Narrative:

I had air carrier X on a 270 degree heading to intercept the sjc 30L localizer, descending to 8000'. Unk small aircraft Y appeared on my scope, sebnd at 9500'. I called this traffic to air carrier X as 2 O'clock, 2 mi, sebnd at 9500'. At this time, air carrier X data block showed him at 10100' descending, so I did not suggest a course of action due to the mode C lag, as the possibility existed that air carrier X flight was already underneath the traffic. I then lost radar contact with the traffic. The air carrier X pilot asked me for an update on the position, and I issued one based on the track as 1 O'clock, 1 mi. The air carrier X pilot stated he had the aircraft in sight and was turning hard right. The ntap data confirmed the absence of a target until just prior to the occurrence. In a phone conversation with the air carrier X pilot, he said that had traffic not been called, 'they would have dusted the other aircraft.' this incident occurred at oak ARTCC, sector ii, in an area where we have been reporting poor radar coverage for at least 2 yrs. There is major nwbnd descending turbojet and turboprop aircraft in this valley (between san jose and hollister), and also substantial nwbnd and sebnd VFR traffic, that come into conflict many times a day. This is not the first instance where traffic has failed to appear on the radar in sufficient time to prevent a problem, even though they are at an altitude sufficient to be seen (normally between 6000 and 12000'). Supplemental information from acn 166376: on descent for landing at sjc, issued a clearance to 8000'. Captain was flying. Issued a traffic alert at 2 O'clock and mileage unverified or not understood by crew--we thought 4 mi. I asked again and traffic was reported at 12 O'clock. Just around 10000' we spotted aircraft very close. Captain rolled into a 60 degree bank right and dove aircraft. It appeared that the other aircraft (a high wing small aircraft) took no evasive action. We understood from the controller he was VFR and his transponder was only giving intermittent readouts. Following the recovery, we descended below 8000' and were cleared for a visibility approach and landing at sjc. I don't think there is anyone at fault. The other aircraft was VFR. The controller issued a traffic advisory. I'm glad we requested another update as to his position--when they are that close, updating really helps, as I was still looking at the 2 O'clock position when he had moved to our 12 O'clock position. From my point of view, the aircraft would have hit if there was no evasive action. It actually helped that the captain was flying by hand, and therefore I think it was easier and quicker to take evasive action.

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Original NASA ASRS Text

Title: ACR X HAD NMAC WITH SMA Y. SEE AND AVOID CONCEPT.

Narrative: I HAD ACR X ON A 270 DEG HDG TO INTERCEPT THE SJC 30L LOC, DSNDING TO 8000'. UNK SMA Y APPEARED ON MY SCOPE, SEBND AT 9500'. I CALLED THIS TFC TO ACR X AS 2 O'CLOCK, 2 MI, SEBND AT 9500'. AT THIS TIME, ACR X DATA BLOCK SHOWED HIM AT 10100' DSNDING, SO I DID NOT SUGGEST A COURSE OF ACTION DUE TO THE MODE C LAG, AS THE POSSIBILITY EXISTED THAT ACR X FLT WAS ALREADY UNDERNEATH THE TFC. I THEN LOST RADAR CONTACT WITH THE TFC. THE ACR X PLT ASKED ME FOR AN UPDATE ON THE POS, AND I ISSUED ONE BASED ON THE TRACK AS 1 O'CLOCK, 1 MI. THE ACR X PLT STATED HE HAD THE ACFT IN SIGHT AND WAS TURNING HARD RIGHT. THE NTAP DATA CONFIRMED THE ABSENCE OF A TARGET UNTIL JUST PRIOR TO THE OCCURRENCE. IN A PHONE CONVERSATION WITH THE ACR X PLT, HE SAID THAT HAD TFC NOT BEEN CALLED, 'THEY WOULD HAVE DUSTED THE OTHER ACFT.' THIS INCIDENT OCCURRED AT OAK ARTCC, SECTOR II, IN AN AREA WHERE WE HAVE BEEN RPTING POOR RADAR COVERAGE FOR AT LEAST 2 YRS. THERE IS MAJOR NWBND DSNDING TURBOJET AND TURBOPROP ACFT IN THIS VALLEY (BTWN SAN JOSE AND HOLLISTER), AND ALSO SUBSTANTIAL NWBND AND SEBND VFR TFC, THAT COME INTO CONFLICT MANY TIMES A DAY. THIS IS NOT THE FIRST INSTANCE WHERE TFC HAS FAILED TO APPEAR ON THE RADAR IN SUFFICIENT TIME TO PREVENT A PROB, EVEN THOUGH THEY ARE AT AN ALT SUFFICIENT TO BE SEEN (NORMALLY BTWN 6000 AND 12000'). SUPPLEMENTAL INFO FROM ACN 166376: ON DSNT FOR LNDG AT SJC, ISSUED A CLRNC TO 8000'. CAPT WAS FLYING. ISSUED A TFC ALERT AT 2 O'CLOCK AND MILEAGE UNVERIFIED OR NOT UNDERSTOOD BY CREW--WE THOUGHT 4 MI. I ASKED AGAIN AND TFC WAS RPTED AT 12 O'CLOCK. JUST AROUND 10000' WE SPOTTED ACFT VERY CLOSE. CAPT ROLLED INTO A 60 DEG BANK RIGHT AND DOVE ACFT. IT APPEARED THAT THE OTHER ACFT (A HIGH WING SMA) TOOK NO EVASIVE ACTION. WE UNDERSTOOD FROM THE CTLR HE WAS VFR AND HIS XPONDER WAS ONLY GIVING INTERMITTENT READOUTS. FOLLOWING THE RECOVERY, WE DSNDED BELOW 8000' AND WERE CLRED FOR A VIS APCH AND LNDG AT SJC. I DON'T THINK THERE IS ANYONE AT FAULT. THE OTHER ACFT WAS VFR. THE CTLR ISSUED A TFC ADVISORY. I'M GLAD WE REQUESTED ANOTHER UPDATE AS TO HIS POS--WHEN THEY ARE THAT CLOSE, UPDATING REALLY HELPS, AS I WAS STILL LOOKING AT THE 2 O'CLOCK POS WHEN HE HAD MOVED TO OUR 12 O'CLOCK POS. FROM MY POINT OF VIEW, THE ACFT WOULD HAVE HIT IF THERE WAS NO EVASIVE ACTION. IT ACTUALLY HELPED THAT THE CAPT WAS FLYING BY HAND, AND THEREFORE I THINK IT WAS EASIER AND QUICKER TO TAKE EVASIVE ACTION.

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.