NMAC OCCURS IN TFC PATTERN AT CRQ WHEN C310 TURNS BASE INTO THE PATH OF AN SMT ON A STRAIGHT-IN ILS APCH.

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I WAS PROVIDING FLT INSTRUCTION TO A NEW OWNER/PLT WHO IS MULTI-ENG RATED WITH SEVERAL HRS OF PREVIOUS INSTRUCTION BUT NO PRIOR TIME IN A 310. WE WERE IN R CLOSED TFC; FULL STOP LNDGS; TAXI BACK. ON THE THIRD TRIP AROUND THE PATTERN; WE WERE ON R DOWNWIND (EBOUND) CONFIGURED FOR LNDG. THERE WAS TFC INBOUND TO THE ARPT FROM ALL DIRECTIONS WHICH WAS KEEPING THE LCL CTLR QUITE BUSY. RWY IN USE WAS RWY 24. WE CONTINUED EBOUND FOR APPROX 3 MI BEYOND THE END OF THE RWY AT 2000 FT MSL. WITH THE CONGESTION ON THE FREQ THERE WAS NO CHANCE TO RPT DOWNWIND ABEAM THE TWR OR ANYWHERE NEAR IT. THERE WERE NO INSTRUCTIONS FROM LCL CTL TO EXTEND DOWNWIND; PROVIDE SEQUENCE; OR OTHER. THERE APPEARED TO BE NO TFC TO FOLLOW AND NONE OBSERVED INBOUND FROM THE E. IN ORDER TO PREVENT STRETCHING THE PATTERN WAY OUT; I TURNED R BASE AND ADVISED LCL CTL WE WERE TURNING R BASE. WE WERE IMMEDIATELY ADVISED TO CLB SBOUND AND CROSS THE FINAL APCH COURSE. THERE WAS A TWIN FLYING THE ILS THAT PRIOR TO TURNING R BASE WE DID NOT OBSERVE. I NEVER HEARD ANYTHING TO ALERT ME THERE WAS TFC INBOUND ON THE ILS. HOWEVER; AS WE CLBED AS INSTRUCTED WE DID SEE THE TWIN PASS DIRECTLY UNDER US PROBABLY 150-200 FT BELOW. OBVIOUSLY; HAD WE OBSERVED THIS TFC WE WOULD HAVE WAITED UNTIL IT WENT BY BEFORE MAKING ANY TURNS. THIS TFC THEN FELT HE HAD A CONFLICT WITH A SINGLE ENG ACFT THAT LCL CTL HAD SEQUENCED IN FRONT OF HIM. THE TFC THEN ANNOUNCED HE WAS DEPARTING THE AREA FOR AN ARPT THAT HAD COMPETENT CTLRS AND THAT HE INTENDED TO FILE A COMPLAINT. THE CTLR LATER TOLD ME ON THE PHONE THAT I SHOULD NEVER TURN BASE WITHOUT HAVING BEEN GIVEN A SEQUENCE. TO MY KNOWLEDGE; THESE TYPES OF SITS ARE NOT COVERED IN AIM. HOWEVER; HE HAS A VALID POINT; BUT THE QUESTION IS; HOW LONG DO YOU WAIT BEFORE DOING SOMETHING? CONTRIBUTING FACTORS: THIS IS A SINGLE RWY BUSY ARPT WITH A MIX OF EVERYTHING FROM STUDENTS TO BIZJETS. THERE IS A LOT OF FREQ CONGESTION AND IT CAN GO FROM VERY LIGHT TO HVY TFC IN MINS. THERE IS A MIX OF STRAIGHT-IN PRACTICE ILS AND GPS APCH TFC WITH BOTH R AND L TFC PATTERNS. THERE ARE ALSO ARRS FROM SEVERAL RPTING POINTS. I THINK THE CTLR SHOULD HAVE INSTRUCTED US TO TURN BACK TO THE DOWNWIND INSTEAD OF XING THE FINAL APCH COURSE. THERE WOULD HAVE BEEN NO OVERFLT AND BETTER SEPARATION EVEN IF IT STILL WAS LESS THAN STANDARD. THE TWIN WAS IN VFR CONDITIONS AND TOOK NO OBSERVABLE EVASIVE ACTION ALTHOUGH IF I WERE HIM; I WOULD HAVE GONE BELOW GS TO GUARANTEE SEPARATION WHICH HE PROBABLY DID. IN THE FUTURE; IT IS MY RULE TO REMAIN IN THE DOWNWIND LEG UNTIL WE ARE SEQUENCED OR OTHERWISE CLRED EVEN IF THE DOWNWIND LEG BECOMES MUCH LONGER THAN IT SHOULD. I DO NOT BELIEVE WE VIOLATED ANY FARS AND WERE IN STRICT COMPLIANCE WITH ANY AND ALL INSTRUCTIONS.