Flight crew reported a TCAS RA event while on approach.

Attributes
ACN	1676383
Time
Date	201908
Local Time Of Day	0601-1200
Place
Locale Reference	ZZZ.Airport
State Reference	US
Environment
Flight Conditions	VMC
Aircraft 1
Make Model Name	B737 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	Descent
Route In Use	Visual Approach
Flight Plan	IFR
Person 1
Function	Pilot Flying Captain
Qualification	Flight Crew Instrument Flight Crew Air Transport Pilot (ATP) Flight Crew Multiengine
Experience	Flight Crew Type 423
Person 2
Function	Captain Pilot Flying
Qualification	Flight Crew Multiengine Flight Crew Air Transport Pilot (ATP) Flight Crew Instrument
Events
Anomaly	Conflict Airborne Conflict

Narrative:

On vectors to ZZZ in the vicinity of zzzzz waypoint cleared to and level at 8;000 ft. Traffic was reported VFR at 7;500 ft. Autopilot was on and we were alt hold at 8;000 ft. Never got a visual of the traffic but TCAS showed it 500 ft. Below. TCAS momentarily showed traffic at 400 ft. Below which triggered a TCAS RA. We disconnected the autopilot and auto throttles and followed the RA climbing approximately 250 ft. Almost immediately TCAS reported clear of traffic. We simultaneously reported the RA to ATC and they also showed us clear of traffic. We then continued our approach. ATC reported the traffic prior to the event and told us to expect lower once clear. It was a light civil that had us in sight but we never got a visual on them. We were coming over the mountains and it is possible thermals might have caused the light civil to momentary climb causing the RA. Our instruments showed us steady at 8;000 ft.

Original NASA ASRS Text

Title: Flight crew reported a TCAS RA event while on approach.

Narrative: On vectors to ZZZ in the vicinity of ZZZZZ waypoint cleared to and level at 8;000 ft. Traffic was reported VFR at 7;500 ft. Autopilot was on and we were Alt Hold at 8;000 ft. Never got a visual of the traffic but TCAS showed it 500 ft. below. TCAS momentarily showed traffic at 400 ft. below which triggered a TCAS RA. We disconnected the autopilot and auto throttles and followed the RA climbing approximately 250 ft. Almost immediately TCAS reported clear of traffic. We simultaneously reported the RA to ATC and they also showed us clear of traffic. We then continued our approach. ATC reported the traffic prior to the event and told us to expect lower once clear. It was a light civil that had us in sight but we never got a visual on them. We were coming over the mountains and it is possible thermals might have caused the light civil to momentary climb causing the RA. Our instruments showed us steady at 8;000 ft.

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.