Date: 201901
Local time of day: 1201-1800
Location: DEN.Airport, CO
Flight conditions: VMC
Aircraft: B757 Undifferentiated or Other Model
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 Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness
Anomaly: Conflict Airborne Conflict; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance
Detector: Person Flight Crew
When detected: Aircraft In Service At Gate
Result: General None Reported / Taken
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: I was the Pilot Flying the PURRL2 RNAV Arrival to Denver. We were cleared to descend via the PURRL2 except maintain 11;000 FT; expect Runway 35L. At some point on the arrival; I believe about RECRD; and approaching/level at 11;000 FT we received a vector toward Runway 35L final (I can't remember the heading) and clearance to descend to 10;000 FT. The First Officer (FO) read back the clearance and I complied. There was also a speed clearance; I believe it was to maintain at or above 210 kt. While on the vector passing 10;400 FT; we received a confusing traffic conflict clearance call from ATC. We were told to climb to 10;000 FT and turn left heading 250 deg. I immediately turned to 250 deg heading but since we were descending through 10;400 FT to 10;000 FT; I didn't know whether to climb or descend. We discussed the altitude clearance briefly inside the flight deck and the First Officer queried the controller and the clearance. Again; the controller said; climb to 10;000 FT and turn left heading 250 deg. I had a choice to descend to 10;000 FT or climb to an unknown altitude; so I opted to go to the cleared altitude of 10;000 FT. As we descended and leveled off at 10;000 FT we got a TCAS RA alert to climb. Simultaneously with the TCAS RA; the First Officer reported the traffic aircraft in sight. I complied with the TCAS RA guidance and the FO reported the TCAS RA altitude deviation to the controller. Once we got the TCAS clear of conflict announcement; we received vector back to final and were cleared for the visual approach. After we landed we contacted ATC for clarification on what happened. While talking to ATC over the phone about the incident; I discovered they showed us about 1000 FT below our assigned altitude throughout the entire arrival and approach. The only thing I could think of that would have caused the altitude discrepancy/deviation was an incorrect altimeter setting. However; I was certain that we set all three altimeters to QNH passing 18;000. I called [Company] Denver Station and found that the aircraft was overnighting and still at the gate. I then contacted a Flight Office Representative and asked him to go to the aircraft to check our altimeter settings on the aircraft. The Flight Operations Representative reported that all three altimeters (Captain; First Officer and Standby) were set to 30.45 inches. The DEN ATIS altimeter was 29.45.




Synopsis: B757 Captain reported discovering after the flight that all three altimeters were incorrectly set by one inch of mercury when they flew the PURRL2 RNAV arrival to DEN.