Date: 201809
Local time of day: 1801-2400
Location: DEN.Airport, CO
Light: Night
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Approach
Airspace: Class B DEN
ATC / advisory: TRACON D01
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Physiological - Other; Situational Awareness
Second reporter function: Pilot Not Flying; Captain
Anomaly: Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Weather / Turbulence
Detector: Person Air Traffic Control; Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Flight Crew Returned To Clearance
Contributing factors: Aircraft; Weather
Primary problem: Ambiguous

Narrative: We were cleared the RNAV RNP Z 16R Approach into DEN. Upon crossing CLFFF; and still maintaining LNAV/ VNAV PATH; I set '0's' in the altitude window. We were instructed to maintain 170 knots until the FAF. As the aircraft turned final (still maintaining LNAV/VNAV PATH and on autopilot) I noticed the aircraft drifting left of course. Prior to turning final; there was a crosswind from the right. As we turned final; the crosswind switched to the left. I assumed the aircraft automation was trying to correct for this sudden change in wind direction; so I let the aircraft continue to fly the approach. It was about then that Tower queried if 'we still had Runway 16R in sight?' and that he showed us left of course. The Captain told the Tower that we did have Runway 16R in sight; and were on course. As Pilot Flying (PF); I ended up clicking the autopilot off to hand fly the approach. I have flown RNAV RNPs; as well as this particular RNAV RNP Z 16R; countless times; and have never seen a course/lateral deviation like what occurred in this instance. Although there was a shift in crosswinds as the aircraft turned final; it was on speed; correctly configured; and displaying LNAV/VNAV PATH. As this is an RNAV RNP Approach; I was hesitant to correct the course deviation by clicking off the autopilot and hand flying; as I am under the impression that the lateral and vertical guidance of the approach are to be strictly adhered to (thus; following the flight director). I can only conclude that there is a navigational; or operational; issue with this particular approach; as it was obvious that it was not maintaining correct lateral guidance.


Second narrative: On the PEEKK Arrival into Denver; we had descent winds out of the west. We were cleared for the RNAV Z Approach 16R. The approach proceeded normally to JETSN with the autopilot engaged. We were cleared to land. Inside JETSN; airport winds were shifting to the southeast approximately 10 knots. The autopilot began to have difficulties maintaining lateral course for a few seconds; drifting a little east of course. There was no apparent traffic conflict; however; the Tower Controller asked if we had the field in sight. I replied we had the field in sight; normal course was resumed and the landing on 16R was normal. Anticipating possible autopilot errors that may occur as winds shift would help. Although to my knowledge it wasn't a factor in this case.  Always remain vigilant of possibly very close traffic on the close parallel approach.


Synopsis: B737-700 flight crew reported a track deviation occurred on the RNAV RNP Z Approach to 16R in DEN when the autopilot appeared to have difficulty dealing with a wind shift.