NMAC BTWN AN MLG ON DEP AND A LEAR JET ON ARR. THE ARR LEAR JET WAS DSNDING HEAD-ON AND THE TCASII PROVIDED A RESOLUTION FOR THE RPTING MLG TO TAKE APPROPRIATE EVASIVE ACTION.

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DURING CLBOUT FROM SBA; FLYING RWY HDG 254 DEGS; CLRED TO 8000 FT MSL. AT APPROX 2500 FT WE RECEIVED AN AUDIBLE TA FROM OUR TCASII SYS. THE SCREEN SHOWED +4 (400 FT ABOVE US) BESIDE AN AMBER TARGET AT 12 O'CLOCK (DIRECTLY AHEAD). I LOOKED; BUT COULD NOT SEE THE TFC. VERY SHORTLY THEREAFTER; TCASII GAVE US AN AUDIBLE 'DSND' ALERT; AND OUR IVSI DIRECTED A DSCNT IN EXCESS OF 1000 FPM. I PULLED THROTTLES TO IDLE AND ESTABLISHED A 1200-1500 FPM DSCNT RATE WHILE DIRECTING THE FO TO TELL SBA DEP (120.55) THAT WE WERE COMPLYING WITH AN RA. THE FO WAS UNABLE TO DO SO IMMEDIATELY BECAUSE OF FREQ CONGESTION. WE ACQUIRED THE TFC VISUALLY. IT WAS AN OFF-WHITE; LOW WING AIRPLANE WITH RETRACTED LNDG GEAR AND TIP TANKS; AND IT PASSED DIRECTLY OVERHEAD; 200-300 FT ABOVE US. THE SMALLEST NUMBER ON THE TCASII SCREEN THAT I SAW WAS +2 (200 FT ABOVE). AFTER TFC HAD CLRED US; WE RESUMED OUR CLB -- WE DSNDED TO APPROX 2000 FT. SBA DEP ASKED IF WE WERE COMPLYING WITH OUR CLB CLRNC. THE FO REPLIED THAT WE HAD RESPONDED TO AN RA BUT WERE CLR OF TFC AND RESUMING OUR CLB. THE CTLR REPLIED; 'SORRY ABOUT THAT.' ONCE WE WERE ABOVE 10000 FT. I MADE A PA ANNOUNCEMENT TO TELL THE PAX WHAT HAD HAPPENED. OUR 'A' FLT ATTENDANT CAME UP TO THE COCKPIT TO TELL US THAT ALL 3 FLT ATTENDANTS WERE STILL SEATED DURING OUR MANEUVER; AND EVERYONE WAS ALL RIGHT. IN ANY CASE; THE MANEUVER WAS DONE SMOOTHLY ENOUGH; SO I DON'T THINK ANYONE WOULD HAVE BEEN INJURED; EVEN IF THEY HAD BEEN STANDING; THOUGH THE UNEXPECTED DSCNT WAS ALARMING. AFTER ARRIVING IN SAN FRANCISCO; I FILED A NMAC RPT WITH FAA CTLR AREA MGR; ZOA. LATER THAT AFTERNOON; I RECEIVED A MESSAGE FROM MY COMPANY TO CALL CTLR AT THE SBA TWR. HE TOLD ME THAT OUR TFC WAS A LIFEGUARD LEARJET INBOUND TO SBA. HE SAID THAT THE CTLR HAD CLRED US FOR A CLB IN VMC TO 17000 FT; MAINTAIN FL230; BUT THAT WE HAD NOT RESPONDED. I TOLD HIM THAT WE WERE PROBABLY COMPLETELY PREOCCUPIED WITH AVOIDING THE LEARJET AND DIDN'T HEAR THE CLRNC. WHEN THE INCIDENT OCCURRED; WE HAD NOT ACCEPTED A VMC CLB CLRNC. ATC CTLR SAID THEY WERE INVESTIGATING AND HAD REPLAYED THE TAPES. WHILE THE CTLR OBSERVED NO LESS THAN 1000-1200 FT VERT SEPARATION; THE REPLAY SHOWED A MINIMUM 300 FT SEPARATION. THE LEARJET WAS BEING HANDLED BY THE SAME CTLR AS OUR ACFT. I'D LIKE TO KNOW WHY 2 ACFT BEING HANDLED BY ATC WERE PERMITTED TO GET HEAD-ON WITH SUCH A SMALL VERT AND NO HORIZ SEPARATION. OUR EVASIVE ACTION WORKED THIS TIME; AND I AM EXTREMELY PLEASED WITH OUR TCASII SYS WHICH WAS ABLE TO DIRECT US AWAY FROM A POSSIBLE COLLISION. FORTUNATELY THE LEARJET HAD A FUNCTIONING ALT ENCODING XPONDER WHICH ENABLED OUR TCASII SYS TO DIRECT AN AVOIDANCE MANEUVER.