Narrative:

Our flight [12x1] was operating as ATC call sign [2x1a]. We followed our taxi instructions out to runway 22L via B and D. Meanwhile; company flight [12y1]; operating in the new paint scheme; was cleared to runway 22L via M. We; 2x1a; had been advised by tower of our sequence for takeoff behind the erj flight in front of us and were told to be ready to go. Tower had been launching 2 to 3 flights between arrivals; we would be the 3rd and last before the next arrival.the erj in front of us was cleared for takeoff; and I heard tower say; '2x1a line up and wait; be ready to go.' as I was turning on the lights for takeoff; I heard my first officer respond to tower and I started to taxi out onto the runway. As I taxied onto the runway I heard tower say; '2 aircraft answered that call; it was only for company 12y1.' I immediately advised tower; 'company 2x1a took that call and now we're in the way.' tower subsequently cleared us; [company 2x1a]; for takeoff and no further discrepancies occurred.I firmly believe that the original takeoff clearance was for us (2x1a). Also; I do not know who the other aircraft that answered was; because company 12y1 (who was sitting off our right and pointed toward us) never turned on any lights; started moving; or made any indication that they thought the call was for them. Further; we had clearly been told our sequence before hand.the cause of this incident was the similarity of flight numbers 12x1 and 12y1 with the non normal call sign thrown in; as well as the close physical proximity of those flights. The tower controller was doing an excellent job of launching aircraft that evening. He had a very long line on the ground; with inbound aircraft to the same runway. I don't know what other aircraft answered the takeoff clearance; but I'm sure this tripped the confusion. I still believe we were cleared for takeoff; but then he just read the next company flight number accidentally when questioning the '2 responses' he had received to the takeoff clearance.

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Original NASA ASRS Text

Title: Similar sounding call signs and high density takeoff/landing operations resulted in communications confusion and a possible runway incursion at ORD.

Narrative: Our flight [12X1] was operating as ATC call sign [2X1A]. We followed our taxi instructions out to Runway 22L via B and D. Meanwhile; Company flight [12Y1]; operating in the new paint scheme; was cleared to Runway 22L via M. We; 2X1A; had been advised by Tower of our sequence for takeoff behind the ERJ flight in front of us and were told to be ready to go. Tower had been launching 2 to 3 flights between arrivals; we would be the 3rd and last before the next arrival.The ERJ in front of us was cleared for takeoff; and I heard Tower say; '2X1A line up and wait; be ready to go.' As I was turning on the lights for takeoff; I heard my First Officer respond to Tower and I started to taxi out onto the runway. As I taxied onto the runway I heard Tower say; '2 aircraft answered that call; it was only for Company 12Y1.' I immediately advised Tower; 'Company 2X1A took that call and now we're in the way.' Tower subsequently cleared us; [Company 2X1A]; for takeoff and no further discrepancies occurred.I firmly believe that the original takeoff clearance was for us (2X1A). Also; I do not know who the other aircraft that answered was; because Company 12Y1 (who was sitting off our right and pointed toward us) never turned on any lights; started moving; or made any indication that they thought the call was for them. Further; we had clearly been told our sequence before hand.The cause of this incident was the similarity of flight numbers 12X1 and 12Y1 with the non normal call sign thrown in; as well as the close physical proximity of those flights. The Tower Controller was doing an excellent job of launching aircraft that evening. He had a very long line on the ground; with inbound aircraft to the same runway. I don't know what other aircraft answered the takeoff clearance; but I'm sure this tripped the confusion. I still believe we were cleared for takeoff; but then he just read the next Company flight number accidentally when questioning the '2 responses' he had received to the takeoff clearance.

Data retrieved from NASA's ASRS site as of July 2013 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.