Narrative:

Upon initial contact with approach control; we were assigned runway 1C at iad. We requested 1L due to its proximity to our gate. Approach control changed our runway to 1R and provided a 090 vector off the gibbz 3 RNAV to a base leg for 1R. We were then given a 060 heading to intercept the 1R localizer and asked to report the airport in sight. After reporting the airport in sight; we were cleared for a visual to 1R and instructed to contact tower. We noted traffic off to our right; but did not consider it a factor. Just outside the FAF; we received a TA at the same time initial contact was made to tower. This was quickly followed by a 'monitor vertical speed' and 'clear of conflict'. Due to the audible volume of the TCAS; as well as the distraction of looking for the traffic; we missed the tower's response to our check-in and asked him to repeat their last transmission. We then received an RA 'descend' with the HSI displaying an aircraft within 400 feet. The pilot flying initiated a descent until we [heard] 'clear of conflict' again. He noted the aircraft was above and to the right of our position; close enough to clearly see their engine nacelles. Tower then informed us he was showing an aircraft above us and asked if we had the traffic in sight. We responded yes. Tower then proceeded to vector the other aircraft away from our position and asked us if we had received an RA. We responded in the affirmative; verified our clearance to land; and landed without further incident. While not totally sure what happened; it appears there was miscommunication between approach control; tower; and the other aircraft. We were cleared for an approach to 1R; but the other aircraft must have thought we were going to 1C. This caused him to turn base early resulting in the very close proximity.

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Original NASA ASRS Text

Title: B737NG Captain reported an NMAC on approach to IAD.

Narrative: Upon initial contact with Approach Control; we were assigned Runway 1C at IAD. We requested 1L due to its proximity to our gate. Approach Control changed our runway to 1R and provided a 090 vector off the Gibbz 3 RNAV to a base leg for 1R. We were then given a 060 heading to intercept the 1R localizer and asked to report the airport in sight. After reporting the airport in sight; we were cleared for a visual to 1R and instructed to contact Tower. We noted traffic off to our right; but did not consider it a factor. Just outside the FAF; we received a TA at the same time initial contact was made to Tower. This was quickly followed by a 'monitor vertical speed' and 'clear of conflict'. Due to the audible volume of The TCAS; as well as the distraction of looking for the traffic; we missed the Tower's response to our check-in and asked him to repeat their last transmission. We then received an RA 'descend' with the HSI displaying an aircraft within 400 feet. The pilot flying initiated a descent until we [heard] 'clear of conflict' again. He noted the aircraft was above and to the right of our position; close enough to clearly see their engine nacelles. Tower then informed us he was showing an aircraft above us and asked if we had the traffic in sight. We responded yes. Tower then proceeded to vector the other aircraft away from our position and asked us if we had received an RA. We responded in the affirmative; verified our clearance to land; and landed without further incident. While not totally sure what happened; it appears there was miscommunication between Approach Control; Tower; and the other aircraft. We were cleared for an approach to 1R; but the other aircraft must have thought we were going to 1C. This caused him to turn base early resulting in the very close proximity.

Data retrieved from NASA's ASRS site 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.