Narrative:

On the arrival into iad; we were told to expect runway 19L by approach control. After being turned off the barrin 1 arrival for a base vector; we were told to fly heading 210 and intercept the localizer. Both myself and the captain heard runway 19L as our assigned runway. The readback of our clearance to intercept the localizer was for runway 19L as well. After turning to intercept the runway 19L localizer; the approach controller told us to make an immediate right turn back to 210 degrees and intercept the runway 19R localizer. We complied; descended into visual conditions; called the airport in sight; and continued the visual approach to land on runway 19R. Either there was not closed loop communication between us and ATC; or ATC mistakenly put us on the wrong approach. We believe that we heard and read back runway 19L each time. During this time; ATC had; under positive control; vectored a VFR aircraft to pass within 500 ft of us without notifying us until we were within a couple miles at the most. ATC also had at least one other non-airline arrival that was making some extra requests and taking up radio time. It could have been easy for ATC to miss our readback or assign us the wrong runway; or for us to miss a last minute runway change.

Related reports:

Google
 

Original NASA ASRS Text

Title: E145 HEARS AND EXECUTES INTERCEPT TO RWY 19L ILS AT IAD. APCH CTLR INTENDED RWY 19R AND RECLEARS APPROPRIATELY.

Narrative: ON THE ARR INTO IAD; WE WERE TOLD TO EXPECT RWY 19L BY APCH CTL. AFTER BEING TURNED OFF THE BARRIN 1 ARR FOR A BASE VECTOR; WE WERE TOLD TO FLY HEADING 210 AND INTERCEPT THE LOC. BOTH MYSELF AND THE CAPT HEARD RWY 19L AS OUR ASSIGNED RWY. THE READBACK OF OUR CLRNC TO INTERCEPT THE LOC WAS FOR RWY 19L AS WELL. AFTER TURNING TO INTERCEPT THE RWY 19L LOC; THE APCH CTLR TOLD US TO MAKE AN IMMEDIATE R TURN BACK TO 210 DEGS AND INTERCEPT THE RWY 19R LOC. WE COMPLIED; DESCENDED INTO VISUAL CONDITIONS; CALLED THE ARPT IN SIGHT; AND CONTINUED THE VISUAL APCH TO LAND ON RWY 19R. EITHER THERE WAS NOT CLOSED LOOP COM BETWEEN US AND ATC; OR ATC MISTAKENLY PUT US ON THE WRONG APCH. WE BELIEVE THAT WE HEARD AND READ BACK RWY 19L EACH TIME. DURING THIS TIME; ATC HAD; UNDER POSITIVE CTL; VECTORED A VFR ACFT TO PASS WITHIN 500 FT OF US WITHOUT NOTIFYING US UNTIL WE WERE WITHIN A COUPLE MILES AT THE MOST. ATC ALSO HAD AT LEAST ONE OTHER NON-AIRLINE ARR THAT WAS MAKING SOME EXTRA REQUESTS AND TAKING UP RADIO TIME. IT COULD HAVE BEEN EASY FOR ATC TO MISS OUR READBACK OR ASSIGN US THE WRONG RWY; OR FOR US TO MISS A LAST MINUTE RWY CHANGE.

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