Date: 202101
Local time of day: 1201-1800
Location: DEN.Airport, CO
Flight conditions: VMC
Aircraft: Commercial Fixed Wing
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Final Approach
Airspace: Class B DEN
ATC / advisory: TRACON D01; Tower DEN
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Human factors: Workload; Time Pressure; Communication Breakdown
Second reporter function: Pilot Not Flying; First Officer
Anomaly: ATC Issue All Types; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Unstabilized Approach
Detector: Automation Aircraft Terrain Warning
When detected: In-flight
Result: Flight Crew Executed Go Around / Missed Approach
Contributing factors: Chart Or Publication; Human Factors; Procedure
Primary problem: Procedure

Narrative: In preparation for our arrival into DEN we briefed for the CLASH 3 RNAV arrival and the ILS to 35R as this was the expected runway from our direction of arrival. We also included in our brief the threat in DEN of runway changes and how we'd handle that especially during the descent phase on the arrival. About 20 NM from the waypoint CLASH on the arrival ATC informed us to expect runway 35L. We then re-set up for that runway and re-briefed. There was still plenty of time for this and there was little time compression. One point of confusion; however; was that the CLASH 3 has a transition for RWY 35R; but doesn't for 35L; which only has a point at which to expect vectors. We spent some time studying the arrival and approach plates to ensure that was the case to make sure we weren't missing something. I asked my F/O to double check as he was the PM and I wanted to concentrate on the descent as the PF. This has always been confusing as some arrivals and approaches have transition points and some don't. There should be some consistency. This was the beginning of some of the time compression which would later result in some workload management issues.Some point after this ATC broke us off of the arrival and instructed us to descend to 9000 feet and gave us a heading. They then assigned us to RWY 35R. We related that we'd just been assigned 35L and they reiterated we were now reassigned to 35R. This resulted in a very rushed set up for 35R as we were now pretty close in to the approach environment. I asked the F/O to set it all up so that I could continue to fly. I quickly dialed in what I thought was the 35R LOC; but it turned out not to be. I could see that he was struggling and tried to help him out. In hindsight; this was a mistake in workload management because I should have let the F/O do all of the set up. I verbalized that we didn't need to re-brief for 35R as we'd already briefed it and so had a plan. Again; in hindsight; I'm not sure this was a correct call; but we literally did not have time to brief at that point.Shortly after this we were cleared the visual approach to 35R.  We were roughly 7 miles from FRONZ and discussed not descending too quickly so as not to bust the floor of the Class B airspace. At about the same time ATC asked us to slow to approach speed for traffic ahead of us for 35R. After slowing we ended up just about 2.5 miles in trail.  At this point we were apprised of traffic for 35L and received a TCAS TA because of the assigned heading we were given to intercept the LOC for 35R. We both saw the traffic and I decided to hand fly from this point on as we had an overshooting crosswind and given our heading there would have been a good chance of an overshoot of the LOC.At FRONZ I asked for gear down and flaps 15. At about this point I noticed that neither the LOC or GS were engaging but I had the runway and the PAPI and decided to continue as we had been cleared for the visual. The FO asked if I'd like him to try and dial in the correct LOC and I said yes if he had the bandwidth to do so. Soon after we passed through 1;000 feet and we both missed the call out as I think he was still trying to dial in the LOC frequency. Also we never accomplished the Landing Checklist as we never got to final flaps. We got to 500 feet and we made those call outs. At around 400 feet AFL we received a Too Low Flaps GPWS warning and my knee jerk reaction was to call for FLAP 30 once I realized we were still at FLAPS 15. I don't know if the FO ever selected them; however. I quickly realize this was the wrong call and called for us to go around and the FO called for it at about the same time. We executed the missed approach with no issues and were vectored expeditiously for a second approach and landing without further incident.The FO is not submitting a report as far as I'm aware as we had no ATC deviations or non-compliance with FARS.


