Medium large transport on approach has TCASII alert. ATC says no aircraft. False TCASII?

Attributes
ACN	274172
Time
Date	199406
Day	Thu
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : sna
State Reference	CA
Altitude	msl bound lower : 5000 msl bound upper : 5300
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : sna
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 : on vectors
Flight Plan	IFR
Aircraft 2
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Operating Under FAR Part	other : unknown
Flight Phase	climbout : intermediate altitude
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 175 flight time total : 15000 flight time type : 3000
ASRS Report	274172
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument pilot : atp
Events
Anomaly	altitude deviation : crossing restriction not met non adherence : clearance other anomaly other
Independent Detector	aircraft equipment other aircraft equipment : unspecified other flight crewa
Resolutory Action	flight crew : took evasive action other
Consequence	faa : reviewed incident with flight crew
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	Pilot Deviation other

Narrative:

Possible false TCASII RA. We were heading 090 on vectors for approach to runway 19 at sna, level at 5000 ft. Had numerous TCASII traffic, one about 5 mi at 12 O'clock 3000 ft below us. As this target drew nearer another target appeared nearly on top of it, but showing 500-600 ft below us. With both targets showing TCASII went to RA for 500 ft one. First officer advised we were climbing as we scanned outside. Aural warning was to climb, so I disconnected autoplt and autothrottles, added power and started climb. Scanned for green lights on vvi and there were no lights. Realized there was also no longer a climb command, so looked at TCASII and there was no 500 ft low target. Still had 3000 ft almost directly below us. Started return to 5000 ft and advised socal of same. Center stated 3000 ft below had set off RA, but we distinctly had 2 targets. Neither first officer nor I ever saw it visually however. After level at 5000 ft, target again appeared, now behind us and 600 ft above. Did not trigger TA or RA. Previous to encounter I did see a large, gray cargo vessel which would have been almost underneath us during this time. ATC should know that traffic 3000 ft below would not trigger TCASII.

Original NASA ASRS Text

Title: MLG ON APCH HAS TCASII ALERT. ATC SAYS NO ACFT. FALSE TCASII?

Narrative: POSSIBLE FALSE TCASII RA. WE WERE HDG 090 ON VECTORS FOR APCH TO RWY 19 AT SNA, LEVEL AT 5000 FT. HAD NUMEROUS TCASII TFC, ONE ABOUT 5 MI AT 12 O'CLOCK 3000 FT BELOW US. AS THIS TARGET DREW NEARER ANOTHER TARGET APPEARED NEARLY ON TOP OF IT, BUT SHOWING 500-600 FT BELOW US. WITH BOTH TARGETS SHOWING TCASII WENT TO RA FOR 500 FT ONE. FO ADVISED WE WERE CLBING AS WE SCANNED OUTSIDE. AURAL WARNING WAS TO CLB, SO I DISCONNECTED AUTOPLT AND AUTOTHROTTLES, ADDED PWR AND STARTED CLB. SCANNED FOR GREEN LIGHTS ON VVI AND THERE WERE NO LIGHTS. REALIZED THERE WAS ALSO NO LONGER A CLB COMMAND, SO LOOKED AT TCASII AND THERE WAS NO 500 FT LOW TARGET. STILL HAD 3000 FT ALMOST DIRECTLY BELOW US. STARTED RETURN TO 5000 FT AND ADVISED SOCAL OF SAME. CTR STATED 3000 FT BELOW HAD SET OFF RA, BUT WE DISTINCTLY HAD 2 TARGETS. NEITHER FO NOR I EVER SAW IT VISUALLY HOWEVER. AFTER LEVEL AT 5000 FT, TARGET AGAIN APPEARED, NOW BEHIND US AND 600 FT ABOVE. DID NOT TRIGGER TA OR RA. PREVIOUS TO ENCOUNTER I DID SEE A LARGE, GRAY CARGO VESSEL WHICH WOULD HAVE BEEN ALMOST UNDERNEATH US DURING THIS TIME. ATC SHOULD KNOW THAT TFC 3000 FT BELOW WOULD NOT TRIGGER TCASII.

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.