SMA X HAD NMAC WITH C310 IN TFC PATTERN. EVASIVE ACTION TAKEN. SEE AND AVOID.

Narrative

SMA X ESTABLISHED ON BASE LEG FOR VAN NUYS ARPT RWY 16R; WE RPTED AT CALIFORNIA STATE NORTHRIDGE (OUR POS) AS INSTRUCTED BY THE TWR. ANOTHER CESSNA SINGLE ALSO RPTED THE SAME POS A MOMENT LATER. THAT ACFT WAS AT OUR 3 O'CLOCK POS (S OF THE RPTING POINT). I WAS INSTRUCTED TO CONTINUE EBOUND WITHOUT A SEQUENCE NUMBER; THE OTHER CESSNA SINGLE WAS TOLD TO TURN TO THE RWY AND LAND. I WAS THEN INSTRUCTED BY THE TWR TO TURN L TO THE DOWNWIND (AT THIS POINT WE WERE NOT AT THE DOWNWIND; WE WERE NEAR FINAL APCH); AND THAT TFC WAS A CESSNA 310 ON FINAL. AT THE MOMENT THAT THE CTLR INSTRUCTED US TO TURN; WE INITIATED THE TURN AND SAW THE CESSNA 310 TFC AS THE CTLR CALLED OUT ITS POS. WE TURNED OUR ACFT STEEPLY AND THE CESSNA 310 PLT ALSO TOOK EVASIVE ACTION BY ALTERING COURSE SLIGHTLY. OUR ACFT WAS CLOSING AT APPROX 200 KTS; AND THE CESSNA 310 PASSED 75 FT ABOVE US AND 200 FT LATERALLY. THE WHOLE OCCURRENCE COULD HAVE BEEN PREVENTED: 1) HAD THE CESSNA SINGLE PROPERLY RPTED ITS POS; 2) HAD THE CTLR ISSUED US A SEQUENCE NUMBER; 3) HAD THE CTLR INSTRUCTED US TO TURN DOWNWIND EARLIER.