Narrative:

Taxiing out for takeoff on 27L, we received what was understood as a takeoff clearance about 2000-3000' on the parallel from takeoff end of runway. The first officer read back (cleared for takeoff). We started a rolling takeoff and at about 60-65 KTS the tower called saying (cancel takeoff clearance). We aborted with no problem and returned to 27L for takeoff. On several occasions later in the flight, ATC noted to us that the first officer xmissions were unreadable, but on checking the radio there seemed to be no problems. Upon returning to ord we were told that the tower had not cleared us for takeoff. Apparently there was radio transmission confusion, since the tower made no comment when the first officer answered with a (cleared for) call. After trying to sort out what had happened, I think part of the problem was the elapsed time from clearance to the end of runway. Since there was no traffic waiting or taxiing to the runway and having flown many takeoffs at ord in this situation, cleared for takeoff was a normal thing. Since the tower made no comment after the first officer answered with (cleared for takeoff) we started our roll. This once again shows the need for good communication. Supplemental information from acn 119570: I answered the call and said back to the tower 'cleared to go' where there was no response.

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Original NASA ASRS Text

Title: FLT CREW OF MLG UNDERSTOOD CLEARED FOR TKOF. TWR CTLR INTERVENED. CANCELLED TKOF. FLT CREW ABORTED TKOF.

Narrative: TAXIING OUT FOR TKOF ON 27L, WE RECEIVED WHAT WAS UNDERSTOOD AS A TKOF CLRNC ABOUT 2000-3000' ON THE PARALLEL FROM TKOF END OF RWY. THE F/O READ BACK (CLRED FOR TKOF). WE STARTED A ROLLING TKOF AND AT ABOUT 60-65 KTS THE TWR CALLED SAYING (CANCEL TKOF CLRNC). WE ABORTED WITH NO PROBLEM AND RETURNED TO 27L FOR TKOF. ON SEVERAL OCCASIONS LATER IN THE FLT, ATC NOTED TO US THAT THE F/O XMISSIONS WERE UNREADABLE, BUT ON CHECKING THE RADIO THERE SEEMED TO BE NO PROBLEMS. UPON RETURNING TO ORD WE WERE TOLD THAT THE TWR HAD NOT CLRED US FOR TKOF. APPARENTLY THERE WAS RADIO XMISSION CONFUSION, SINCE THE TWR MADE NO COMMENT WHEN THE F/O ANSWERED WITH A (CLRED FOR) CALL. AFTER TRYING TO SORT OUT WHAT HAD HAPPENED, I THINK PART OF THE PROBLEM WAS THE ELAPSED TIME FROM CLRNC TO THE END OF RWY. SINCE THERE WAS NO TFC WAITING OR TAXIING TO THE RWY AND HAVING FLOWN MANY TKOFS AT ORD IN THIS SITUATION, CLRED FOR TKOF WAS A NORMAL THING. SINCE THE TWR MADE NO COMMENT AFTER THE F/O ANSWERED WITH (CLRED FOR TKOF) WE STARTED OUR ROLL. THIS ONCE AGAIN SHOWS THE NEED FOR GOOD COM. SUPPLEMENTAL INFORMATION FROM ACN 119570: I ANSWERED THE CALL AND SAID BACK TO THE TWR 'CLRED TO GO' WHERE THERE WAS NO RESPONSE.

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.