Flight crew of an mdt descended from assigned altitude in response to a false TCASII warning.

Attributes
ACN	314349
Time
Date	199508
Day	Tue
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : vkz airport : pbi
State Reference	FL
Altitude	msl bound lower : 6500 msl bound upper : 8000
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Controlling Facilities	tracon : pbi tower : mie
Operator	common carrier : air carrier
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 121
Navigation In Use	Other Other
Flight Phase	cruise other descent other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 200 flight time total : 14000 flight time type : 4000
ASRS Report	314349
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : excursion from assigned altitude non adherence : clearance other anomaly other
Independent Detector	aircraft equipment other aircraft equipment : unspecified other flight crewa
Resolutory Action	flight crew : overcame equipment problem
Consequence	faa : reviewed incident with flight crew Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

During instrument scan, noted open diamond (aircraft symbol) with zero altitude overlying our aircraft (center of TCASII display). This was followed by TA alert, then an RA alert to descend! This was re-enforced quickly by increase command. During this time conflicting target continued to show zero separation with a descent. ATC advised and were told 'there was no traffic within 5 mi.' after descending 1500 ft, had reason to doubt the TCASII. Selected 'standby' and back on unit didn't have any conflict. Reported malfunction to company maintenance.

Original NASA ASRS Text

Title: FLC OF AN MDT DSNDED FROM ASSIGNED ALT IN RESPONSE TO A FALSE TCASII WARNING.

Narrative: DURING INST SCAN, NOTED OPEN DIAMOND (ACFT SYMBOL) WITH ZERO ALT OVERLYING OUR ACFT (CTR OF TCASII DISPLAY). THIS WAS FOLLOWED BY TA ALERT, THEN AN RA ALERT TO DSND! THIS WAS RE-ENFORCED QUICKLY BY INCREASE COMMAND. DURING THIS TIME CONFLICTING TARGET CONTINUED TO SHOW ZERO SEPARATION WITH A DSCNT. ATC ADVISED AND WERE TOLD 'THERE WAS NO TFC WITHIN 5 MI.' AFTER DSNDING 1500 FT, HAD REASON TO DOUBT THE TCASII. SELECTED 'STANDBY' AND BACK ON UNIT DIDN'T HAVE ANY CONFLICT. RPTED MALFUNCTION TO COMPANY MAINT.

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.