Narrative:

We were on the shnon two arrival into iad. When we first checked on with approach we were assigned runway 19C. This approach was loaded into the rmus (as well as the FMS for reference) and briefed. We turned from elsin to mattc; which is downwind for this approach. At this time potomac approach changed our runway assignment to 19L. I (pilot not flying) changed the rmus; FMS; and re-briefed the approach to reflect this. Prior to mattc we were turned off the arrival for the base leg. We were then given a turn to the south; approximately 160 heading to intercept the localizer. I read back 'heading 160; join the 19L localizer'. The first officer (pilot flying) said; 'I think he said 19C'. I realized that approach had in fact said 19C; despite changing us to 19L. Was this a mistake? Or did he in fact mean 19C? I called approach 'verify intercept 19L'. The frequency was very busy and I did not hear a reply; I don't know if my call was blocked or approach's reply was blocked. I made a second call. At this point we see we are catching up to another aircraft clearly on 19L approach and decide that approach did mean 19C. I guess he forgot that he changed our approach. We start turning to the south to maintain separation. I can't remember if I made a third call to approach but in any case we finally heard from him saying '19C'. He then gave us a heading of 220 to rejoin the 19C localizer. The field was not in sight yet; and I was very busy re-entering the 19C ILS frequencies. Approach then said we were lined up on the 19R. I told him we were on the 220 heading. At that point I had the 19C frequency set up and we got the field in sight at the same time; we were indeed right of course. He then gave us vectors back around and we rejoined 19C and landed without further complications. At no time did we compromise separation with any aircraft. ATC forgot he had changed our approach. At very busy airports (not coincidentally; the airports with multiple parallel approaches) it is a crap shoot whether we the pilots will be able to get a word in edgewise to get verification when we need it at such a critical time. If they are going to change our approaches they need some kind of reminder to themselves so they don't put us in a position where we are intercepting the wrong localizer.

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Original NASA ASRS Text

Title: An air carrier crew described a misunderstood runway change clearance during an IAD SHNON TWO arrival. High arrival traffic radio calls delayed verification about the confusion.

Narrative: We were on the SHNON Two arrival into IAD. When we first checked on with approach we were assigned Runway 19C. This approach was loaded into the RMUs (as well as the FMS for reference) and briefed. We turned from ELSIN to MATTC; which is downwind for this approach. At this time Potomac Approach changed our runway assignment to 19L. I (Pilot Not Flying) changed the RMUs; FMS; and re-briefed the approach to reflect this. Prior to MATTC we were turned off the arrival for the base leg. We were then given a turn to the south; approximately 160 heading to intercept the LOC. I read back 'heading 160; join the 19L LOC'. The First Officer (pilot flying) said; 'I think he said 19C'. I realized that approach had in fact said 19C; despite changing us to 19L. Was this a mistake? Or did he in fact mean 19C? I called approach 'verify intercept 19L'. The frequency was very busy and I did not hear a reply; I don't know if my call was blocked or approach's reply was blocked. I made a second call. At this point we see we are catching up to another aircraft clearly on 19L approach and decide that approach did mean 19C. I guess he forgot that he changed our approach. We start turning to the south to maintain separation. I can't remember if I made a third call to approach but in any case we finally heard from him saying '19C'. He then gave us a heading of 220 to rejoin the 19C LOC. The field was not in sight yet; and I was very busy re-entering the 19C ILS frequencies. Approach then said we were lined up on the 19R. I told him we were on the 220 heading. At that point I had the 19C frequency set up and we got the field in sight at the same time; we were indeed right of course. He then gave us vectors back around and we rejoined 19C and landed without further complications. At no time did we compromise separation with any aircraft. ATC forgot he had changed our approach. At very busy airports (not coincidentally; the airports with multiple parallel approaches) it is a crap shoot whether we the pilots will be able to get a word in edgewise to get verification when we need it at such a critical time. If they are going to change our approaches they need some kind of reminder to themselves so they don't put us in a position where we are intercepting the wrong localizer.

Data retrieved from NASA's ASRS site as of April 2012 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.