Narrative:

The incident was primarily caused by flight crew complacency; in my opinion; and not enough attention paid on the part of the flight crew to all ATC xmissions. Situation would have been averted if we had heard and understood that the md-80 was cleared for takeoff from runway 9L. It also would have helped significantly if the ground controller had told us to 'pass behind the MD80' instead of 'follow the MD80;' or if he had mentioned to us that the MD80 was on his way to runway 9L. Taxi clearance was 'bravo-juliet-whiskey to runway 4L; follow the MD80 from left to right.' we waited for the MD80 to pass and then followed it up bravo and juliet. It was a relatively short taxi route; and there was no line-up for departure; which was unusual. We had taxied out single-engine; so we were scrambling a little to get the other engine started and run the checklists. We knew we were taxiing to runway 4L; but we did not realize that the MD80 was taxiing to runway 9L. I was preoccupied and too relaxed; and simply followed the MD80 to runway 9L. We did not have time to think things over; because the MD80 was immediately cleared for takeoff on runway 9L; and seconds later we were cleared into position and hold runway 4L. We assumed we were being cleared into position behind the MD80. I cleared final as we taxied out; and I was certain there was no one on final. As we began to taxi out; the picture began to seem wrong. I braked to a stop while still on the portion of the taxiway perpendicular to the runway; but over the hold short line. At this point tower called and asked us if we were taxiing onto runway 9L. I replied that we were. Tower said 'ok; just continue into position and hold runway 9L.' we did so; and held for about 20 seconds on the runway before being cleared for takeoff. It was clear that there was no traffic conflict; and tower had no further comments to us about the situation. I braked to a stop upon realizing that the runway environment did not look right. Tower also made a call to us at about this same time. As stated above; I think ground control should be careful about advising aircraft to 'follow' other aircraft that are taxiing to different runways. However; I am aware of this practice; and I should have caught it.

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Original NASA ASRS Text

Title: CL65 FLT CREW RECEIVES CONFUSING TAXI INSTRUCTIONS FROM ORD GROUND AND TAXIES TO 9L INSTEAD OF 4L. FLT CREW CROSSES HOLD LINE AT 9L BEFORE REALIZING THEIR MISTAKE.

Narrative: THE INCIDENT WAS PRIMARILY CAUSED BY FLT CREW COMPLACENCY; IN MY OPINION; AND NOT ENOUGH ATTN PAID ON THE PART OF THE FLT CREW TO ALL ATC XMISSIONS. SITUATION WOULD HAVE BEEN AVERTED IF WE HAD HEARD AND UNDERSTOOD THAT THE MD-80 WAS CLRED FOR TKOF FROM RWY 9L. IT ALSO WOULD HAVE HELPED SIGNIFICANTLY IF THE GND CTLR HAD TOLD US TO 'PASS BEHIND THE MD80' INSTEAD OF 'FOLLOW THE MD80;' OR IF HE HAD MENTIONED TO US THAT THE MD80 WAS ON HIS WAY TO RWY 9L. TAXI CLRNC WAS 'BRAVO-JULIET-WHISKEY TO RWY 4L; FOLLOW THE MD80 FROM L TO R.' WE WAITED FOR THE MD80 TO PASS AND THEN FOLLOWED IT UP BRAVO AND JULIET. IT WAS A RELATIVELY SHORT TAXI RTE; AND THERE WAS NO LINE-UP FOR DEP; WHICH WAS UNUSUAL. WE HAD TAXIED OUT SINGLE-ENG; SO WE WERE SCRAMBLING A LITTLE TO GET THE OTHER ENG STARTED AND RUN THE CHKLISTS. WE KNEW WE WERE TAXIING TO RWY 4L; BUT WE DID NOT REALIZE THAT THE MD80 WAS TAXIING TO RWY 9L. I WAS PREOCCUPIED AND TOO RELAXED; AND SIMPLY FOLLOWED THE MD80 TO RWY 9L. WE DID NOT HAVE TIME TO THINK THINGS OVER; BECAUSE THE MD80 WAS IMMEDIATELY CLRED FOR TKOF ON RWY 9L; AND SECONDS LATER WE WERE CLRED INTO POS AND HOLD RWY 4L. WE ASSUMED WE WERE BEING CLRED INTO POS BEHIND THE MD80. I CLRED FINAL AS WE TAXIED OUT; AND I WAS CERTAIN THERE WAS NO ONE ON FINAL. AS WE BEGAN TO TAXI OUT; THE PICTURE BEGAN TO SEEM WRONG. I BRAKED TO A STOP WHILE STILL ON THE PORTION OF THE TXWY PERPENDICULAR TO THE RWY; BUT OVER THE HOLD SHORT LINE. AT THIS POINT TWR CALLED AND ASKED US IF WE WERE TAXIING ONTO RWY 9L. I REPLIED THAT WE WERE. TWR SAID 'OK; JUST CONTINUE INTO POS AND HOLD RWY 9L.' WE DID SO; AND HELD FOR ABOUT 20 SECONDS ON THE RWY BEFORE BEING CLRED FOR TKOF. IT WAS CLR THAT THERE WAS NO TFC CONFLICT; AND TWR HAD NO FURTHER COMMENTS TO US ABOUT THE SITUATION. I BRAKED TO A STOP UPON REALIZING THAT THE RWY ENVIRONMENT DID NOT LOOK RIGHT. TWR ALSO MADE A CALL TO US AT ABOUT THIS SAME TIME. AS STATED ABOVE; I THINK GND CTL SHOULD BE CAREFUL ABOUT ADVISING ACFT TO 'FOLLOW' OTHER ACFT THAT ARE TAXIING TO DIFFERENT RWYS. HOWEVER; I AM AWARE OF THIS PRACTICE; AND I SHOULD HAVE CAUGHT IT.

Data retrieved from NASA's ASRS site as of January 2009 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.