VFR PA28 TRANSITIONING SNA'S CLASS C AIRSPACE; WITH SCT; EXPERIENCED NMAC WITH B737 BELIEVED TO BE SNA ARRIVAL NO TRAFFIC ISSUED BY ATC; EVASIVE DESCENT INITIATED.

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OCT/XA/08 AT APPROX XA00 LCL TIME I WAS A CFI FLYING WITH A STUDENT PLT. WE WERE FLYING IN VFR CONDITIONS NWBOUND -- VFR ON THE LAX 115 DEG RADIAL FLYING FROM CRQ (CARLSBAD-PALOMAR) TO SMO (SANTA MONICA MUNICIPAL ARPT; SANTA MONICA; CA). WE WERE IN RADIO CONTACT WITH SOCAL APCH CTL OPERATING VFR UTILIZING VFR FLT FOLLOWING SVCS FROM SOCAL FOR THE ENTIRE FLT FROM CONTACTING SOCAL IN CLBOUT FROM CRQ UNTIL RADAR SVC WAS TERMINATED BY SOCAL OVER FUL (APPROX) AS WE ENTERED THE LAX SPECIAL RULES AIRSPACE TO CROSS LAX CLASS B AIRSPACE. IT WAS BEAUTIFUL VMC WITH NO LIMITATIONS TO VISIBILITY AND FEW HIGH SCATTERED CLOUDS. VISIBILITY WAS 20+ MI IN DAYLIGHT CONDITIONS. OUR ACFT A PIPER PA28-161/U WARRIOR II. WE WERE LEVEL AT 4500 FT MSL -- WITH THE LCL ALTIMETER SETTING PROVIDED BY SOCAL AND WERE ON THE DISCRETE XPONDER CODE ASSIGNED BY SOCAL APCH ATC. WE WERE XMITTING MODE C. WE HAD A FEW TA'S PRIOR TO THE INCIDENT BY SOCAL. SHORTLY BEFORE THE INCIDENT I CALLED SOCAL AND ASKED WHETHER THEY WOULD PREFER THAT WE CLB OR DSND FOR FLT THROUGH SNA CLASS C AS ON THE FLT EARLIER IN THE DAY SMO-CRQ ON THE SAME RTE WE HAD BEEN ASSIGNED 5000 FT MSL IN THE LAX CLASS B AND THEN REQUESTED TO FLY 5500 FT THROUGH SNA CLASS C AREA TO AVOID SNA DEPS. THE SOCAL CTLR ADVISED THAT 4500 FT WOULD BE OK (NOT SURE THE ACTUAL TERMINOLOGY USED). WE CONTINUED LEVEL AT 4500 FT. AT APPROX XB00 AFTER A TFC CALL FOR TFC AT '1 O'CLOCK' (A DIFFERENT CONFLICT) A B737 APPEARED AT OUR 10 O'CLOCK POS APPARENTLY DSNDING TOWARDS US. WE WERE ON A HDG OF 295 DEGS MAGNETIC ON THE LAX 115 DEG RADIAL INBOUND TO THE LAX VOR. THE AIRLINER WAS ON A HDG OF APPROX 025 DEGS (NEBOUND FROM OVER THE PACIFIC OCEAN -- OUR POS AND LIKELY ENTERING THE DOWNWIND FOR SNA). I KEYED UP THE MIKE AND INDICATED TO SOCAL THAT WE HAD A VISUAL ON THE AIRLINER AT 10 O'CLOCK POS ABOVE OUR ALT. THE PLT INDICATED THAT HE DID NOT SEE THE TFC (AT THIS POINT WE WERE LESS THAN A MI NEARLY DIRECTLY IN FRONT OF HIM AND ABOUT 500 FT BELOW HIS ALT). I DIRECTED THE STUDENT TO DSND -- WE REDUCED PWR AND ADJUSTED PITCH FOR A FAIRLY HEALTHY DSCNT DOWN TO ABOUT 4000 FT WHEN THE ACFT PASSED ABOUT 500 FT ABOVE US AT APPROX 4500 FT MSL -- THE ALT THAT WE VACATED TO AVOID THE COLLISION. THE ACFT TOOK NO EVASIVE ACTION. WE LIKELY WOULD HAVE COME VERY CLOSE TO HITTING IF WE HAD NOT SEEN THE ACFT AND TAKEN PROMPT EVASIVE ACTION. WHAT I DON'T UNDERSTAND IS WHY THE SOCAL CTLR ALLOWED THE IFR ARR TO DSND OUT OF 5000 FT WHEN THERE WAS A CLR TARGET -- TAGGED AND BEING FLT FOLLOWED AT 4500 FT MSL VFR. WE OF COURSE WERE VFR ON THE PROPER VFR CRUISING ALT FOR A NW MAGNETIC HDG. WE HAD LONG BEEN TALKING TO SOCAL ON THE EXACT FREQ AND SECTOR THAT HAD CTL OF THE B737 IFR ARR INTO SNA. I BELIEVE THAT WE WERE IN SNA CLASS C AIRSPACE AT THE TIME. WE WERE IN COMPLIANCE WITH 16CFR RULES WITH REGARD TO MAINTAINING RADIO COM WITH ATC. FORTUNATELY 'SEE AND AVOID' WORKED HERE -- BUT THOSE PAX ON THE FLT DESERVED MORE DILIGENT ATTN AT THE ATC POS WHO HAD BOTH OUR TARGET AND THE IFR ARR UNDER HIS CTL. THIS COULD HAVE HAD A TOTALLY DIFFERENT OUTCOME IF WE HAD NOT SEEN THE BRIGHTLY PAINTED B737 DSNDING RIGHT FOR US.