Date: 202106
Local time of day: 0001-0600
Location: D01.TRACON, CO
Flight conditions: VMC
Aircraft: Widebody; Low Wing; 2 Turbojet Eng
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Approach
Airspace: Class B DEN
ATC / advisory: TRACON D01
Aircraft component: Autoflight System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; Captain
Reporter qualification: Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Human factors: Time Pressure; Situational Awareness; Fatigue; Confusion; Human-Machine Interface
Second reporter function: Pilot Flying; First Officer
Anomaly: Aircraft Equipment Problem Less Severe; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance
Detector: Automation Air Traffic Control; Person Flight Crew; Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Flight Crew Overrode Automation; Flight Crew Became Reoriented
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: As this report is not timely; it is to the best of my recollection. Approach phase of the flight to DEN. Briefed and installed the SSKII 2 RNAV/ILS 16R. On arrival we a received multiple speed and heading changes but we were back on course and LNAV/VNAV path were engaged. At some point prior to CLIFF; for whatever reason ATC changed clearance to RNAV 16R for us and the Aircraft (AC) behind us. I was Pilot Not Flying (PNF) so I installed the approach/transition. The AC ahead of us was in sight and was on the arrival but still assigned the ILS 16R. The approach was verified by both the First Officer (FO) and myself. LNAV/VNAV were still engaged and the speed window was open as ATC had already been assigning speed changes. The FO located the Approach briefing guide for the RNAV/RNP approach and RNP values for the approach were line selected and installed. All altitudes for the approach were verified.  I made the comment that 'Since ATC has been messing with our speed so much; the speed window is already open'. So far the approach was flawless and the aircraft (AC) was on the vertical and lateral profile.Upon reaching CLIFF the AC began a descent as it should have. TDZ had been set and VNAV Path was engaged. As we approached CEPEE the MCP/FMA's had not changed SPD/LNAV/VNAV path were engaged. The magenta track depicted CLIFF;CEPEE;AAGEE;JABRO;JETSN; RW 16R; as it should. It was upon arrival at CEPEE that the event occurred. The AC continued to descend and VNAV path remained engaged. But; the AC continued to track straight ahead. CEPEE had not become active and the vector leg out of CLIFF was still engaged. I had been so concerned with the vertical profile that I failed to notice CEPEE was not active. Realizing what had happened I went to the FMC to LS AAGEE; but ATC noticed at the same time and assigned us either a 080 or 090 heading. ATC also canceled the RNAV approach clearance for the AC behind us. We landed without further issue. Once inside the terminal; still pissed that I allowed this to happen; I located the Captain of the AC that landed behind us to apologize. He informed us that ATC had mistakenly given him the # to call. I took the number (#) and called immediately. After a conversation with the supervisor I was informed that there was no safety issue; no loss of separation; and no need to file a report.Looking back; how this could have been avoided... first of all; I could have just said 'unable' when ATC switched us to the RNAV RNP 16R. Although not a big deal at the time...after flying all night; briefing and installing one arrival and approach then; at the last minute being reassigned to a RNAV/ RNP approach which requires lots of briefing and changes; and programming; was probably not the thing to do. What else could I have done...I allowed myself to get distracted. I should have continually monitored the lateral path to insure the AC was doing what we thought it was going to do. It was very clear and it was easy to look outside to follow traffic. It was also the FO's first Pilot Flying leg so there was instruction taking place and attention was diverting to multiple places.The FO did a great job when we were reassigned to the RNAV RNP Z 16R. He did not miss a checklist; procedure; or setting.I'm disgusted that I allowed this to happen. I am unfortunately or fortunately very familiar with the airspace and lateral/ vertical deviation issues in the area. I'm not the first and sure won't be the last; but I know what I will and won't do next time.


Second narrative: During the SSKII 2 RNAV Arrival phase; we were advised by ATC to expect the ILS 16R approach. A few minutes prior to CLFFF; the last point on the SSKII 2 arrival; we were assigned the RNAV (RNP) Z R16R approach. We were on a northerly heading in HDG mode at this time for vectors for the previously expected ILS. The Captain; as Pilot Monitoring (PM); selected the correct RNAV Z 16R approach as well as the correct transition (CLFFF transition) in the FMC. I verified the selected approach and transition after he did so. I then set up the approach per all steps of the Approach Briefing Guide and briefed the approach. We were then given direct CLFFF and selected LNAV. We were cleared for the RNAV RNP Z R16R approach shortly before arriving at CLFFF. Upon arriving at CLFFF; I verified that the CLFFF waypoint sequenced in the FMC to ensure we were now on a published segment of the approach. I selected LNAV; VNAV; opened the speed window for the appropriate speed; requested the PM set touchdown zone elevation (which he did); and verified that the ENGAGED FMA's were in green LNAV and VPATH. The aircraft began a descent per the VPATH glide slope. I ran through the items again to verify that the approach was proceeding correctly. I verified again that the FMA's showed an engaged green LNAV and engaged green VPATH. The approach was proceeding as expected. At the CEPEE waypoint; the aircraft should have begun a right turn towards the AAGEE as part of the track towards the final approach point and final approach course. The aircraft did not turn and continued in a straight course after passing CEPEE. I immediately checked the FMA's again to ensure it did not revert to HDG mode. The FMA's were still in green; engaged LNAV and VPATH. The aircraft was also still in an expected VPATH descent to the TDZE. Within a few seconds; and before we could figure out why the aircraft did not turn; ATC called to cancel the RNAV approach and gave us a right turn for a visual approach to runway 16R and gave clearance for the visual approach. We proceeded uneventfully for the visual approach and landing. After arriving at the gate and leaving the aircraft; a pilot from another aircraft that was behind us during the arrival and approach advised us that he was mistakenly given a phone number by ATC to call regarding the RNAV Z 16R turning issue that we actually experienced. We got the phone number from him and the Captain called. The ATC supervisor said that no report would be required; there was no loss of separation; there was no safety issue; and how just wanted to know what happened. The Captain and I reviewed everything we did in setting up; preparing; and executing that approach and could not figure any reason why the aircraft did not initiate a turn at CEPEE and continued straight while engaged in LNAV with the proper approach waypoints; altitudes; and sequencing verified in the FMC.


Synopsis: Air Carrier flight crew reported a track deviation caused by an automation error while on final approach to DEN. Both pilots and ATC intervened to correct the deviation.