A B767-ER HITS THE TOW BAR OF THE TUG THAT HAD JUST PUSHED THEM BACK FROM THE GATE DURING AN ENG START PROC AT ORD; IL.

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I WAS THE RELIEF PLT ON THIS FLT AND MONITORING BOTH INTERPHONE AND RAMP CTL. PUSHBACK WAS COMPLICATED BY NUMEROUS RADIO CALLS REGARDING 2 OTHER ACFT PUSHING BACK WITH US. AFTER WE HAD BEEN PUSHED BACK AND STARTED THE L ENG; THE RAMP CTLR ASKED IF WE COULD PUSH BACK FURTHER. THE CAPT ASKED THE TUG DRIVER IF WE WERE STILL CONNECTED; AND SHE SAID WE WERE ALREADY DISCONNECTED. WE THEN STARTED THE R ENG. I WAS MONITORING THE ENG START AND WATCHING THE FO ACCOMPLISH THE INITIAL ITEMS ON THE TAXI CHKLIST WHEN THE TUG DRIVER SAID 'BRAKES; BRAKES' AS THE ACFT STARTED A SLIGHT ROLL. I THEN NOTICED THE PARKING BRAKE WAS NOT SET AS THE CAPT DEPRESSED THE BRAKES AND STOPPED THE ACFT. I DID NOT HEAR EITHER THE TUG DRIVER ASK FOR 'PARKING BRAKE SET' OR THE CAPT REPLY. THERE WERE MULTIPLE INPUTS FROM THE RAMP CTLR DURING THIS PERIOD SAYING WE WERE OK TO STAY WHERE WE WERE AND THE CAPT AND FO DISCUSSING WHETHER WE NEEDED TO BE PUSHED MORE. IT IS POSSIBLE THAT THE TUG DRIVER ASKED FOR PARKING BRAKES SET AND THE CAPT REPLIED; BUT I DIDN'T HEAR EITHER BECAUSE OF A RADIO CALL FROM RAMP OR DISCUSSION BTWN THE CAPT AND FO. WE EVIDENTLY CONTACTED THE TOW BAR. AFTER SOME DIFFICULTY; THE TOW BAR WAS DISCONNECTED FROM THE AIRPLANE; AND WE LEARNED THE TAXI LIGHT WAS DAMAGED. WE TAXIED BACK TO THE GATE. THE TAXI LIGHT WAS REMOVED BY MAINT; AND WE WERE DISPATCHED WITHOUT IT. SUPPLEMENTAL INFO FROM ACN 537813: I DO NOT RECALL HEARING A PARKING BRAKES PARKED CALL. HOWEVER; IT WAS DIFFICULT TO HEAR THE TUG DRIVER. I WAS CONCENTRATING ON COM WITH THE RAMP AND THE ENG START. I THINK A POOR CONNECTION BTWN THE TUG AND OUR ACFT AND SEVERAL THINGS OCCURRING AT ONCE CONTRIBUTED TO THE INCIDENT. SUPPLEMENTAL INFO FROM ACN 537636: RAMP REQUESTED WE PUSH BACK FARTHER. THEN RELAYED THE REQUEST TO THE TUG DRIVER WHO RESPONDED THAT THE TOW BAR WAS ALREADY DISCONNECTED. I SAID 'STANDBY' SO AS TO ASSESS THE SIT WITH OTHER GATE PUSH. I CANNOT HONESTLY SAY IF THE TUG DRIVER HAD REQUESTED BRAKES PARKED OR NOT. I ALSO CANNOT REMEMBER RESPONDING TO SUCH A REQUEST. IT IS POSSIBLE THE XMISSIONS WERE GARBLED; SINCE THERE WAS LOTS OF ENG NOISE ON THE HEADSET. ALSO; IT IS POSSIBLE MY RESPONSE WAS GIVEN AUTOMATICALLY AS MY ATTN WAS DRAWN ELSEWHERE. THE OTHER MD80 PUSH WAS AN ADDITIONAL CONCERN FOR MYSELF AND; I AM SURE; THE TUG DRIVER; ADDING ADDITIONAL NOISE TO THE SIT DUE TO ITS CLOSE POS. I SHOULD NOT HAVE ALLOWED MYSELF TO BE DISTR UNTIL THIS MOST IMPORTANT PROC WAS COMPLETE.