Narrative:

I was the pilot monitoring conducting a visual approach to runway 01 at dca via the mt. Vernon river visual. Pre-descent brief was unremarkable and briefed a flaps 40; brakes 3 landing. While with potomac approach we were told to maintain 160 kts until 5 miles on the mount vernon visual approach to runway 1. When handed off to tower and at about 6-7 miles and approximately 1;700-1;800 ft.; tower told us to slow to approach speed for multiple departures. While I was querying the tower about the speed; the first officer was asking for continued lowering of the flaps and target speed. During these two conversations I did not hear if he called for flaps 30 or 40 and only lowered them to 30 setting. Maybe due to expectation bias (very few first officers fly flaps 40 approaches). I had completely forgotten that we were planning a 40 flap landing.I finished the landing checklist (with the 30 flaps) just prior to the bridge. By the time we passed the woodrow wilson bridge we made a late 1;000 ft. Call out. This is when we began to get behind the aircraft. First officer asked for 'another knot' which confused me because I thought I had set the correct speed. I reduced the target speed to 137 kts. Thinking that's what he wanted. He noticed that I lowered the speed instead of increasing and asked for one knot faster not slower. I was not sure what our planned target speed was at that point and was unable to see in the dark my note I had made during the descent plan. I think I did allow this distraction to get me behind a bit. The 500 ft. Call out was made and we confirmed the prior landing aircraft was clear.around 100 ft. The first officer called for a go-around which did in-fact catch me off guard. I originally thought an aircraft waiting to take off had entered the runway. Then the first officer stated that we were still at flaps 30 instead of 40. The go-around was pretty busy but uneventful. We set up for another visual backed up with the ILS to the same runway.national tower will always give the same missed approach instructions for the mt. Vernon visual approach. They should publish the 3;000 ft. And heading 270 so we know what to expect. Crews are very busy executing the missed and making sure to stay out of the P-56 prohibited area. Better coordination between potomac and tower so one is not keeping you fast when the other will immediately slow you back down very late in the approach. Clearer cockpit communication when in high workload environment.

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Original NASA ASRS Text

Title: B737 flight crew reported an incorrect flap setting on short final that resulted in a go-around.

Narrative: I was the Pilot Monitoring conducting a Visual approach to Runway 01 at DCA via the Mt. Vernon River Visual. Pre-descent brief was unremarkable and briefed a flaps 40; brakes 3 landing. While with Potomac approach we were told to maintain 160 kts until 5 miles on the Mount Vernon visual approach to Runway 1. When handed off to Tower and at about 6-7 miles and approximately 1;700-1;800 ft.; Tower told us to slow to approach speed for multiple departures. While I was querying the Tower about the speed; the First Officer was asking for continued lowering of the flaps and target speed. During these two conversations I did not hear if he called for flaps 30 or 40 and only lowered them to 30 setting. Maybe due to expectation bias (very few First Officers fly flaps 40 approaches). I had completely forgotten that we were planning a 40 flap landing.I finished the landing checklist (with the 30 flaps) just prior to the bridge. By the time we passed the Woodrow Wilson Bridge we made a late 1;000 ft. call out. This is when we began to get behind the aircraft. First Officer asked for 'another knot' which confused me because I thought I had set the correct speed. I reduced the target speed to 137 kts. thinking that's what he wanted. He noticed that I lowered the speed instead of increasing and asked for one knot faster not slower. I was not sure what our planned target speed was at that point and was unable to see in the dark my note I had made during the descent plan. I think I did allow this distraction to get me behind a bit. The 500 ft. call out was made and we confirmed the prior landing aircraft was clear.Around 100 ft. the First Officer called for a go-around which did in-fact catch me off guard. I originally thought an aircraft waiting to take off had entered the runway. Then the First Officer stated that we were still at flaps 30 instead of 40. The go-around was pretty busy but uneventful. We set up for another visual backed up with the ILS to the same runway.National Tower will always give the same missed approach instructions for the Mt. Vernon visual approach. They should publish the 3;000 ft. and heading 270 so we know what to expect. Crews are VERY busy executing the missed and making sure to stay out of the P-56 Prohibited Area. Better coordination between Potomac and Tower so one is not keeping you fast when the other will immediately slow you back down very late in the approach. Clearer cockpit communication when in high workload environment.

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.