A FLC OF A C208 CARAVAN ATTEMPTS A VISUAL SEPARATION CLB BUT GETS TOO CLOSE TO A LEVEL ACR BA3200 AND OVERFLIES THAT ACFT BY 100-300 FT 10 MI W OF SMF; CA.

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WHILE DEPARTING SMF; VFR TO THE W; WE WERE RECEIVING RADAR ADVISORIES FROM SACRAMENTO DEP CTL. DURING OUR CLB; WE WERE INSTRUCTED TO REMAIN AT OR BELOW 3500 FT FOR AN ARRIVING COMMUTER XING AT 4000 FT AND TO RPT THE TFC IN SIGHT. WE LEVELED AT 3500 FT AND SUBSEQUENTLY ADVISED ATC THAT WE HAD THE COMMUTER IN SIGHT. WE WERE THEN INSTRUCTED TO MAINTAIN VISUAL SEPARATION AND CLB AT OUR DISCRETION. THE COMMUTER APPEARED TO BE DSNDING AND MOVING TO OUR REAR AND WE DETERMINED THAT A CLB WOULD RESULT IN US BEING WELL ABOVE AND AHEAD OF HIM. AS WE GOT CLOSER; WE REALIZED THAT THE COMMUTER WAS STILL AT ABOUT 4000 FT AND THAT THERE WAS NOT GOING TO BE AS MUCH SEPARATION AS WE INITIALLY THOUGHT; SO I INCREASED OUR RATE OF CLB. AS THE COMMUTER PASSED BELOW US; I NOTED OUR ALT CLBING THROUGH 4500 FT. IT WAS THEN THAT THE PLT OF THE COMMUTER STATED THAT HE HAD US IN SIGHT; THAT HE HAD TO 'DEVIATE SLIGHTLY' FROM HIS ALT; AND THAT HIS TCASII SHOWED ONLY 100 FT OF SEPARATION. FROM MY VISUAL OBSERVATION; I BELIEVE THAT AT THE CLOSEST; THE COMMUTER PASSED ABOUT 400-500 FT BELOW AND 400-500 FT BEHIND US. WHILE I DO NOT BELIEVE THERE WAS EVER ANY SERIOUS COLLISION HAZARD; I DO NOT THINK THAT THERE WAS AS MUCH SEPARATION AS ANY OF US WOULD HAVE LIKED. I FEEL THAT ATC SHOULD HAVE EITHER HELD US AT 3500 FT UNTIL PAST THE COMMUTER; OR DSNDED THE COMMUTER WHEN WE WERE GIVEN PERMISSION TO CLB. I ALSO FEEL THAT THE CREW OF THE COMMUTER SHOULD HAVE HAD US IN SIGHT EARLIER THAN THEY DID; AND THAT THEY MADE A SLIGHT COURSE ADJUSTMENT THAT DIMINISHED LATERAL SEPARATION. A SIGNIFICANT CONTRIBUTING FACTOR WAS MY ERRONEOUS INITIAL IMPRESSION THAT THE COMMUTER WAS IN A DSCNT. EVEN FROM THE 3500 FT ALT AND A DISTANCE OF ABOUT 3 MI; WE APPEARED TO BE LOOKING AT THE TOP OF THE COMMUTER ACFT; A PERSPECTIVE THAT REMAINED UNTIL OUR PASSING. HAD I REALIZED THAT THE COMMUTER WAS IN LEVEL FLT; I WOULD HAVE INITIATED THE CLB MORE AGGRESSIVELY. I HAVE LEARNED THE IMPORTANCE OF MAINTAINING VISUAL CONTACT WITH POTENTIALLY CONFLICTING TFC -- THAT EVEN WHEN IT APPEARS THAT THE POTENTIAL IS MINIMAL; IT DOES NOT TAKE MUCH FOR THE SIT TO CHANGE RAPIDLY.