NMAC ALTDEV ALT EXCURSION.

Narrative

WHILE BEING VECTORED FOR AN APCH TO OAK ARPT; WE WERE CLRED TO MAINTAIN 11000 FT MSL. LEVEL AT 11000 FT MSL WE GOT A TA ON OUR TCASII. AS WE LOOKED FOR THE TFC; WE GOT AN RA TO CLB. TFC WAS AT OUR 1-2 O'CLOCK POS SHOWING 200 FT BELOW AND CLBING. WE LOOKED; ATTEMPTED TO VERIFY WITH OAK APCH; AND FOLLOWED THE RA; CLBING TO 11500 MSL. BY THIS TIME OAK APCH HAD BEEN NOTIFIED OF ALT EXCURSION; AND THEY INDICATED NO TFC HAD BEEN OBSERVED. WE WONDERED WHY THE ACFT SHOWED A RESPONSE ON OUR TCASII; BUT NOT ON APCH CTL RADAR; BUT GOT NO ANSWER. WE OBSERVED SINGLE ENG SMA Y PASS BELOW OUR ACFT WITH THE TCASII; INDICATING A 200 FT SEPARATION. AFTER COMPLYING WITH RA; WE RETURNED TO 11000 MSL AND FURTHER RPTED SITUATION TO OAK ATC. SINCE A PREVIOUS EXPERIENCE; SIMILAR TO THIS ONE; OCCURRED A MONTH PREVIOUSLY; I WANTED TO KNOW HOW THE OTHER CREW MEMBERS FELT. THEY (FO AND SO) INDICATED A DISBELIEF OF OAK'S RADAR CTLR FAILURE OF PICKING UP THE SMA Y TRANSPONDER REPLY; SINCE OUR TCASII RECEIVED SUFFICIENT DATA TO GIVE US THE APPROPRIATE WARNING. IN ADDITION; WE ALL AGREED THAT THE TCASII HAD SAVED OUR ACFT AND OUR LIVES FROM A MIDAIR COLLISION (NO EXAGGERATION!). WE THINK THE 2 COLOR TRIM SMA WAS FLYING VFR UNAWARE OF HIS CLOSE PROX TO THE INBOUND ARR GATE USED BY OAK (AND SFO) AND WE HAD TO WONDER IF THIS SMA PLT WAS AS SHAKEN BY THE NEAR MISS AS WE WERE. IF THE CTLED AIRSPACE SYS IS GOING TO BE USED AND RELIED ON; THEN ATC MUST HAVE A BETTER GRIP ON 'THEIR' AIRSPACE -- OTHERWISE; DO AWAY WITH THE CTLED AIRSPACE AND RETURN TO A COMPLETELY 'SEE AND AVOID -- EVERYONE TAKES THEIR CHANCES' SYS.