Narrative:

The copilot was flying the aircraft being vectored for ILS 9L at ord. We were on downwind descending from 6000 ft to 4000 ft on a 230 degree heading. I was tuning the ILS when we got a call from approach control to 'slow to 170/heading 260.' I'm sure I read back the controller's instructions. But, after identing the ILS and watching the copilot set 170 in the speed window, I was no longer sure of the heading. I asked him what the heading was and he replied, '150 degrees.' he seemed sure and it was a good heading for a base leg, so we began a turn toward final approach. For some reason, I didn't feel comfortable and decided to confirm the heading with the controller. The controller paused, said '270 -- what is your heading -- climb and maintain 5000.' he then instructed another flight to descend immediately to 3000 ft. Separation was achieved and we were put back in the line-up for landing. Strictly human error. I should have confirmed the instructions with the controller at the first instance of doubt. Not the copilot! Excellent work by the controller to remain calm and separate the aircraft. Although it was crew error, I would say that multiple clrncs tend to run together -- ie, the descend, turn, slow, etc, type of clearance. It would make it easier to remember and understand if the instructions were separated by readback. This may not be feasible in areas like ord, but it's all I can come up with to reduce misunderstandings and errors.

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Original NASA ASRS Text

Title: HDG TRACK DEV.

Narrative: THE COPLT WAS FLYING THE ACFT BEING VECTORED FOR ILS 9L AT ORD. WE WERE ON DOWNWIND DSNDING FROM 6000 FT TO 4000 FT ON A 230 DEG HDG. I WAS TUNING THE ILS WHEN WE GOT A CALL FROM APCH CTL TO 'SLOW TO 170/HDG 260.' I'M SURE I READ BACK THE CTLR'S INSTRUCTIONS. BUT, AFTER IDENTING THE ILS AND WATCHING THE COPLT SET 170 IN THE SPD WINDOW, I WAS NO LONGER SURE OF THE HDG. I ASKED HIM WHAT THE HDG WAS AND HE REPLIED, '150 DEGS.' HE SEEMED SURE AND IT WAS A GOOD HDG FOR A BASE LEG, SO WE BEGAN A TURN TOWARD FINAL APCH. FOR SOME REASON, I DIDN'T FEEL COMFORTABLE AND DECIDED TO CONFIRM THE HDG WITH THE CTLR. THE CTLR PAUSED, SAID '270 -- WHAT IS YOUR HDG -- CLB AND MAINTAIN 5000.' HE THEN INSTRUCTED ANOTHER FLT TO DSND IMMEDIATELY TO 3000 FT. SEPARATION WAS ACHIEVED AND WE WERE PUT BACK IN THE LINE-UP FOR LNDG. STRICTLY HUMAN ERROR. I SHOULD HAVE CONFIRMED THE INSTRUCTIONS WITH THE CTLR AT THE FIRST INSTANCE OF DOUBT. NOT THE COPLT! EXCELLENT WORK BY THE CTLR TO REMAIN CALM AND SEPARATE THE ACFT. ALTHOUGH IT WAS CREW ERROR, I WOULD SAY THAT MULTIPLE CLRNCS TEND TO RUN TOGETHER -- IE, THE DSND, TURN, SLOW, ETC, TYPE OF CLRNC. IT WOULD MAKE IT EASIER TO REMEMBER AND UNDERSTAND IF THE INSTRUCTIONS WERE SEPARATED BY READBACK. THIS MAY NOT BE FEASIBLE IN AREAS LIKE ORD, BUT IT'S ALL I CAN COME UP WITH TO REDUCE MISUNDERSTANDINGS AND ERRORS.

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.