Narrative:

Aircraft X and aircraft Y were both departures off of iah on the LOA5 SID. Aircraft X was approximately 15 NM in front of aircraft Y and at a lower altitude. Both aircraft had speeds assigned by the low altitude controller. Aircraft Y was requesting a higher final altitude (FL380) than aircraft X (FL360). I assigned a slightly faster airspeed to aircraft Y (300 KTS from previously assigned 280 KTS) in order to facilitate spacing with subsequent aircraft. I allowed aircraft X to resume normal speed. Given the aircraft types and expected performance; I believed that aircraft Y would continue to out climb the E140 (aircraft X) and longitudinal separation would never be factor. After handing off both aircraft to ZFW and transferring communications; it appears that aircraft X significantly reduced its airspeed and significantly increased its climb rate. Conflict alert activated just inside ZFW airspace with aircraft X climbing out of 316 and aircraft Y only out of 330. I erred in my assumption that a B737 (aircraft Y) would continue to out climb the E140. By not ensuring that the vertical separation remained established; it created a situation where the ZFW controller more than likely had to take some type of action to prevent the E140 from climbing into the B737. If I was to speculate; the E140 probably reduced speed from the previously assigned 280 to below 240 KTS in the climb - thus accounting for the speed reduction (aircraft Y had a 70 KT overtake; thereabouts) and the increased vertical rate. Typically such regional jets; especially the E140 as opposed to the E145; climb at speeds comparable to the 'regular' jets. I was not expecting the 300 KTS assigned to aircraft Y to result in such a significant overtake with aircraft X who was 'resumed' from 280. As always; positive control is necessary to prevent aberrations from occurring. Recommendation; ensure positive control; rather than basing separation on expected aircraft performance.

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Original NASA ASRS Text

Title: ZHU Controller described over take event when misjudging the climb rate and in trail distances of two successive departure aircraft; the event triggering a conflict alert in another controller's airspace.

Narrative: Aircraft X and Aircraft Y were both departures off of IAH on the LOA5 SID. Aircraft X was approximately 15 NM in front of Aircraft Y and at a lower altitude. Both aircraft had speeds assigned by the low altitude controller. Aircraft Y was requesting a higher final altitude (FL380) than Aircraft X (FL360). I assigned a slightly faster airspeed to Aircraft Y (300 KTS from previously assigned 280 KTS) in order to facilitate spacing with subsequent aircraft. I allowed Aircraft X to resume normal speed. Given the aircraft types and expected performance; I believed that Aircraft Y would continue to out climb the E140 (Aircraft X) and longitudinal separation would never be factor. After handing off both aircraft to ZFW and transferring communications; it appears that Aircraft X significantly reduced its airspeed and significantly increased its climb rate. Conflict alert activated just inside ZFW airspace with Aircraft X climbing out of 316 and Aircraft Y only out of 330. I erred in my assumption that a B737 (Aircraft Y) would continue to out climb the E140. By not ensuring that the vertical separation remained established; it created a situation where the ZFW Controller more than likely had to take some type of action to prevent the E140 from climbing into the B737. If I was to speculate; the E140 probably reduced speed from the previously assigned 280 to below 240 KTS in the climb - thus accounting for the speed reduction (Aircraft Y had a 70 KT overtake; thereabouts) and the increased vertical rate. Typically such regional jets; especially the E140 as opposed to the E145; climb at speeds comparable to the 'regular' jets. I was not expecting the 300 KTS assigned to Aircraft Y to result in such a significant overtake with Aircraft X who was 'resumed' from 280. As always; positive control is necessary to prevent aberrations from occurring. Recommendation; ensure positive control; rather than basing separation on expected aircraft performance.

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.