Narrative:

On the modesto 2 arrival, going to sfo, we (aircraft X) noted a yellow TA symbol on TCASII at the 1:30 O'clock position, traveling toward us rapidly. This symbol was accompanied by the 'traffic, traffic!' advisory. We sighted the aircraft Y traffic, a low wing experimental type, approximately 1/4 SM and 100 ft below. I (aircraft X) then initiated a rapid climb to 9400 ft, missing the aircraft Y by about 200 ft, as it passed directly below. The aircraft Y involved did not have mode C transponder capabilities, because the yellow TA had no altitude information. The aircraft Y appeared to be in cruise flight, at an IFR altitude, headed in the wrong direction. I do not believe that the aircraft Y was under the class B veil. ATC knew nothing of this aircraft Y. It would be to the flying public's benefit if all aircraft require mode C capabilities. Also, if the GA group were more educated on arrival/departure rtes in congested airspace, this incident could have been prevented.

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Original NASA ASRS Text

Title: FLC IN SMALL ACR INBOUND TO SFO VIA THE MOD RTE ENCOUNTER SMA ACFT WITH NO MODE C AND ARE REQUIRED TO TAKE EVASIVE ACTION.

Narrative: ON THE MODESTO 2 ARR, GOING TO SFO, WE (ACFT X) NOTED A YELLOW TA SYMBOL ON TCASII AT THE 1:30 O'CLOCK POS, TRAVELING TOWARD US RAPIDLY. THIS SYMBOL WAS ACCOMPANIED BY THE 'TFC, TFC!' ADVISORY. WE SIGHTED THE ACFT Y TFC, A LOW WING EXPERIMENTAL TYPE, APPROX 1/4 SM AND 100 FT BELOW. I (ACFT X) THEN INITIATED A RAPID CLB TO 9400 FT, MISSING THE ACFT Y BY ABOUT 200 FT, AS IT PASSED DIRECTLY BELOW. THE ACFT Y INVOLVED DID NOT HAVE MODE C XPONDER CAPABILITIES, BECAUSE THE YELLOW TA HAD NO ALT INFO. THE ACFT Y APPEARED TO BE IN CRUISE FLT, AT AN IFR ALT, HEADED IN THE WRONG DIRECTION. I DO NOT BELIEVE THAT THE ACFT Y WAS UNDER THE CLASS B VEIL. ATC KNEW NOTHING OF THIS ACFT Y. IT WOULD BE TO THE FLYING PUBLIC'S BENEFIT IF ALL ACFT REQUIRE MODE C CAPABILITIES. ALSO, IF THE GA GROUP WERE MORE EDUCATED ON ARR/DEP RTES IN CONGESTED AIRSPACE, THIS INCIDENT COULD HAVE BEEN PREVENTED.

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.