Narrative:

I was the controller in this event. I initially saw that an EMB170 and a BE40 were going to be tied at the larks gate; which they both needed to cross at 17;000 ft MSL. When the EMB170 checked on I issued 250 KTS as a speed restriction. I also issued 280 KTS or greater initially to the BE40. I then came back and issued 300 KTS or greater to the BE40; with a crossing restriction at larks of 250 KTS to keep that aircraft ahead of the EMB170. The speeds seemed to be working and the BE40 was pulling ahead of the EMB170. As the two aircraft converged toward larks; I knew the speeds would not be enough. I then issued a turn of 30 degrees left to the EMB170. At this point it was apparent that the BE40 was beginning their speed reduction to 250 KTS for the crossing restriction. Even at 250 KTS; the EMB170 was still getting too close to the BE40. I then issued an additional 30 degrees left turn to the EMB170. At this point; the EMB170 was not heading toward the powdr arrival stream and an EMB145. Once the EMB170 was clear of the BE40; I cleared the EMB170 back direct larks. In the mean time; the EMB170 was uncomfortably close to the EMB145's protected airspace. The EMB170 eventually made it back in trail with the BE40 and did not come in conflict with the EMB145.there were several actions I could have taken to prevent such a close event. To begin; I should have kept vertical separation between the EMB170 and the BE40 the whole time. Instead; I had relied on the speeds to maintain lateral separation by the time they converged. Next; when issuing both 30 degree left turns to the EMB170; I should have stopped the decent at FL200 to remain above the EMB145. One last action would have been to use our LOA with D01 allowing for the BE40 (apa landing) to cross larks at 16;000 ft; which would have gotten that aircraft below the EMB170.

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Original NASA ASRS Text

Title: ZDV Controller described a loss of separation event when first attempting to use speed then lateral actions to secure clearance; both of which failed.

Narrative: I was the Controller in this event. I initially saw that an EMB170 and a BE40 were going to be tied at the LARKS gate; which they both needed to cross at 17;000 FT MSL. When the EMB170 checked on I issued 250 KTS as a speed restriction. I also issued 280 KTS or greater initially to the BE40. I then came back and issued 300 KTS or greater to the BE40; with a crossing restriction at LARKS of 250 KTS to keep that aircraft ahead of the EMB170. The speeds seemed to be working and the BE40 was pulling ahead of the EMB170. As the two aircraft converged toward LARKS; I knew the speeds would not be enough. I then issued a turn of 30 degrees left to the EMB170. At this point it was apparent that the BE40 was beginning their speed reduction to 250 KTS for the crossing restriction. Even at 250 KTS; the EMB170 was still getting too close to the BE40. I then issued an additional 30 degrees left turn to the EMB170. At this point; the EMB170 was not heading toward the POWDR arrival stream and an EMB145. Once the EMB170 was clear of the BE40; I cleared the EMB170 back direct LARKS. In the mean time; the EMB170 was uncomfortably close to the EMB145's protected airspace. The EMB170 eventually made it back in trail with the BE40 and did not come in conflict with the EMB145.There were several actions I could have taken to prevent such a close event. To begin; I should have kept vertical separation between the EMB170 and the BE40 the whole time. Instead; I had relied on the speeds to maintain lateral separation by the time they converged. Next; when issuing both 30 degree left turns to the EMB170; I should have stopped the decent at FL200 to remain above the EMB145. One last action would have been to use our LOA with D01 allowing for the BE40 (APA landing) to cross LARKS at 16;000 FT; which would have gotten that aircraft below the EMB170.

Data retrieved from NASA's ASRS site as of April 2012 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.