Date: 201812
Local time of day: 1201-1800
Location: DEN.Airport, CO
Light: Night
Aircraft: B737-800
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: FMS/FMC
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Anomaly: Aircraft Equipment Problem Less Severe; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Air Traffic Control Issued Advisory / Alert; Flight Crew Returned To Clearance
Contributing factors: Human Factors; Aircraft
Primary problem: Ambiguous

Narrative: Denver was landing south on 17R; 16L and 16R. Weather was VFR and arriving aircraft were being vectored for visuals. We descended on the JAGGR 3 RNAV Arrival and were given the RNAV Z 17R Approach. We selected the approach and the First Officer (F/O) asked me which transition to choose. I looked it up on the chart and responded with the 'QWIKE' waypoint; which the F/O selected and executed. We both visually verified this. We then fully briefed the arrival and the approach on the FMS and the Jepps Approach plate. Shortly thereafter; we were cleared both the arrival and the approach and I selected 7000 FT in the altitude window since we were not yet established on a segment past the IF/IAF point on the approach. We both verbalized 'LNAV/VNAV Path' on the MCP panel and the PFD display. We crossed QWIKE at 11;000 FT and the aircraft slowed to 210 knots as published. Established on the approach; I selected '0' in the altitude window. We were both watching/monitoring the approach and as we approached the STAAM waypoint where the RF turn to final starts; we noticed the aircraft did not commence a turn to final and instead just continued straight ahead. I looked up to verify LNAV/VNAV PATH were selected and they were. Even stranger was the fact that the leg from STAAM to DAVTE (the arc of the approach we were on) was magenta in color (the active leg) yet the aircraft was not following it. The FMS flight plan had no 'DISCO' and correctly contained the next three waypoints as well as the missed approach routing. I immediately disengaged the autopilot and started a left turn to follow the published arc routing of the approach. Our cross track error was perhaps a mile at the most; probably less; since I had started the turn manually within four or five seconds after we noticed the aircraft was not turning at STAAM. We were halfway through the turn and almost back on course when ATC said we 'lost the RNAV Approach' canceled our clearance and gave us a heading and altitude to intercept a visual final. We complied and intercepted final for 17R and flew the Visual Approach. There were no traffic conflicts with any other aircraft during the event. While intercepting final; he gave us a number to call due to a 'possible deviation' and we wrote it down. The rest of the approach was uneventful. Upon landing; we contacted (Name); Company ATC Specialist; who spoke with Denver Approach on our behalf to explain our FMS navigation problem with the aircraft. As a Crew; we discussed the situation in depth and we cannot determine what would have caused the aircraft to continue straight ahead since we both verified all the waypoints in the FMS; both verified 'LNAV/VNAV' was active; both verified there was no 'DISCO' inserted; and we both noted the magenta line was highlighted in our PFD showing the correct course that should have been flown by the aircraft. We both felt we had done an excellent job checking the waypoints and briefing the approach. We could have perhaps let the controller know earlier that our aircraft had not commenced the turn but we were busy maintaining aircraft control and navigating.




Synopsis: B737-800 Captain reported ATC issued a new clearance when the aircraft failed to accurately track the RNAV Z 17R Approach to DEN.