Narrative:

I cleared the BA46 for takeoff sbound, on a southern departure SID. The BA46 began what appeared to be a normal climb on the SID, which calls for a slight right turn at 3 DME to join the 211 degree radial (off runway 19L). When the BA46 was approximately a mi south of the airport, I cleared air carrier Y, a B747, for takeoff on the same runway on a diverging SID. Course divergence, however, would not occur until air carrier Y reached 3 DME, then turned right and crossed the 211 degree radial nwbound. At approximately 4000 ft altitude, the BA46 slowed its rate of climb considerably, due probably to the temperature outside (over 100 degree F). Air carrier Y, however, climbed extremely well, and at 3 DME, when air carrier Y began his right turn, their altitudes were right at 1000 ft. As air carrier Y turned, their lateral separation diminished to 2.5 mi, and altitudes reached 800 ft apart. Once air carrier Y crossed the 211 degree radial, diverging courses existed. Density altitude played a significant role in this operational error. No doubt the BA46 pilot knew he was not going to be able to climb well. There is a crossing restr on the SID at 15 DME which requires approximately 550 ft per NM. Once established on the 211 degree radial, the pilot asked for and received a VFR climb reference the terrain, which would remove the crossing restr. The pilot never let ATC know, however, that he was going to be climbing at such an unusually slow rate. It was anticipated that the climb would be normal, and even though the B757 climbs much better, if the BA46 had climbed just 550 ft per NM (the minimum to make the crossing restr) vertical separation would have been maintained between the 2 aircraft until air carrier Y was beyond the 211 degree radial and diverging courses existed. Normally, the B757 climbs at a rate twice the BA46, but on this occasion, the rate of climb was triple, due to the B46 reducing, either by choice or density altitude effect. Controllers can only provide additional separation if we know there is going to be a problem in the climb. Had the BA46 pilot indicated prior to takeoff that he was heavy, or that he was unsure if he'd be able to make the crossing restr, a few more mi would have been added to ensure lateral separation, instead of depending on the vertical.

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Original NASA ASRS Text

Title: CTLR CLRED A BA46 OFF ON A SID CLBING TO 4000 FT, THEN SHORTLY THEREAFTER CLRED A B757 OFF ON A SID THAT DIVERGED FROM THE BA46 AFTER THE LAS 211 DEG RADIAL AT 3 NM. BA46 WAS SLOW IN CLBING AND THE B757 OVERTOOK THE BA46 WITH LTSS.

Narrative: I CLRED THE BA46 FOR TKOF SBOUND, ON A SOUTHERN DEP SID. THE BA46 BEGAN WHAT APPEARED TO BE A NORMAL CLB ON THE SID, WHICH CALLS FOR A SLIGHT R TURN AT 3 DME TO JOIN THE 211 DEG RADIAL (OFF RWY 19L). WHEN THE BA46 WAS APPROX A MI S OF THE ARPT, I CLRED ACR Y, A B747, FOR TKOF ON THE SAME RWY ON A DIVERGING SID. COURSE DIVERGENCE, HOWEVER, WOULD NOT OCCUR UNTIL ACR Y REACHED 3 DME, THEN TURNED R AND CROSSED THE 211 DEG RADIAL NWBOUND. AT APPROX 4000 FT ALT, THE BA46 SLOWED ITS RATE OF CLB CONSIDERABLY, DUE PROBABLY TO THE TEMP OUTSIDE (OVER 100 DEG F). ACR Y, HOWEVER, CLBED EXTREMELY WELL, AND AT 3 DME, WHEN ACR Y BEGAN HIS R TURN, THEIR ALTS WERE RIGHT AT 1000 FT. AS ACR Y TURNED, THEIR LATERAL SEPARATION DIMINISHED TO 2.5 MI, AND ALTS REACHED 800 FT APART. ONCE ACR Y CROSSED THE 211 DEG RADIAL, DIVERGING COURSES EXISTED. DENSITY ALT PLAYED A SIGNIFICANT ROLE IN THIS OPERATIONAL ERROR. NO DOUBT THE BA46 PLT KNEW HE WAS NOT GOING TO BE ABLE TO CLB WELL. THERE IS A XING RESTR ON THE SID AT 15 DME WHICH REQUIRES APPROX 550 FT PER NM. ONCE ESTABLISHED ON THE 211 DEG RADIAL, THE PLT ASKED FOR AND RECEIVED A VFR CLB REF THE TERRAIN, WHICH WOULD REMOVE THE XING RESTR. THE PLT NEVER LET ATC KNOW, HOWEVER, THAT HE WAS GOING TO BE CLBING AT SUCH AN UNUSUALLY SLOW RATE. IT WAS ANTICIPATED THAT THE CLB WOULD BE NORMAL, AND EVEN THOUGH THE B757 CLBS MUCH BETTER, IF THE BA46 HAD CLBED JUST 550 FT PER NM (THE MINIMUM TO MAKE THE XING RESTR) VERT SEPARATION WOULD HAVE BEEN MAINTAINED BTWN THE 2 ACFT UNTIL ACR Y WAS BEYOND THE 211 DEG RADIAL AND DIVERGING COURSES EXISTED. NORMALLY, THE B757 CLBS AT A RATE TWICE THE BA46, BUT ON THIS OCCASION, THE RATE OF CLB WAS TRIPLE, DUE TO THE B46 REDUCING, EITHER BY CHOICE OR DENSITY ALT EFFECT. CTLRS CAN ONLY PROVIDE ADDITIONAL SEPARATION IF WE KNOW THERE IS GOING TO BE A PROB IN THE CLB. HAD THE BA46 PLT INDICATED PRIOR TO TKOF THAT HE WAS HVY, OR THAT HE WAS UNSURE IF HE'D BE ABLE TO MAKE THE XING RESTR, A FEW MORE MI WOULD HAVE BEEN ADDED TO ENSURE LATERAL SEPARATION, INSTEAD OF DEPENDING ON THE VERT.

Data retrieved from NASA's ASRS site as of July 2007 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.