Narrative:

Issued takeoff sequence by ord tower behind B-757. As 757 took position on the runway, a dc-9 blocked our path to the runway. Tower asked them to pull ahead to allow us access to the runway and then turn around to follow us. As they were moving out of the way, the 757 was issued a takeoff clearance and as he began rolling, we were cleared into position and hold. I was distracted by watching the dc-9 swing around close behind us as we rolled onto the runway, while listening to the final takeoff checklist items being completed, and mentally preparing for a heavy weight takeoff versus departure clearance limitations. We were given a takeoff clearance as the 757 lifted off and while we were still 90 degrees to the centerline, just starting to line up. I wanted to make use of our forward momentum and began to add power as the aircraft became close to runway alignment. At this point I was concentrating on setting maximum takeoff power as soon as possible while being smooth with the nosewheel steering so that by the time I looked up and realized that we should have 4 mi separation behind a 757 we were already through 80 KTS. The 757 made a very short takeoff roll and then made an immediate left turn. Our clearance was to maintain runway heading. Rather than risk a high speed abort , I chose to continue the takeoff because I knew we would be below vr as we passed through his liftoff point and because of his left turn, I felt the chances of encountering his wake were remote. The takeoff was normal and no wake turbulence encountered. As we climbed out I pointed out our mistake to the first officer and so and they agreed we should probably have questioned the separation, but it hadn't occurred to them until I mentioned it. Actually, I'm not sure if the 4 mi separation behind a 757 is an actual rule or just an ATC guideline, and if a short takeoff roll and a steep, immediate left turn affects the 4 mi rule. If the aircraft was a heavy 747 no question I would not have accepted the clearance and probably would not have gotten that takeoff clearance, but obviously the B-757 issue is not clear to me or the controller working tower.

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Original NASA ASRS Text

Title: AN ACR LGT CAPT FRETS ABOUT HIS SEPARATION ON TKOF BEHIND A B-757.

Narrative: ISSUED TKOF SEQUENCE BY ORD TWR BEHIND B-757. AS 757 TOOK POS ON THE RWY, A DC-9 BLOCKED OUR PATH TO THE RWY. TWR ASKED THEM TO PULL AHEAD TO ALLOW US ACCESS TO THE RWY AND THEN TURN AROUND TO FOLLOW US. AS THEY WERE MOVING OUT OF THE WAY, THE 757 WAS ISSUED A TKOF CLRNC AND AS HE BEGAN ROLLING, WE WERE CLRED INTO POS AND HOLD. I WAS DISTRACTED BY WATCHING THE DC-9 SWING AROUND CLOSE BEHIND US AS WE ROLLED ONTO THE RWY, WHILE LISTENING TO THE FINAL TKOF CHKLIST ITEMS BEING COMPLETED, AND MENTALLY PREPARING FOR A HVY WT TKOF VERSUS DEP CLRNC LIMITATIONS. WE WERE GIVEN A TKOF CLRNC AS THE 757 LIFTED OFF AND WHILE WE WERE STILL 90 DEGS TO THE CTRLINE, JUST STARTING TO LINE UP. I WANTED TO MAKE USE OF OUR FORWARD MOMENTUM AND BEGAN TO ADD PWR AS THE ACFT BECAME CLOSE TO RWY ALIGNMENT. AT THIS POINT I WAS CONCENTRATING ON SETTING MAX TKOF PWR AS SOON AS POSSIBLE WHILE BEING SMOOTH WITH THE NOSEWHEEL STEERING SO THAT BY THE TIME I LOOKED UP AND REALIZED THAT WE SHOULD HAVE 4 MI SEPARATION BEHIND A 757 WE WERE ALREADY THROUGH 80 KTS. THE 757 MADE A VERY SHORT TKOF ROLL AND THEN MADE AN IMMEDIATE L TURN. OUR CLRNC WAS TO MAINTAIN RWY HDG. RATHER THAN RISK A HIGH SPD ABORT , I CHOSE TO CONTINUE THE TKOF BECAUSE I KNEW WE WOULD BE BELOW VR AS WE PASSED THROUGH HIS LIFTOFF POINT AND BECAUSE OF HIS L TURN, I FELT THE CHANCES OF ENCOUNTERING HIS WAKE WERE REMOTE. THE TKOF WAS NORMAL AND NO WAKE TURB ENCOUNTERED. AS WE CLBED OUT I POINTED OUT OUR MISTAKE TO THE FO AND SO AND THEY AGREED WE SHOULD PROBABLY HAVE QUESTIONED THE SEPARATION, BUT IT HADN'T OCCURRED TO THEM UNTIL I MENTIONED IT. ACTUALLY, I'M NOT SURE IF THE 4 MI SEPARATION BEHIND A 757 IS AN ACTUAL RULE OR JUST AN ATC GUIDELINE, AND IF A SHORT TKOF ROLL AND A STEEP, IMMEDIATE L TURN AFFECTS THE 4 MI RULE. IF THE ACFT WAS A HVY 747 NO QUESTION I WOULD NOT HAVE ACCEPTED THE CLRNC AND PROBABLY WOULD NOT HAVE GOTTEN THAT TKOF CLRNC, BUT OBVIOUSLY THE B-757 ISSUE IS NOT CLR TO ME OR THE CTLR WORKING TWR.

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.