Narrative:

On approach to sfo in VMC conditions, under radar vectors for a visual to runway 28R, flew downwind at 6000. At approximately 13 DME we were given right turn to 190 degrees and descend to 4000 ft. Made turn and leveled at 4000 ft. Observed lights of another aircraft about 11 O'clock approximately same altitude. We then got a TA from TCASII. First officer inquired about traffic to approach control, who commented he had been trying to turn us, then gave us a descent to 3000. At same time we got an RA from TCASII commanding a descent. We went under other aircraft, who was on vectors for visual to 28L. We then were given right turn back to runway 28R. Other aircraft landed visually on 28L ahead of us. We heard no change in our vector heading, before our query about traffic. Either controller was in error and missed giving us our turn to final (we had not yet been cleared for the visual) or possibly our aircraft had a stuck microphone blocking out our reception. However had had no previous problem during a 7 hour flight, and had no subsequent problem of that nature. Since we are not really sure what caused the problem, I cannot make a recommendation for correction. TCASII worked as advertised and would have prevented accident if controller had not caught situation when he did. I am convinced of the value of TCASII and the necessity to follow its command/guidance. Supplemental information from acn 251176: in excellent visibility, we observed traffic in our 10-11 O'clock position. Shortly thereafter we received a TA on our TCASII. I questioned approach control about the traffic and almost immediately, received an RA on our TCASII. The controller responded to my query, and implied that he had tried to turn us, 'a couple of times.' we, the entire cockpit crew, heard no calls from approach control after our turn to 190 degrees. My best guess: either approach control didn't call us or we had a momentary communication failure, stuck microphone, etc. Approach made no mention of evidence of a stuck microphone once communication was re- established.

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Original NASA ASRS Text

Title: POTENTIAL CONFLICT RESOLVED BY APCH CTLR AFTER RADIO CONTACT RE-ESTABLISHED WITH ACR WDB ON RADAR VECTORING. TCASII RA CHIMES IN AT MOMENT OF AMENDED CLRNC ALT CHANGE.

Narrative: ON APCH TO SFO IN VMC CONDITIONS, UNDER RADAR VECTORS FOR A VISUAL TO RWY 28R, FLEW DOWNWIND AT 6000. AT APPROX 13 DME WE WERE GIVEN R TURN TO 190 DEGS AND DSND TO 4000 FT. MADE TURN AND LEVELED AT 4000 FT. OBSERVED LIGHTS OF ANOTHER ACFT ABOUT 11 O'CLOCK APPROX SAME ALT. WE THEN GOT A TA FROM TCASII. FO INQUIRED ABOUT TFC TO APCH CTL, WHO COMMENTED HE HAD BEEN TRYING TO TURN US, THEN GAVE US A DSCNT TO 3000. AT SAME TIME WE GOT AN RA FROM TCASII COMMANDING A DSCNT. WE WENT UNDER OTHER ACFT, WHO WAS ON VECTORS FOR VISUAL TO 28L. WE THEN WERE GIVEN R TURN BACK TO RWY 28R. OTHER ACFT LANDED VISUALLY ON 28L AHEAD OF US. WE HEARD NO CHANGE IN OUR VECTOR HDG, BEFORE OUR QUERY ABOUT TFC. EITHER CTLR WAS IN ERROR AND MISSED GIVING US OUR TURN TO FINAL (WE HAD NOT YET BEEN CLRED FOR THE VISUAL) OR POSSIBLY OUR ACFT HAD A STUCK MIKE BLOCKING OUT OUR RECEPTION. HOWEVER HAD HAD NO PREVIOUS PROB DURING A 7 HR FLT, AND HAD NO SUBSEQUENT PROB OF THAT NATURE. SINCE WE ARE NOT REALLY SURE WHAT CAUSED THE PROB, I CANNOT MAKE A RECOMMENDATION FOR CORRECTION. TCASII WORKED AS ADVERTISED AND WOULD HAVE PREVENTED ACCIDENT IF CTLR HAD NOT CAUGHT SIT WHEN HE DID. I AM CONVINCED OF THE VALUE OF TCASII AND THE NECESSITY TO FOLLOW ITS COMMAND/GUIDANCE. SUPPLEMENTAL INFO FROM ACN 251176: IN EXCELLENT VISIBILITY, WE OBSERVED TFC IN OUR 10-11 O'CLOCK POS. SHORTLY THEREAFTER WE RECEIVED A TA ON OUR TCASII. I QUESTIONED APCH CTL ABOUT THE TFC AND ALMOST IMMEDIATELY, RECEIVED AN RA ON OUR TCASII. THE CTLR RESPONDED TO MY QUERY, AND IMPLIED THAT HE HAD TRIED TO TURN US, 'A COUPLE OF TIMES.' WE, THE ENTIRE COCKPIT CREW, HEARD NO CALLS FROM APCH CTL AFTER OUR TURN TO 190 DEGS. MY BEST GUESS: EITHER APCH CTL DIDN'T CALL US OR WE HAD A MOMENTARY COM FAILURE, STUCK MIKE, ETC. APCH MADE NO MENTION OF EVIDENCE OF A STUCK MIKE ONCE COM WAS RE- ESTABLISHED.

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.