Narrative:

The two air carrier aircraft were on parallel RNAV departures westbound. Aircraft X was on the northern westbound departure RNAV. I turned aircraft X southwest bound so I could climb him otherwise he would have had to level at 10;000 for 15 miles while he waited to pass the inbound arrival at 11;000. Aircraft Y was direct to a fix that had him on about a 240 track or so and I turned aircraft X to a 220 heading. I knew this was a slightly converging heading but they were both right around 10;000 and I expected them both to increase speed so; although the space would close up slightly; it would happen slowly. The space closed up quicker than expected and I could not turn aircraft X back to the right because I was still waiting for him to get above the arrival that was to the northeast of aircraft X and heading southeast bound at 11000. I turned aircraft Y to a 220 heading to parallel aircraft X. As soon aircraft X was separated from the arrival; I turned aircraft X to the right but he didn't acknowledge. He answered when I turned him the second time. Aircraft X and aircraft Y were on parallel headings but I don't think there was 3 miles laterally and I'm not sure how much altitude space was between them as I planned on keeping them laterally separated and was focused on increasing the lateral separation between them. I noticed aircraft Y was showing a very slow speed and asked him what his speed was. He said 200. He was at 11;000' or so doing 200 kts. There is a speed on the RNAV that says 230 kts until 5000' so he either didn't comply or slowed after passing 5000' when I was expecting him to increase his speed which would have helped him stay more parallel with aircraft X. I have seen aircraft Y climb out at 200 kts before but because of the speed on the RNAV departure procedure; I didn't expect him to slow down after passing 5000'. I'd estimate that separation got to about 2-2.5 miles lateral but I'm not sure on altitude. I don't think aircraft Y's speed caused the possible loss of separation; but it definitely was not expected and between that and the delay of having to turn aircraft X twice because he didn't acknowledge the first time; it was enough to lose lateral separation. I've worked the same situation many times but it just didn't work today because of all the factors involved. I pride myself on not messing up and even though aircraft Y's speed and aircraft X not taking the first turn; it was my plan and I take responsibility for it not working. I could've just let aircraft X sit at 10;000 instead for 15 nm instead of trying to get him up and out of the airspace so quickly but I was trying to help him out by getting him around and above the arrival and on his way. I probably could have turned aircraft Y parallel immediately instead of waiting until it was getting close and then his speed wouldn't have come into play.

Related reports:

Google
 

Original NASA ASRS Text

Title: D10 Controller experienced a loss of separation event involving two Air Carrier aircraft when utilizing minimally lateral spacing techniques and a less than expected departure speed.

Narrative: The two Air Carrier aircraft were on parallel RNAV departures Westbound. Aircraft X was on the Northern Westbound departure RNAV. I turned Aircraft X Southwest bound so I could climb him otherwise he would have had to level at 10;000 for 15 miles while he waited to pass the inbound arrival at 11;000. Aircraft Y was direct to a fix that had him on about a 240 track or so and I turned Aircraft X to a 220 heading. I knew this was a slightly converging heading but they were both right around 10;000 and I expected them both to increase speed so; although the space would close up slightly; it would happen slowly. The space closed up quicker than expected and I could not turn Aircraft X back to the right because I was still waiting for him to get above the arrival that was to the Northeast of Aircraft X and heading Southeast bound at 11000. I turned Aircraft Y to a 220 heading to parallel Aircraft X. As soon Aircraft X was separated from the arrival; I turned Aircraft X to the right but he didn't acknowledge. He answered when I turned him the second time. Aircraft X and Aircraft Y were on parallel headings but I don't think there was 3 miles laterally and I'm not sure how much altitude space was between them as I planned on keeping them laterally separated and was focused on increasing the lateral separation between them. I noticed Aircraft Y was showing a very slow speed and asked him what his speed was. He said 200. He was at 11;000' or so doing 200 kts. There is a speed on the RNAV that says 230 kts until 5000' so he either didn't comply or slowed after passing 5000' when I was expecting him to increase his speed which would have helped him stay more parallel with Aircraft X. I have seen Aircraft Y climb out at 200 kts before but because of the speed on the RNAV departure procedure; I didn't expect him to slow down after passing 5000'. I'd estimate that separation got to about 2-2.5 miles lateral but I'm not sure on altitude. I don't think Aircraft Y's speed caused the possible loss of separation; but it definitely was not expected and between that and the delay of having to turn Aircraft X twice because he didn't acknowledge the first time; it was enough to lose lateral separation. I've worked the same situation many times but it just didn't work today because of all the factors involved. I pride myself on not messing up and even though Aircraft Y's speed and Aircraft X not taking the first turn; it was my plan and I take responsibility for it not working. I could've just let Aircraft X sit at 10;000 instead for 15 nm instead of trying to get him up and out of the airspace so quickly but I was trying to help him out by getting him around and above the arrival and on his way. I probably could have turned Aircraft Y parallel immediately instead of waiting until it was getting close and then his speed wouldn't have come into play.

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.