Narrative:

While working the local north position; I cleared a B737 for takeoff too soon behind a B757 off a parallel runway separated by 3565 ft. I was advised by another position of the B757 departing and my sequence was to follow. Since I had an arrival I was not able to taxi my aircraft into position and hold. I observed the B757 in position on runway 7L while waiting for the arrival on runway 8 to land. After the arrival landed; I put my B737 into position. I also had an airbus on a 4 mi final for runway 8. Once the landing aircraft exited the runway; I cleared aircraft X for an immediate takeoff to ensure appropriate arrival/departure separation. As aircraft X began takeoff roll; I scanned upwind and observed the B757 upwind off runway 7L. The initial departure routing of the 2 aircraft initially were to converge so I gave aircraft X a runway heading vector to avoid a path that would not put it directly behind the B757. I also restr the climb out speed to 210 KTS to try and meet the 4 mi separation standard. I issued the B757 traffic to aircraft X. Aircraft X reported the B757 in sight. I asked aircraft X if they could maintain visual and they replied affirmative. At approximately 4 mi east of the airport; aircraft X was on a converging course less than 4 mi behind a B757 with visual separation. At this point; the aircraft was switched to departure control.

Related reports:

Google
 

Original NASA ASRS Text

Title: PHX CTLR ALLOWED A B737 TO DEP FROM PARALLEL RWY AND LOST SEPARATION WITH PRECEDING B757.

Narrative: WHILE WORKING THE LCL N POS; I CLRED A B737 FOR TKOF TOO SOON BEHIND A B757 OFF A PARALLEL RWY SEPARATED BY 3565 FT. I WAS ADVISED BY ANOTHER POS OF THE B757 DEPARTING AND MY SEQUENCE WAS TO FOLLOW. SINCE I HAD AN ARR I WAS NOT ABLE TO TAXI MY ACFT INTO POS AND HOLD. I OBSERVED THE B757 IN POS ON RWY 7L WHILE WAITING FOR THE ARR ON RWY 8 TO LAND. AFTER THE ARR LANDED; I PUT MY B737 INTO POS. I ALSO HAD AN AIRBUS ON A 4 MI FINAL FOR RWY 8. ONCE THE LNDG ACFT EXITED THE RWY; I CLRED ACFT X FOR AN IMMEDIATE TKOF TO ENSURE APPROPRIATE ARR/DEP SEPARATION. AS ACFT X BEGAN TKOF ROLL; I SCANNED UPWIND AND OBSERVED THE B757 UPWIND OFF RWY 7L. THE INITIAL DEP ROUTING OF THE 2 ACFT INITIALLY WERE TO CONVERGE SO I GAVE ACFT X A RWY HDG VECTOR TO AVOID A PATH THAT WOULD NOT PUT IT DIRECTLY BEHIND THE B757. I ALSO RESTR THE CLBOUT SPD TO 210 KTS TO TRY AND MEET THE 4 MI SEPARATION STANDARD. I ISSUED THE B757 TFC TO ACFT X. ACFT X RPTED THE B757 IN SIGHT. I ASKED ACFT X IF THEY COULD MAINTAIN VISUAL AND THEY REPLIED AFFIRMATIVE. AT APPROX 4 MI E OF THE ARPT; ACFT X WAS ON A CONVERGING COURSE LESS THAN 4 MI BEHIND A B757 WITH VISUAL SEPARATION. AT THIS POINT; THE ACFT WAS SWITCHED TO DEP CTL.

Data retrieved from NASA's ASRS site as of January 2009 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.