Second narrative: This is a report regarding Flight X ZZZ-DEN regarding the changes and ATC clearances we received during our descent and approach to land in DEN.When we reviewed the flight plan in ZZZ we loaded the CLASH3 STAR with runway 35R being our selected landing runway. The flight plan had 17R; but DEN was landing North so that was the only change made to the plan on our part versus what dispatch had planned.Prior to TOD the CA fully reviewed and briefed the STAR and approach that we were expecting. We were fully prepared to execute this plan. Upon checking in with a close in DEN center controller we were advised to expect runway 35L with the EBBLR transition. There was some discussion regarding the EBBLR transition by myself and the CA as we were attempting to figure out if the transition and how the transition linked up with the approach. However; we did this again prior to TOD and were able to be prepared for the new arrival transition and approach. As we descended into Denver; we were handed off to DEN APP. This controller changed our runway back to 35R. We were well into the descent at this point and fairly close to the airport. The CA instructed me to load up the new approach and as I was attempting to do so ATC took us off the arrival for vectors and speed changes. At this point we were South of DEN but still fairly close in. I was still working on getting the FMC setup when ATC vectored us due West and then turned us on to final for 35R. The vector was vey poorly planned by the controller and both the CA and I could see that we were going to blow through the localizer if we allowed the A/P to handle the intercept. The CA disengaged the A/P and made a manual turn to join the localizer. At this point we could both see that the vector had us close in behind another airplane in final. ATC requested that we slow to 160 knots. The CA asked for me to deploy the gear and the flaps to down and 15. Shortly after ATC then requested us to slow to the slowest possible speed. Both the CA and I were closely monitoring the separation between us and the preceding aircraft. At this point I noticed there was a message on the FMC scratchpad that I had not seen before and did not recognize. I don't remember exactly what the message was but it did indicate that there was a conflict between the FMC selected approach and what the VHF radios were tuned to. I was rapidly trying to get the correct frequency on the NAV radios but my JEPP app on the iPad was not responding quickly as I was attempting to zoom in to view the frequency box. At this point we were somewhere around 1;000 feet AGL. The CA announced that he had the runway and the PAPIs in sight and was going to land and just disregard the FMC He was busy attempting to slow and again we were both concerned about the separation between us the preceding aircraft. That was what are focus was on at this point. I cannot recall if either the 1;000 ft or the 500 ft call outs were made. We were both totally task saturated at this time. At some altitude below 500 ft we received a warning message 'too low-flaps'. We both looked at the flap position and realized we were still at flaps 15. The CA called for flaps 30 and I monetarily selected flaps 30 before advocating that we needed to go around. The CA also realized we had to go around at almost the same instant I was stating that we needed to go around. So we executed the go around. We did have the 10;000 ft foot missed approach altitude selected in the MCP so we were climbing to that altitude. However; while just climbing through 7;000 the tower told us to maintain 7;000. I advised tower that we were at 7;200. Tower instructed us to maintain 7;100. We descended back to 7;100 feet and flew runway heading and completed the rest of the go around profile. Tower asked us for the reason for the GA and I replied 'configuration'. They handed us off to approach and we were vectored back around for another ILS to runway 35R. We ensured everything was properly setup and landed without further incident.Some comments about this incident. Both the CA and I had discussed the threat of Denver and how they often make changes to arrivals and approaches during cruise. In fact the CA had been reading in the flight ops newsletter on his iPad about the problems we have been having with arrivals into Denver. We were both fully aware that Denver presented a threat and thought that we had planned and were prepared for our arrival. Had we been cleared as planned per our original plan we would have been fine. Even after the first change to runway 35L we had time to plan and brief. It was only after the final switch back to 35R that we both began to become task saturated with the speed changes; the off route vectors; the FMC programming; the NAV radios; the poor vector to join final; and finally the separation between us and the preceding aircraft and the speed changes made while on final in regards to that aircraft. The amount of items needed to be accomplished while this was occurring in the terminal area was long and complex. We both became so overcome that we must have failed to accomplish the landing checklist; which would have had both of us realizing that we were not in the landing configuration.We made errors along the way; failing to get some of the call outs accomplished; FMC programming; and NAV radio frequency selection among some of the errors. However; many of these errors were driven by the nature of the changes being made by ATC in regards to arrival vectors; speed changes; runway and approach changes among other demands being made by ATC in a very time compressed period while flying within (or near) the terminal area.One more note about the runway changes in Denver. I know Denver and am aware of its threat even on departure. So while we were getting ready to depart and head to ZZZ2 I asked the ramp controller which runway was in use for the ZZZZZ SID. He stated XXR. Yet; when we received our push clearance 15-20 minutes later he told us to expect runway XY. I don't know if he was given bad information by the ATC tower or what happened; but yet again here we are in a high workload situation having to make changes to the FMC and request new TO data.Because of our short turn time in Denver the CA decided to debrief the entire arrival event into Denver while in cruise flying back to ZZZ2. We spent a good 30-40 minutes discussing the flight. Threat and errors were discussed; we used the briefing cards and identified what had occurred on the flight. I am not going to make excuses for the errors that we made; even if they occurred during a task saturated situation. However; a couple of my observations from this event were that we had talked about Denver ATC being a threat. We thought that we had addressed this threat by a good solid brief for the originally planned arrival; even after the change to the 35L arrival; we had time to plan and brief. We were ready; but ATC in Denver threw us some many changes and curve balls that we both were task saturated to the point leading to an undesired aircraft state. Fortunately; both the CA and I followed our training and SOP. safely executed a missed approach; and subsequently landed safely and uneventfully on the next approach.


Synopsis: Air carrier flight crew reported a high workload environment on approach into DEN airport; which included several runway changes; confusion over arrival procedures; and an unstable approach resulted in a go-around.