Narrative:

At 5000 ft on downwind to ILS runway 27L at ord I heard and first officer heard and read back instructions to turn to 010 degrees, descend to 4000 ft. While rolling out on 010 degree heading at about 4500 ft we got a 'traffic' alert on TCASII followed shortly by an RA 'monitor vertical speed.' at the same time we got instructions to turn to 120 degrees, maintain 5000 ft. I disengaged the autoplt and made a climbing turn to 120 degrees and 5000 ft. At about the time of the RA and instructions to turn to 120 degrees we acquired visual contact with a heavy. My best guess would be that he was about 500 ft below us at a range of about 1 mi. During the turn to 120 degrees the controller said we had been assigned a heading of 100 degrees. The first officer and I were both sure we heard 010 degrees, and that is what was read back by the first officer. Maybe we just heard what we expected to hear at this point on the approach and the controller didn't catch our readback. After filling out the other side of this form I called ord TRACON and spoke with a supervisor. He said the tapes had been reviewed and that my first officer had read back a heading of 100 degrees. I'm sure the first officer will be amazed by that as after the incident he was positive he had read back 010 degrees. I find it very hard to understand how this happened. I always repeat all assigned headings and altitudes to be sure that the first officer and I agree on what we heard. Somehow this time even though the first officer read back 100 degrees to the controller we were both 'thinking' 010 degrees. In the past I have experienced times where the first officer or myself would mistake 010 degrees, 020 degrees, or 030 degrees for 100 degrees, 200 degrees, 300 degrees but one of us always corrected or questioned the other and the heading was reconfirmed with the controller. This time neither of us had any doubt we were correct when in fact we were wrong. I really have no suggestions on how to avoid this in the future. I will certainly redouble my efforts to be sure all clrncs are heard correctly. The TRACON supervisor said he would have to file a report on loss of separation as we were as close as 500 ft and 2 mi to the other aircraft. The TCASII worked as it should and would have kept us from getting any closer to the other aircraft if the controller had missed the situation.

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Original NASA ASRS Text

Title: AN ACR MD80 SUFFERS A HDG DEV AND A POTENTIAL CONFLICT WITH ANOTHER ACR HVY ON APCH TO RWY 27L AT ORD.

Narrative: AT 5000 FT ON DOWNWIND TO ILS RWY 27L AT ORD I HEARD AND FO HEARD AND READ BACK INSTRUCTIONS TO TURN TO 010 DEGS, DSND TO 4000 FT. WHILE ROLLING OUT ON 010 DEG HDG AT ABOUT 4500 FT WE GOT A 'TFC' ALERT ON TCASII FOLLOWED SHORTLY BY AN RA 'MONITOR VERT SPD.' AT THE SAME TIME WE GOT INSTRUCTIONS TO TURN TO 120 DEGS, MAINTAIN 5000 FT. I DISENGAGED THE AUTOPLT AND MADE A CLBING TURN TO 120 DEGS AND 5000 FT. AT ABOUT THE TIME OF THE RA AND INSTRUCTIONS TO TURN TO 120 DEGS WE ACQUIRED VISUAL CONTACT WITH A HVY. MY BEST GUESS WOULD BE THAT HE WAS ABOUT 500 FT BELOW US AT A RANGE OF ABOUT 1 MI. DURING THE TURN TO 120 DEGS THE CTLR SAID WE HAD BEEN ASSIGNED A HDG OF 100 DEGS. THE FO AND I WERE BOTH SURE WE HEARD 010 DEGS, AND THAT IS WHAT WAS READ BACK BY THE FO. MAYBE WE JUST HEARD WHAT WE EXPECTED TO HEAR AT THIS POINT ON THE APCH AND THE CTLR DIDN'T CATCH OUR READBACK. AFTER FILLING OUT THE OTHER SIDE OF THIS FORM I CALLED ORD TRACON AND SPOKE WITH A SUPVR. HE SAID THE TAPES HAD BEEN REVIEWED AND THAT MY FO HAD READ BACK A HDG OF 100 DEGS. I'M SURE THE FO WILL BE AMAZED BY THAT AS AFTER THE INCIDENT HE WAS POSITIVE HE HAD READ BACK 010 DEGS. I FIND IT VERY HARD TO UNDERSTAND HOW THIS HAPPENED. I ALWAYS REPEAT ALL ASSIGNED HEADINGS AND ALTS TO BE SURE THAT THE FO AND I AGREE ON WHAT WE HEARD. SOMEHOW THIS TIME EVEN THOUGH THE FO READ BACK 100 DEGS TO THE CTLR WE WERE BOTH 'THINKING' 010 DEGS. IN THE PAST I HAVE EXPERIENCED TIMES WHERE THE FO OR MYSELF WOULD MISTAKE 010 DEGS, 020 DEGS, OR 030 DEGS FOR 100 DEGS, 200 DEGS, 300 DEGS BUT ONE OF US ALWAYS CORRECTED OR QUESTIONED THE OTHER AND THE HEADING WAS RECONFIRMED WITH THE CTLR. THIS TIME NEITHER OF US HAD ANY DOUBT WE WERE CORRECT WHEN IN FACT WE WERE WRONG. I REALLY HAVE NO SUGGESTIONS ON HOW TO AVOID THIS IN THE FUTURE. I WILL CERTAINLY REDOUBLE MY EFFORTS TO BE SURE ALL CLRNCS ARE HEARD CORRECTLY. THE TRACON SUPVR SAID HE WOULD HAVE TO FILE A RPT ON LOSS OF SEPARATION AS WE WERE AS CLOSE AS 500 FT AND 2 MI TO THE OTHER ACFT. THE TCASII WORKED AS IT SHOULD AND WOULD HAVE KEPT US FROM GETTING ANY CLOSER TO THE OTHER ACFT IF THE CTLR HAD MISSED THE 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.