Narrative:

While holding short of runway 13C in mdw, we (flight 5XY) heard tower clear flight 3XY into position and hold on runway 13C. We assumed the call sign to be in error and the first officer read back the clearance with emphasis on the 5XY to correct the call sign. Tower did not respond and we had no indication we were blocked, so we proceeded on to runway 13C with a slight back taxi to get full length. During this someone was cleared for takeoff. The first officer and I were confused by the clearance but noticed landing lights at the other end of the runway when we got aligned, so we began to exit the runway when tower called for the aircraft on the taxiway (us) to exit. Contributing factors include: 1) similar call signs that we were not aware of. 2) similar sounding runway, ie, runway 13C and runway 31C. 3) opposite direction departure without notice to us. 4) assuming that we would be next for release and not getting positive confirmation of the clearance. Supplemental information from acn 456449: assuming that the transmission was meant for us, we answered inquisitively 'roger, that's air carrier flight 5XY, position and hold runway 13C,' and cautiously proceeded onto runway 13C. Aircraft flight 3XY continued takeoff roll and departed normally. Contributing factors: the use of runway 31C for departures when runway 13C was the active, and no correction by tower when we repeated what we thought was instruction to position and hold on runway 13L. I would assume that it was stepped on as other flight responded simultaneously. A higher degree of situational awareness by us and a no assumptions attitude would have prevented this situation.

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Original NASA ASRS Text

Title: THE FLC OF A B737 FINDS THEMSELVES NOSE TO NOSE ON TO A COMPANY FLT ON THE TKOF ROLL, OPPOSITE DIRECTION, ON RWY 31C AT MDW, IL.

Narrative: WHILE HOLDING SHORT OF RWY 13C IN MDW, WE (FLT 5XY) HEARD TWR CLR FLT 3XY INTO POS AND HOLD ON RWY 13C. WE ASSUMED THE CALL SIGN TO BE IN ERROR AND THE FO READ BACK THE CLRNC WITH EMPHASIS ON THE 5XY TO CORRECT THE CALL SIGN. TWR DID NOT RESPOND AND WE HAD NO INDICATION WE WERE BLOCKED, SO WE PROCEEDED ON TO RWY 13C WITH A SLIGHT BACK TAXI TO GET FULL LENGTH. DURING THIS SOMEONE WAS CLRED FOR TKOF. THE FO AND I WERE CONFUSED BY THE CLRNC BUT NOTICED LNDG LIGHTS AT THE OTHER END OF THE RWY WHEN WE GOT ALIGNED, SO WE BEGAN TO EXIT THE RWY WHEN TWR CALLED FOR THE ACFT ON THE TXWY (US) TO EXIT. CONTRIBUTING FACTORS INCLUDE: 1) SIMILAR CALL SIGNS THAT WE WERE NOT AWARE OF. 2) SIMILAR SOUNDING RWY, IE, RWY 13C AND RWY 31C. 3) OPPOSITE DIRECTION DEP WITHOUT NOTICE TO US. 4) ASSUMING THAT WE WOULD BE NEXT FOR RELEASE AND NOT GETTING POSITIVE CONFIRMATION OF THE CLRNC. SUPPLEMENTAL INFO FROM ACN 456449: ASSUMING THAT THE XMISSION WAS MEANT FOR US, WE ANSWERED INQUISITIVELY 'ROGER, THAT'S ACR FLT 5XY, POS AND HOLD RWY 13C,' AND CAUTIOUSLY PROCEEDED ONTO RWY 13C. ACFT FLT 3XY CONTINUED TKOF ROLL AND DEPARTED NORMALLY. CONTRIBUTING FACTORS: THE USE OF RWY 31C FOR DEPS WHEN RWY 13C WAS THE ACTIVE, AND NO CORRECTION BY TWR WHEN WE REPEATED WHAT WE THOUGHT WAS INSTRUCTION TO POS AND HOLD ON RWY 13L. I WOULD ASSUME THAT IT WAS STEPPED ON AS OTHER FLT RESPONDED SIMULTANEOUSLY. A HIGHER DEG OF SITUATIONAL AWARENESS BY US AND A NO ASSUMPTIONS ATTITUDE WOULD HAVE PREVENTED THIS SIT.

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