Date: 201405
Local time of day: 0601-1200
Location: D01.TRACON, CO
Flight conditions: VMC
Aircraft: B737-800
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: Pilot Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Time Pressure; Workload; Communication Breakdown; Confusion
Second reporter function: Pilot Not Flying; First Officer
Anomaly: ATC Issue All Types; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Air Traffic Control; Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Flight Crew Requested ATC Assistance / Clarification
Contributing factors: Airport; Procedure; Human Factors; Environment - Non Weather Related
Primary problem: Ambiguous

Narrative: Original clearance was the TSHNR ONE RNAV arrival into Denver. After checking ATIS; we asked if we should expect the MOLTN TWO arrival since Denver was landing north. We were then re-cleared for the MOLTN TWO arrival to Wooky. I had briefed the visual backed up by the ILS to 34R at 80 miles before TOD. The ATIS said to expect visual or ILS to 34R; 35L; RNPZ approach available. When we checked in with approach near RAMMS we were told to expect the RNAV RNPZ 34R approach. We had to hurry to change the FMC from the ILS 34R to the RNAV RNPZ 34R with little time to brief and reset everything at a very busy time of the arrival. Approaching HIMOM we were cleared to descend to 9;000 feet. At HIMOM is where things became more confusing. While descending to 9;000 feet and just past HIMOV; we were given a turn to 80 heading. They called traffic for 35L which we reported in sight. In my recollection; this turn occurred before MCMUL or KALME. In either case; we thought we were still on the 173 track. At this point; we didn't know if we were cleared for the RNPZ approach or not. A suggestion that would help the pilots as well as the controllers would be to get a definite clearance for the RNAV RNPZ approach well before HIMOM to prevent unstable approaches or inability to even fly the approach properly and to avoid any confusion as to what the clearance is.


Second narrative: Arrival was MOLTN2 and cleared to descend via. PF had originally briefed and built in the FMC ILS 34R prior to TOD (80 miles or so). Checked in with Denver Approach at or around RAMMS at which point ATC tells us to expect RNAV-Z 34RRebuilt approach in FMC. Continued descent via VNAV/LNAV. Level at 11;000 feet at HIMOM intersection. ATC instructs to descend and maintain 9;000 feet. Recollection was just prior to MCMUL ATC gives us 80 degree heading assignment. PF turns to heading 080 from our 173 degree track. ATC inquires if we have 35L traffic in sight. We reply yes we do. The assigned turn put us nearly on the RNAV-Z 34L arc but just a bit inside. ATC clears us to 8;000 feet and then asks if we are on the RNAV-Z. As we were essentially on the arc I replied 'yes; that's what we were told to expect.'ATC seemed surprised at this. Do not know if he knew what our original instructions are.


Synopsis: A B737-800 crew reported changing the FMS to the MOLTN TWO RNAV and the approach from the ILS 34R to the RNAV RNPZ 34R and while on a vector the crew and ATC was uncertain if they were cleared for the approach.