Narrative:

Descending into sdf with approach controller. Clearance was descend to 4000 ft, speed 190 KTS on downwind vector. We got a turn to base 270 degree heading. We were getting vectored to ILS runway 17 at sdf. Inbound course for runway was 168 degrees. During base vector we got a clearance to descend to 3500 ft speed 180 KTS, which my first officer read back as he was PNF. I entered 3500 ft in the altitude alerter and asked if it was heading 180 degrees, which he acknowledged. I began the turn and descent. During the turn the approach controller asked if we had begun our turn in. We said yes 3500 ft heading 180 degrees. We were told to turn back heading 270 degrees which we did. Established on 270 degree heading we told the controller that we screwed up the instructions. He then cleared us for visual approach. This clearance was given at a perfect time in the approach vector where a 180 degree heading was a workable vector for final at a time when one would expect a turn in, and just after a speed reduction to 190 KTS. The events leading up to my turn into airport did not lead me to believe that my 180 degree heading was in error because it all fit into parameters for a final vector.

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Original NASA ASRS Text

Title: AN E145 CREW, ON APCH TO SDF, MISINTERPRETED A SPD ASSIGNMENT AS A HEADING VECTOR, DEVIATING FROM ASSIGNED COURSE.

Narrative: DSNDING INTO SDF WITH APCH CTLR. CLRNC WAS DSND TO 4000 FT, SPD 190 KTS ON DOWNWIND VECTOR. WE GOT A TURN TO BASE 270 DEG HDG. WE WERE GETTING VECTORED TO ILS RWY 17 AT SDF. INBOUND COURSE FOR RWY WAS 168 DEGS. DURING BASE VECTOR WE GOT A CLRNC TO DSND TO 3500 FT SPD 180 KTS, WHICH MY FO READ BACK AS HE WAS PNF. I ENTERED 3500 FT IN THE ALT ALERTER AND ASKED IF IT WAS HEADING 180 DEGS, WHICH HE ACKNOWLEDGED. I BEGAN THE TURN AND DSCNT. DURING THE TURN THE APCH CTLR ASKED IF WE HAD BEGUN OUR TURN IN. WE SAID YES 3500 FT HEADING 180 DEGS. WE WERE TOLD TO TURN BACK HEADING 270 DEGS WHICH WE DID. ESTABLISHED ON 270 DEG HDG WE TOLD THE CTLR THAT WE SCREWED UP THE INSTRUCTIONS. HE THEN CLRED US FOR VISUAL APCH. THIS CLRNC WAS GIVEN AT A PERFECT TIME IN THE APCH VECTOR WHERE A 180 DEG HDG WAS A WORKABLE VECTOR FOR FINAL AT A TIME WHEN ONE WOULD EXPECT A TURN IN, AND JUST AFTER A SPD REDUCTION TO 190 KTS. THE EVENTS LEADING UP TO MY TURN INTO ARPT DID NOT LEAD ME TO BELIEVE THAT MY 180 DEG HDG WAS IN ERROR BECAUSE IT ALL FIT INTO PARAMETERS FOR A FINAL VECTOR.

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.