Narrative:

On taxi for takeoff at ord using runway 23L and departing from the T1 intersection, the captain elected to continue to the e-w taxiway and depart from that intersection to have an extra margin of safety on our takeoff on runway 32. We were cleared to taxi into position, and during this same time frame, a company large transport was cleared into position at T1 ahead of us. The company aircraft was cleared for takeoff. As he started his takeoff roll, we were given 'cleared for immediate takeoff.' at first impression, this appeared to be no problem--but as the takeoff roll progressed, the aircraft in front seemed to accelerate very slowly and our sep seemed to diminish some what--although this was not a close call and probably was not as close as it looked from our perspective. I am aware that tower can give takeoff clearance based on projected sep; however, in this case, I suspect the large transport in front was on a stretched aircraft which has considerably less power than the advanced large transport we were flying. I can see where the controller would think with 2 identical airplanes, and they do look identical, that he could clear one for takeoff approximately 1000' down runway and accelerating and then clear the second plane, and this should present no problem.

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Original NASA ASRS Text

Title: 2 ACR'S WERE SIMULTANEOUSLY TAKING OFF ON SAME RWY.

Narrative: ON TAXI FOR TKOF AT ORD USING RWY 23L AND DEPARTING FROM THE T1 INTXN, THE CAPT ELECTED TO CONTINUE TO THE E-W TXWY AND DEPART FROM THAT INTXN TO HAVE AN EXTRA MARGIN OF SAFETY ON OUR TKOF ON RWY 32. WE WERE CLRED TO TAXI INTO POS, AND DURING THIS SAME TIME FRAME, A COMPANY LGT WAS CLRED INTO POS AT T1 AHEAD OF US. THE COMPANY ACFT WAS CLRED FOR TKOF. AS HE STARTED HIS TKOF ROLL, WE WERE GIVEN 'CLRED FOR IMMEDIATE TKOF.' AT FIRST IMPRESSION, THIS APPEARED TO BE NO PROB--BUT AS THE TKOF ROLL PROGRESSED, THE ACFT IN FRONT SEEMED TO ACCELERATE VERY SLOWLY AND OUR SEP SEEMED TO DIMINISH SOME WHAT--ALTHOUGH THIS WAS NOT A CLOSE CALL AND PROBABLY WAS NOT AS CLOSE AS IT LOOKED FROM OUR PERSPECTIVE. I AM AWARE THAT TWR CAN GIVE TKOF CLRNC BASED ON PROJECTED SEP; HOWEVER, IN THIS CASE, I SUSPECT THE LGT IN FRONT WAS ON A STRETCHED ACFT WHICH HAS CONSIDERABLY LESS PWR THAN THE ADVANCED LGT WE WERE FLYING. I CAN SEE WHERE THE CTLR WOULD THINK WITH 2 IDENTICAL AIRPLANES, AND THEY DO LOOK IDENTICAL, THAT HE COULD CLR ONE FOR TKOF APPROX 1000' DOWN RWY AND ACCELERATING AND THEN CLR THE SECOND PLANE, AND THIS SHOULD PRESENT NO PROB.

Data retrieved from NASA's ASRS site as of August 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.