Narrative:

Runway 25L and runway 25R are less than 2500 ft apart. Runway 25L had a displaced threshold; making all of those departures intersection departures; because both runways are considered as one for wake turbulence separation. I departed a B757 from runway 25R. I put a second B757 in position for departure on runway 25L. The ATC handbook states that the second aircraft does not start takeoff roll until at least 3 mins after the other B757 has taken off. The traffic picture was moderate and complex. I departed the second B757; 1 1/2 mins after the first aircraft. At the time I was thinking like aircraft; all I need is 4 mi radar separation. When the second aircraft was airborne; radar separation between both aircraft was 4.75 mi and 1700 ft vertical. Human performance consideration: I was aware of the 3 min rule. However; at the time what stuck in my mind was like aircraft and performance; different departure sids and I was going to have a minimum of 4 mi between 2 B757's.

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Original NASA ASRS Text

Title: PHX TWR CTLR EXPERIENCED OPERROR WHEN SUFFICIENT TIME INTERVAL WAS NOT USED WITH SUCCESSIVE B757 DEP ACFT.

Narrative: RWY 25L AND RWY 25R ARE LESS THAN 2500 FT APART. RWY 25L HAD A DISPLACED THRESHOLD; MAKING ALL OF THOSE DEPS INTXN DEPS; BECAUSE BOTH RWYS ARE CONSIDERED AS ONE FOR WAKE TURB SEPARATION. I DEPARTED A B757 FROM RWY 25R. I PUT A SECOND B757 IN POS FOR DEP ON RWY 25L. THE ATC HANDBOOK STATES THAT THE SECOND ACFT DOES NOT START TKOF ROLL UNTIL AT LEAST 3 MINS AFTER THE OTHER B757 HAS TAKEN OFF. THE TFC PICTURE WAS MODERATE AND COMPLEX. I DEPARTED THE SECOND B757; 1 1/2 MINS AFTER THE FIRST ACFT. AT THE TIME I WAS THINKING LIKE ACFT; ALL I NEED IS 4 MI RADAR SEPARATION. WHEN THE SECOND ACFT WAS AIRBORNE; RADAR SEPARATION BTWN BOTH ACFT WAS 4.75 MI AND 1700 FT VERT. HUMAN PERFORMANCE CONSIDERATION: I WAS AWARE OF THE 3 MIN RULE. HOWEVER; AT THE TIME WHAT STUCK IN MY MIND WAS LIKE ACFT AND PERFORMANCE; DIFFERENT DEP SIDS AND I WAS GOING TO HAVE A MINIMUM OF 4 MI BTWN 2 B757'S.

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.