Narrative:

I own FL240 and up. I had a d-side. I had been sitting on position approaching 2 hours. The rides were crappy; continuous light turbulence all altitudes; VFR with layers. I had taken the handoff on a gulfstream climbing to FL270 with a speed of 290 plus. When he checked in; I told him leveling at FL270; resume normal speed. I took the handoff on an airbus climbing to FL270; 290 KTS. When he checked in; I continued his climb to FL310 and told him not to exceed 270 KTS until through FL280. There was a slight overtake but I thought with the decrease in speed and the airbus' climb that it wouldn't be an issue. I went on working the rest of my traffic. Came back around on my scan; noticed the overtake was getting worse. Turned the airbus and stopped off the GA at FL260. Once the airbus cleared the altitude I put him back on course. After everything was resolved I went back to the GA and questioned his speed. He said he slowed for the turbulence; I tried to diplomatically explain that next time he shall give ATC a heads up before he does that; as I was spacing; and separating based on the speed he was assigned. No error resulted.I am not sure how to impress on the pilots how important it is to comply with speed assignments and if unable to let us know so we can adjust from where I am sitting. I do not want to file a report on the pilot either; I think calling him out on the frequency was good enough. But they should know; we can absolutely adjust to what we know exists. If I had been busier with sequencing and volume; I am not sure I would have caught the drop-off in his speed.

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Original NASA ASRS Text

Title: Controller issued speed control assignments to two aircraft; the lead aircraft slowed due to turbulence but did not advise ATC. Controller observed the speed difference and corrected the potential conflict.

Narrative: I own FL240 and up. I had a D-Side. I had been sitting on position approaching 2 hours. The rides were crappy; continuous light turbulence all altitudes; VFR with layers. I had taken the handoff on a Gulfstream climbing to FL270 with a speed of 290 plus. When he checked in; I told him leveling at FL270; resume normal speed. I took the handoff on an Airbus climbing to FL270; 290 KTS. When he checked in; I continued his climb to FL310 and told him not to exceed 270 KTS until through FL280. There was a slight overtake but I thought with the decrease in speed and the Airbus' climb that it wouldn't be an issue. I went on working the rest of my traffic. Came back around on my scan; noticed the overtake was getting worse. Turned the Airbus and stopped off the GA at FL260. Once the Airbus cleared the altitude I put him back on course. After everything was resolved I went back to the GA and questioned his speed. He said he slowed for the turbulence; I tried to diplomatically explain that next time he shall give ATC a heads up before he does that; as I was spacing; and separating based on the speed he was assigned. No error resulted.I am not sure how to impress on the pilots how important it is to comply with speed assignments and if unable to let us know so we can adjust from where I am sitting. I do not want to file a report on the pilot either; I think calling him out on the frequency was good enough. But they should know; we can absolutely adjust to what we know exists. If I had been busier with sequencing and volume; I am not sure I would have caught the drop-off in his speed.

Data retrieved from NASA's ASRS site 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.