Air carrier aircraft descends out of assigned altitude when receiving a TCASII RA. Approach control said, 'no traffic on scope.' assumed to be a false warning.

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Attributes
ACN	364884
Time
Date	199703
Day	Wed
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : mzb airport : san
State Reference	CA
Altitude	msl bound lower : 9000 msl bound upper : 9000
Environment
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : san
Operator	common carrier : air carrier
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 121
Flight Phase	cruise other
Route In Use	enroute other
Flight Plan	IFR
Aircraft 2
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Operating Under FAR Part	other : unknown
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Experience	flight time last 90 days : 200 flight time total : 4000 flight time type : 500
ASRS Report	364884
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : excursion from assigned altitude conflict : airborne less severe non adherence : clearance other anomaly other
Independent Detector	aircraft equipment other aircraft equipment : unspecified other controllera
Resolutory Action	flight crew : returned to intended course or assigned course other
Consequence	Other
Miss Distance	horizontal : 12000 vertical : 500
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

On mar/wed/97, while flying to san on the mzb 320 degree radial we received a TCASII RA with traffic depicted at 10-11 O'clock position. As required by company operations we responded to the RA by following the commands to descend. When the frequency was clear I advised the controller of the situation. We followed company operations for RA's but the controller questioned why we descended. There may be a misunderstanding of our responsibility to respond to an RA. The controller said there wasn't any traffic on his radar. Our TCASII said otherwise.

Original NASA ASRS Text

Title: ACR ACFT DSNDS OUT OF ASSIGNED ALT WHEN RECEIVING A TCASII RA. APCH CTL SAID, 'NO TFC ON SCOPE.' ASSUMED TO BE A FALSE WARNING.

Narrative: ON MAR/WED/97, WHILE FLYING TO SAN ON THE MZB 320 DEG RADIAL WE RECEIVED A TCASII RA WITH TFC DEPICTED AT 10-11 O'CLOCK POS. AS REQUIRED BY COMPANY OPS WE RESPONDED TO THE RA BY FOLLOWING THE COMMANDS TO DSND. WHEN THE FREQ WAS CLR I ADVISED THE CTLR OF THE SIT. WE FOLLOWED COMPANY OPS FOR RA'S BUT THE CTLR QUESTIONED WHY WE DSNDED. THERE MAY BE A MISUNDERSTANDING OF OUR RESPONSIBILITY TO RESPOND TO AN RA. THE CTLR SAID THERE WASN'T ANY TFC ON HIS RADAR. OUR TCASII SAID OTHERWISE.

Data retrieved from NASA's ASRS site as of July 2007 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.