate descent through 20000' and then gave another aircraft a 30 degree heading change. All of this west/O warning and the controller did sound concerned. The controller then asked us for our airspeed. We replied with 290 KIAS. He reminded us that the previous clearance was to maintain 310 KIAS or greater. (He was correct.) we then heard him slowing some other aircraft, presumably (sic) behind us. WX that day: numerous layers of clouds, light to moderate chop, essentially from 10000' MSL through 33000' MSL. We had just encountered some heavier turbulence which necessitated a speed reduction within a 5 min time period just prior to the controller's immediate descent clearance. We had reduced to 290 KIAS from 310 KIAS (285 KIAS being our turbulence penetration speed at this particular altitude), but did not notify the controller of this thinking we would be able to resume our speed as soon as the turbulence subsided. We have no idea how close the other aircraft may have been or how our 20 KT speed reduction could have caused a conflict. Our altitude was never in question.

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Original NASA ASRS Text

Title: ACR MLG SLOWS BELOW ASSIGNED SPEED WITHOUT ADVISING ATC WITH PROBABLE LOSS OF STANDARD SEPARATION WITH FOLLOWING ACFT.

Narrative: DSNDING INTO BOS AND LEVEL AT 21000', CTLR GAVE US AN IMMEDIATE DSCNT THROUGH 20000' TO LEVEL AT 19000'. WE RESPONDED AND HE THEN REPEATED THE IMMEDIATE DSCNT THROUGH 20000' AND THEN GAVE ANOTHER ACFT A 30 DEG HDG CHANGE. ALL OF THIS W/O WARNING AND THE CTLR DID SOUND CONCERNED. THE CTLR THEN ASKED US FOR OUR AIRSPD. WE REPLIED WITH 290 KIAS. HE REMINDED US THAT THE PREVIOUS CLRNC WAS TO MAINTAIN 310 KIAS OR GREATER. (HE WAS CORRECT.) WE THEN HEARD HIM SLOWING SOME OTHER ACFT, PRESUMABLY (SIC) BEHIND US. WX THAT DAY: NUMEROUS LAYERS OF CLOUDS, LIGHT TO MODERATE CHOP, ESSENTIALLY FROM 10000' MSL THROUGH 33000' MSL. WE HAD JUST ENCOUNTERED SOME HEAVIER TURB WHICH NECESSITATED A SPD REDUCTION WITHIN A 5 MIN TIME PERIOD JUST PRIOR TO THE CTLR'S IMMEDIATE DSCNT CLRNC. WE HAD REDUCED TO 290 KIAS FROM 310 KIAS (285 KIAS BEING OUR TURB PENETRATION SPD AT THIS PARTICULAR ALT), BUT DID NOT NOTIFY THE CTLR OF THIS THINKING WE WOULD BE ABLE TO RESUME OUR SPD AS SOON AS THE TURB SUBSIDED. WE HAVE NO IDEA HOW CLOSE THE OTHER ACFT MAY HAVE BEEN OR HOW OUR 20 KT SPD REDUCTION COULD HAVE CAUSED A CONFLICT. OUR ALT WAS NEVER IN QUESTION.

Data retrieved from NASA's ASRS site as of August 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.