B737-800 captain reported TCAS indicating near midair collision which resulted in evasive action alt...

Narrative:

40 miles north of rno at 10;000 feet MSL; in a holding pattern assigned by ATC; as we were accomplishing the checklist for a flap asymmetry malfunction. A TCAS advisory had traffic at our 12:30 and 12 miles; indicating 1000 ft. Above our altitude and heading toward us/converging flight paths. At five miles traffic was now 12:00 and 500 feet above our altitude. At 3 miles and 12:00 traffic was descending and indicating 300 feet above us. We took evasive action; turning away 90 degrees and descending over 700 feet. Traffic ultimately crossed to our right 500 feet above our altitude while we executed the evasive maneuver. TCAS gave a visual RA after we executed our evasive maneuver; but it gave no aural RA. We notified ATC of our actions and our subsequent return to heading and altitude. ATC stated that according to his radar traffic was 1000 feet above our altitude. I believe oncoming traffic was a general aviation aircraft off his altitude. We never acquired the traffic visually. We called out traffic early to ATC; but his radar showed a 1000 feet of clearance. We on the other hand could not risk such a near miss encounter and chose to take aggressive and decisive action to avoid the oncoming traffic; ultimately violating our ATC clearance. Conditions were VFR and no other traffic was in our area.

Original NASA ASRS Text

Title: B737-800 Captain reported TCAS indicating NMAC which resulted in evasive action although ATC stated traffic indicated 1000 feet vertical separation.

Narrative: 40 miles north of RNO at 10;000 feet MSL; in a holding pattern assigned by ATC; as we were accomplishing the checklist for a flap asymmetry malfunction. A TCAS advisory had traffic at our 12:30 and 12 miles; indicating 1000 ft. above our altitude and heading toward us/converging flight paths. At five miles traffic was now 12:00 and 500 feet above our altitude. At 3 miles and 12:00 traffic was descending and indicating 300 feet above us. We took evasive action; turning away 90 degrees and descending over 700 feet. Traffic ultimately crossed to our right 500 feet above our altitude while we executed the evasive maneuver. TCAS gave a visual RA after we executed our evasive maneuver; but it gave no aural RA. We notified ATC of our actions and our subsequent return to heading and altitude. ATC stated that according to his radar traffic was 1000 feet above our altitude. I believe oncoming traffic was a general aviation aircraft off his altitude. We never acquired the traffic visually. We called out traffic early to ATC; but his radar showed a 1000 feet of clearance. We on the other hand could not risk such a near miss encounter and chose to take aggressive and decisive action to avoid the oncoming traffic; ultimately violating our ATC clearance. Conditions were VFR and no other traffic was in our area.

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	1670855
Time
Date	201908
Local Time Of Day	1201-1800
Place
Locale Reference	RNO.Airport
State Reference	NV
Environment
Flight Conditions	VMC
Aircraft 1
Make Model Name	B737-800
Operating Under FAR Part	Part 121
Flight Phase	Cruise
Flight Plan	IFR
Person 1
Function	Captain Pilot Not Flying
Qualification	Flight Crew Air Transport Pilot (ATP) Flight Crew Instrument Flight Crew Multiengine
Experience	Flight Crew Total 23000
Events
Anomaly	ATC Issue All Types Aircraft Equipment Problem Less Severe Conflict NMAC Deviation - Procedural Published Material / Policy

B737-800 captain reported TCAS indicating near midair collision which resulted in evasive action although ATC stated traffic indicated 1000 feet vertical separation.