Date: 201909
Local time of day: 1801-2400
Location: DCA.Tower, DC
Flight conditions: VMC
Light: Night
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Final Approach
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Human factors: Confusion; Communication Breakdown
Second reporter function: Pilot Flying; First Officer
Anomaly: Deviation - Speed All Types; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Unstabilized Approach
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Executed Go Around / Missed Approach
Contributing factors: Human Factors
Primary problem: Human Factors

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.


Second narrative: I was the Pilot Flying 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. All was well and approach control had asked to keep speed up as much as possible during approach. My recollection is that speed was slightly high but slowing (Approximately 190 kts.) and gear was called late (1;800 ft.) for we were approaching 1;600 ft. while still slowing to 180 kts. While working to reduce speed (autothrottle still engaged and autopilot off) and get flap calls and aircraft configured; Tower radio calls started to add to my reduction in situational awareness. Whether I errantly called for flaps 30 and landing checklist or flaps 40 landing checklist and the Pilot Monitoring didn't hear I can't say with certainty. As we were coming through 900 ft.; we made a late 1;000 ft. call and I noticed our speed bug was not at the 5 kts above Vref so I asked the Pilot Monitoring to give me another knot. Poor verbiage for he lowered the speed another knot (wrong direction) but I reiterated I wanted 5kt above Vref and it was fixed but this showed we were both having timeline and potentially situational awareness issues. A 500 ft. callout was replied as stable but I somehow did not catch the incorrect flap configuration per the flight manual. At 200 ft. (a point that I personally take one-last look at primarily the gear but also the flap setting) is where I noticed we had the incorrect flap setting of 30. We had briefed a flaps 40 landing; set flaps 40 target speed but were in a flaps 30 configuration. I made a comment to the Pilot Monitoring that we were at the wrong flap setting but do not believe that he heard what I said and at 100 ft. executed a go-around stating we were at the wrong flap setting. We executed the wave-off uneventfully and re-entered the landing pattern and recovered uneventfully via a flaps 40 landing.Areas of Fault-Both got behind timeline and had moments of lost situational awareness.-May have errantly called for Flaps 30 Landing Checklist or didn't speak loudly and clearly enough for the Pilot Monitoring to understand I stated flaps 40 landing checklist.-Errantly did not verify flap 40 setting by required final configuration check altitude of 500 ft.-Failed to plan accordingly by slowing to on-speed earlier vice at final approach fix or 1;500 ft. given increased speed reduction requirement when landing at flaps 40 vs flaps 30.-Perhaps some complacency worked in because flight had been going well but failed to adequately realize potential threats associated with this approach which would be different. It was the only nighttime approach landing of the trip and at flaps 40 setting.


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