Narrative:

I was working final center. Iah was on an east flow; and we were bringing aircraft into runways 8L; 8R; and 09. Final north was getting visual approaches below 4;100 feet for 8L. I was vectoring aircraft around heavy thunderstorms on the final; which had associated microburst alerts. I had two RNAV yankee rnp approaches on the south downwind for runway 9. I was trying to thread the needle so to speak with aircraft Y from the southwest inbound for runway 8R. When I realized that I was going to lose both lateral and vertical separation with aircraft X; I cancelled aircraft X's approach clearance; and gave a descent to 2;000 feet and a heading of 070. I thought that was the only initial divergence that I'd need; because I was expecting aircraft X to call the airport in sight almost immediately. Previous aircraft in the same position; had seen the airport; and I was expecting the 070 heading to be enough once it was combined with a visual approach clearance. When I realized that the pilot was not going to see the airport; I turned further to a 090 degree heading; and stopped my subsequent downwind aircraft at 5;000 feet. I then immediately turned aircraft X to a right turn heading 180; and assigned the aircraft 3;000 feet. I believe the separation was 2.56 lateral; and 400 feet vertical (estimated). I shouldn't have descended aircraft Y until I had ensured aircraft X was lower; or assured a visual approach. The convective activity on the final; made the situation extremely difficult; however I should not have given up altitude. In the moment; I thought it would work; and upon realizing that it wasn't I took action; however separation was shortly lost thereafter.

Related reports:

Google
 

Original NASA ASRS Text

Title: Houston TRACON Controller reported a loss of separation between two aircraft on a RNAV approach.

Narrative: I was working Final Center. IAH was on an East Flow; and we were bringing aircraft into Runways 8L; 8R; and 09. Final North was getting visual approaches below 4;100 feet for 8L. I was vectoring aircraft around heavy thunderstorms on the final; which had associated microburst alerts. I had two RNAV Yankee RNP Approaches on the south downwind for Runway 9. I was trying to thread the needle so to speak with Aircraft Y from the southwest inbound for Runway 8R. When I realized that I was going to lose both lateral and vertical separation with Aircraft X; I cancelled Aircraft X's approach clearance; and gave a descent to 2;000 feet and a heading of 070. I thought that was the only initial divergence that I'd need; because I was expecting Aircraft X to call the airport in sight almost immediately. Previous aircraft in the same position; had seen the airport; and I was expecting the 070 heading to be enough once it was combined with a visual approach clearance. When I realized that the pilot was not going to see the airport; I turned further to a 090 degree heading; and stopped my subsequent downwind aircraft at 5;000 feet. I then immediately turned Aircraft X to a right turn heading 180; and assigned the aircraft 3;000 feet. I believe the separation was 2.56 lateral; and 400 feet vertical (estimated). I shouldn't have descended Aircraft Y until I had ensured Aircraft X was lower; or assured a visual approach. The convective activity on the final; made the situation extremely difficult; however I should not have given up altitude. In the moment; I thought it would work; and upon realizing that it wasn't I took action; however separation was shortly lost thereafter.

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.