Date: 201703
Local time of day: 1801-2400
Location: BZN.Airport, MT
Flight conditions: VMC
Light: Night
Aircraft: Large Transport
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent; Initial Approach
Airspace: Class E BOI
ATC / advisory: TRACON BOI
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Confusion; Troubleshooting; Situational Awareness
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Inflight Event / Encounter Other / Unknown
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Took Evasive Action
Contributing factors: Environment - Non Weather Related
Primary problem: Environment - Non Weather Related

Narrative: BZN was landing RWY 12; night VMC conditions below 12;000 MSL; with visibility unlimited. Approach was thoroughly briefed including plotting terrain in case of vectors to east of field. Proceeding direct to BZN from southeast; on initial contact with Big Sky Approach at 17;000 feet we were cleared to 13;000 feet and complied with clearance to fly heading 10 degrees left of current course for vectors visual approach to RWY 12. Our next assigned altitude was 11;000 feet. In cruise flight at 11;000; autopilot on; we could see the mountains ahead and below us and the city/valley with unrestricted visibility. ATC was setting us up for a west downwind over the valley. At 11;000 feet; prior to the valley we received a radar altimeter '2500' voice audible that cleared itself. Approximately 1 minute later while still on a vector heading; in level cruise flight at 11;000 feet; with no indications of terrain on the HSI we received a GPWS warning of 'terrain; terrain; pull up; pull up.' We could see the terrain below us but it was night therefore I initiated the GPWS terrain escape by selecting the TOGA thrust and brought the stick full aft; Auto Pilot off; speed brakes were not out. Warning was short lived; and passing 12;000 I told First Officer (FO) to notify ATC we were climbing for a terrain warning. ATC said (paraphrasing) there was no terrain issue and we could return to 11;000 feet. We ensured we had the correct altimeter settings; which we did (29.38). Then he cleared us to 7;000 feet where we remained with vectors until established on the approach and landed uneventfully. Previously to this event I had selected the BZN VOR in the PROG page and after the event I checked our position and estimate the terrain warning occurred approximately 140 deg / 25 miles from the BZN VOR on an extended downwind heading northwest. We also queried the controller as to why we may have received the warning. He stated (paraphrasing) he did not know; we were clear of terrain at the MVA of 11;000; and highest terrain was approximately 8;700 feet. He also said there was terrain to our west that was about 11;500 feet but was not a factor. FO and I thoroughly debriefed event.


Second narrative: [Report narrative contained no additional information.]


Synopsis: Air Carrier flight crew reported experiencing a possible false terrain warning while in the initial phase of approach in a mountainous terrain area in clear VMC night conditions. An escape maneuver was executed as a precaution.