INSTRUCTOR WITH SPI IN TFC PATTERN HAS NMAC WITH ACFT ON IFR APCH INTO CLOSE BY ARPT.

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I WAS PRACTICING TOUCH AND GO LNDGS WITH A STUDENT. WE WERE FLYING A CESSNA 152 IN A L CLOSED TFC PATTERN AT CAMARILLO; CALIFORNIA; ARPT. WE HAD COMPLETED SEVERAL PATTERNS WHEN THE TWR INSTRUCTED US TO 'EXTEND OUR UPWIND 1/2 MI FOR SPACING.' WE ACKNOWLEDGED THIS INSTRUCTION; AND COMPLIED WITH THE INSTRUCTION. AFTER PASSING OUR 1/2 MI EXTENSION; WE TURNED L XWIND. IMMEDIATELY AFTER TURNING XWIND; THE TWR CTLR ISSUED AN URGENT TFC ALERT FOR A TWIN CESSNA 'ON THE VOR 25 APCH TO OXNARD ARPT.' BEFORE I COULD ACKNOWLEDGE THIS ALERT; I SAW A CESSNA 425 ACFT PASS OVER MY ACFT; APPROX 50 FT DIRECTLY ABOVE ME. I WAS ABLE TO OBSERVE THIS BECAUSE MY ACFT WAS EQUIPPED WITH OVERHEAD SKYLIGHT WINDOWS. THERE WAS NO POSSIBLE TIME TO TAKE EVASIVE ACTION. THIS WAS CAUSED BY SEVERAL FACTORS. WX WAS MARGINAL VFR (1400 FT BROKEN; VISIBILITY 10 MI). LCL TWR CTLR WAS DISTRACTED BY OTHER TOUCH AND GO PATTERN TFC. THE ACFT THAT ALMOST HIT ME (CESSNA 425) WAS ON THE VOR 25 APCH TO OXNARD; CA. THIS APCH USES THE CAMARILLO VOR; AND ACFT OVERFLY CAMARILLO ON THIS APCH. THIS APCH MANDATES AN ALT OF 1700 FT MSL UNTIL PASSING THE CMA VOR. AFTER THAT; AN ACFT MAY DSND DOWN TO 480 FT MSL. BECAUSE WE WERE EXTENDED; THE C425 ON APCH HAD DSNDED SEVERELY DOWN TO APPROX 800 FT MSL. CAMARILLO DOES NOT HAVE RADAR IN THE TWR CAB. THEY HAVE TO RELY ON 'STRIPS' ACQUIRED OVER THE PHONE OR RADIO FROM NEARBY TWRS/APCH CTL. MY SUGGESTION IS TO CHANGE THE VOR 25 APCH TO OXNARD IN THE FOLLOWING MANNER. 1) MDA TO CMA VOR UNCHANGED -- 1700 FT MSL. 2) MDA TO CMA DME 1000 FT MSL. 3) MDA TO CMA DME 480 FT MSL. THIS WOULD PROTECT BOTH VFR/IFR ACFT OPERATING IN THE CMA/OXR AREAS; ESPECIALLY CONFLICTS BTWN VFR PATTERN ACFT AND IFR ACFT GOING INTO OXR ON THE VOR 25 OR ILS RWY 25 APCH. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATES THAT HE WENT UP TO THE TWR AFTER THIS INCIDENT AND SPOKE WITH THE CTLRS. THEY OVERLAID THE TFC PATTERN AND THE APCH INTO OXR AND THE CTLRS WERE SHOCKED TO SEE HOW THEY OVERLAPPED EACH OTHER. THE CTLRS HAVE SINCE BEEN MUCH MORE AGGRESSIVE ABOUT CONTACTING APCH FOR OXR TO ASSURE THAT THE APCH CTLRS NOTIFY CMA CTLRS OF TFC ON THE VOR OR ILS APCHS. THIS IS NOT THE WHOLE SOLUTION OF COURSE; BUT MAY HELP SOME. THE BEST SOLUTION WOULD BE TO PUT A REMOTE OF THE RADAR FOR OXR IN THE CMA TWR. RPTR FEELS HIS SUGGESTED SOLUTION FOR A STEP DOWN DSCNT WOULD BE OF GREAT HELP.