Date: 201809
Local time of day: 1201-1800
Location: DEN.Airport, CO
Aircraft: B737 Next Generation Undifferentiated
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Final Approach
Airspace: Class B DEN
ATC / advisory: TRACON D01
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness; Distraction; Workload
Anomaly: ATC Issue All Types; Conflict Airborne Conflict; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Became Reoriented
Contributing factors: Human Factors; Procedure
Primary problem: Procedure

Narrative: Too many approach assignment changes resulted in very high workload and failure to tune the last ILS assigned. Root issue: ATIS information did not include the RNAV/RNP as an advertised approach in use. And scrambling to accommodate the last change distracted us from warning the flight attendants to prep for landing. Before top of descent we selected the advertised ILS for the most likely runway (we guessed right); prepped and briefed. Denver Approach advised us to expect the RNAV/RNP for the selected runway even though it had not been advertised as an approach in use. We accepted the change; prepped the cockpit and re-briefed for the RNAV/RNP. Then the next-to-last controller said we were going to be changed back to the ILS that we had previously briefed. I think this was just before we turned downwind on the FRNCH 3 Arrival. I pushed back and said we'd like to stay on the RNAV/RNP because we'd changed approaches once; re-prepped for the more complex approach; and it was getting late in the arrival. His response was that the next controller would make the ultimate decision and to contact her. We changed frequencies abeam the field and were told we would be flying the ILS. I pushed again because it was very late to be making those changes. [Controller] said they were unable [to issue] the RNAV/RNP and that we would be flying the ILS. Somewhere in that process we had a TCAS event (you can hear it on the radio as my First Officer (FO) was responding to the controller) which didn't help the workload. It wasn't an RA - it was a TA; but it still required our attention until we understood the threat. After considering the big picture I elected to accept the ILS and we started the process of setting the cockpit back up for the ILS. Before everything was set; we were given a base leg which in retrospect we should not have accepted. We weren't ready for the approach. But we turned anyway because the controller was 'explaining' her reason for giving us the ILS and we couldn't respond right away and I didn't want to continue on an unassigned heading without her knowing. [Controller] then turned us dogleg and cleared us for the approach. I hit the APP button but the approach wouldn't arm because it wasn't tuned. We caught it before we flew through final; but it was a near thing. And it was about that time that I realized I had not warned the flight attendants to prep for landing. Not good. From there the approach and landing were uneventful. I regret taking the turn to base leg. That done; I regret not abandoning the approach as soon as I realized we weren't tuned/briefed and fully ready for the approach. I should have gone around from outside the FAF and gone back to the radar pattern to set it up again. I wish I had.




Synopsis: B737NG Captain reported high workload resulted in some missed SOPs on arrival into DEN following multiple ATC clearance changes.