Air carrier flight crew reported experiencing a possible false terrain warning while in the initial ...

Narrative:

Bzn was landing runway 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 runway 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 (first officer) 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. First officer and I thoroughly debriefed event.

Original NASA ASRS Text

Title: 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.

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.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

Attributes
ACN	1429551
Time
Date	201703
Local Time Of Day	1801-2400
Place
Locale Reference	BZN.Airport
State Reference	MT
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Make Model Name	Large Transport
Operating Under FAR Part	Part 121
Flight Phase	Descent Initial Approach
Route In Use	Vectors
Flight Plan	IFR
Person 1
Function	Captain Pilot Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 195 Flight Crew Type 2634
Person 2
Function	Pilot Not Flying First Officer
Qualification	Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Type 557
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Altitude Excursion From Assigned Altitude Inflight Event / Encounter Other / Unknown

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.