Narrative:

During taxi out to runway 32R at O'hare, a F/a came up to ask if I could check the left (aft) service door for security. When I came back to the cockpit, we were very close to being cleared for takeoff and had not started our 3RD engine or completed taxi checks. Although rushed, we smartly started the engine and completed the after start check. As we were starting the taxi check, tower either cleared us for takeoff or on to hold. We finished the taxi check and began with the before takeoff checks, when the captain asked the first officer if we were cleared for takeoff. First officer replied yes, so we finished the check and began our roll (first officer flying). About 200' into the roll (very low airspeed, 20 KTS), tower said 'you're not cleared for takeoff!' at that time a widebody transport Y flew just in front of us (just beginning his flare for landing on cross runway 27R). We stopped immediately and tower then quickly cleared us for takeoff. The captain recalled hearing only 'cleared on to hold', the first officer distinctly remembered tower clearing us for takeoff, and I am not sure since I was reading checklists. I do know thee was a lot of urgency. There was another similar call sign holding with us which possibly added to the confusion. The captain said that tower made a comment that we were out there together after they had answered for us or vice versa. Although all checklist items were complete, the captain was uncertain about our clearance, but the first officer was 'sure' the tower had cleared us. Maybe a quick check with tower (even during this urgency) would have been best especially when there was a difference of opinions.

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Original NASA ASRS Text

Title: AIRLINER BEGINNING TKOF IS TOLD TO ABORT BY TWR CTLR. CONFUSION WHETHER CTLR ISSUED CLRNC OR IF FLT CREW WAS IN ERROR.

Narrative: DURING TAXI OUT TO RWY 32R AT O'HARE, A F/A CAME UP TO ASK IF I COULD CHECK THE LEFT (AFT) SERVICE DOOR FOR SECURITY. WHEN I CAME BACK TO THE COCKPIT, WE WERE VERY CLOSE TO BEING CLRED FOR TKOF AND HAD NOT STARTED OUR 3RD ENGINE OR COMPLETED TAXI CHECKS. ALTHOUGH RUSHED, WE SMARTLY STARTED THE ENGINE AND COMPLETED THE AFTER START CHECK. AS WE WERE STARTING THE TAXI CHECK, TWR EITHER CLRED US FOR TKOF OR ON TO HOLD. WE FINISHED THE TAXI CHECK AND BEGAN WITH THE BEFORE TKOF CHECKS, WHEN THE CAPT ASKED THE F/O IF WE WERE CLRED FOR TKOF. F/O REPLIED YES, SO WE FINISHED THE CHECK AND BEGAN OUR ROLL (F/O FLYING). ABOUT 200' INTO THE ROLL (VERY LOW AIRSPEED, 20 KTS), TWR SAID 'YOU'RE NOT CLRED FOR TKOF!' AT THAT TIME A WDB Y FLEW JUST IN FRONT OF US (JUST BEGINNING HIS FLARE FOR LNDG ON CROSS RWY 27R). WE STOPPED IMMEDIATELY AND TWR THEN QUICKLY CLRED US FOR TKOF. THE CAPT RECALLED HEARING ONLY 'CLRED ON TO HOLD', THE F/O DISTINCTLY REMEMBERED TWR CLEARING US FOR TKOF, AND I AM NOT SURE SINCE I WAS READING CHECKLISTS. I DO KNOW THEE WAS A LOT OF URGENCY. THERE WAS ANOTHER SIMILAR CALL SIGN HOLDING WITH US WHICH POSSIBLY ADDED TO THE CONFUSION. THE CAPT SAID THAT TWR MADE A COMMENT THAT WE WERE OUT THERE TOGETHER AFTER THEY HAD ANSWERED FOR US OR VICE VERSA. ALTHOUGH ALL CHECKLIST ITEMS WERE COMPLETE, THE CAPT WAS UNCERTAIN ABOUT OUR CLRNC, BUT THE F/O WAS 'SURE' THE TWR HAD CLRED US. MAYBE A QUICK CHECK WITH TWR (EVEN DURING THIS URGENCY) WOULD HAVE BEEN BEST ESPECIALLY WHEN THERE WAS A DIFFERENCE OF OPINIONS.

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.