Narrative:

This aircraft is 1 of 2 we have equipped with the new TCAS ii traffic avoidance system. We were vectored for ILS 19R sna at 3000' MSL, 180 KTS IAS. Just prior to G/south intercept we received a resolution advisory on the TCAS ii system. The unknown unreported aircraft first indicated it was relative position 1 O'clock position, approximately 1/2 mi and 300' below us climbing. It came an estimated 100' below and 500' to our right. We did not ever see it. The TCAS ii directed us not to descend, which we didn't. Actually, we climbed slightly. We did capture the G/south until we were clear of target. We inquired with the tower about the traffic he said there was only one 6 mi behind us. Sna is an extremely hazardous airport with numerous light aircraft in the area. This airport has numerous problems with heavy light aircraft traffic landing on a parallel runway very close to airline traffic. Short runway. Routine reduced visibility in smog. Also the required noise abatement procedures tend to keep the pilot's eyes in the cockpit instead of watching for small aircraft. Callback conversation with reporter revealed the following: the reporter states that they had no other advisory from TCAS ii or any ATC traffic advisory. He states that in his experience the TCAS ii has been extremely reliable and is known not to give false warnings. It is his opinion that the flight crew follow any advisories given by TCAS ii above 500' AGL and he believes that the ability of TCAS ii to issue advisories below 500' AGL is being disabled. He states that there is so much traffic around orange county, ca, that they are constantly being alerted by the TCAS ii equipment. It is interesting to note that the ATC could not identify the intruder aircraft either before or after the incident.

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Original NASA ASRS Text

Title: FLT CREW RECEIVED A TCAS II RESOLUTION ALERT INDICATING VERY CLOSE PROX TRAFFIC AND TCAS II ISSUED A DO NOT DESCEND TYPE ADVISORY. FLT CREW FOLLOWED ADVISORY AND NO FURTHER ACTION WAS NECESSARY.

Narrative: THIS ACFT IS 1 OF 2 WE HAVE EQUIPPED WITH THE NEW TCAS II TFC AVOIDANCE SYS. WE WERE VECTORED FOR ILS 19R SNA AT 3000' MSL, 180 KTS IAS. JUST PRIOR TO G/S INTERCEPT WE RECEIVED A RESOLUTION ADVISORY ON THE TCAS II SYS. THE UNKNOWN UNREPORTED ACFT FIRST INDICATED IT WAS RELATIVE POS 1 O'CLOCK POS, APPROX 1/2 MI AND 300' BELOW US CLBING. IT CAME AN ESTIMATED 100' BELOW AND 500' TO OUR RIGHT. WE DID NOT EVER SEE IT. THE TCAS II DIRECTED US NOT TO DSND, WHICH WE DIDN'T. ACTUALLY, WE CLBED SLIGHTLY. WE DID CAPTURE THE G/S UNTIL WE WERE CLR OF TARGET. WE INQUIRED WITH THE TWR ABOUT THE TFC HE SAID THERE WAS ONLY ONE 6 MI BEHIND US. SNA IS AN EXTREMELY HAZARDOUS ARPT WITH NUMEROUS LIGHT ACFT IN THE AREA. THIS ARPT HAS NUMEROUS PROBS WITH HEAVY LIGHT ACFT TFC LNDG ON A PARALLEL RWY VERY CLOSE TO AIRLINE TFC. SHORT RWY. ROUTINE REDUCED VISIBILITY IN SMOG. ALSO THE REQUIRED NOISE ABATEMENT PROCS TEND TO KEEP THE PLT'S EYES IN THE COCKPIT INSTEAD OF WATCHING FOR SMA. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: THE RPTR STATES THAT THEY HAD NO OTHER ADVISORY FROM TCAS II OR ANY ATC TFC ADVISORY. HE STATES THAT IN HIS EXPERIENCE THE TCAS II HAS BEEN EXTREMELY RELIABLE AND IS KNOWN NOT TO GIVE FALSE WARNINGS. IT IS HIS OPINION THAT THE FLT CREW FOLLOW ANY ADVISORIES GIVEN BY TCAS II ABOVE 500' AGL AND HE BELIEVES THAT THE ABILITY OF TCAS II TO ISSUE ADVISORIES BELOW 500' AGL IS BEING DISABLED. HE STATES THAT THERE IS SO MUCH TFC AROUND ORANGE COUNTY, CA, THAT THEY ARE CONSTANTLY BEING ALERTED BY THE TCAS II EQUIP. IT IS INTERESTING TO NOTE THAT THE ATC COULD NOT IDENT THE INTRUDER ACFT EITHER BEFORE OR AFTER THE INCIDENT.

Data retrieved from NASA's ASRS site as of August 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.