CONFLICT AIRBORNE.

Narrative

ACFT #1 (ACR 1); C172RG/A FLYING MULTIPLE IFR APCHS IN VFR CONDITIONS INTO CNO ARPT. ON BOARD: PLT (ME) AND RATED CHK PLT IN CONTACT WITH ONTARIO APCH ON 135.4 ACFT #2 (ACR #2); MODERATE SIZE TWIN OR SMALL COMMUTER; POSSIBLY TWIN TURBO-PROP. ACR #1 WAS EBOUND TOWARD PDZ (APPROX 5 NM W) LEVEL AT 3000 FT MSL ON RADAR VECTORS FOR THE VOR-B INTO CNO. TFC WAS CALLED AT 5 O'CLOCK; 3 MI; 3800 FT DSNDING. THE CHK PLT OF ACR #1 ATTEMPTED TO GET A VISUAL ON ACR #2 BUT WAS HINDERED BY THE RELATIVE POS (SLIGHTLY HIGH; TO THE REAR; IN THE SETTING SUN. APPROX 1 MIN LATER; ACR #2 WAS CALLED AT 5 O'CLOCK; 1/2 MI; 3200 FT DSNDING. THE ONTARIO CTLR ALSO RECOMMENDED AN IMMEDIATE DSCNT. ACR #1 INITIATED AN IMMEDIATE DSCNT TO 2200 FT AND SPOTTED ACR #2 APPROX 800 FT ABOVE AND LESS THAN 500 FT OFF THE L WING OF MY ACFT. THE TWIN (ACR #2) SUBSEQUENTLY LANDED AT RIVERSIDE (RAL) ARPT. OTHER OBSERVATIONS: IT DID NOT SEEM THAT ACR #2 WAS IN CONTACT WITH THE SECTOR CTLR (ONT APCH) IN CHARGE OF THAT AIRPLANE. PERSONAL OPINION: THE INCIDENT WAS CAUSED BY POOR COM BTWN ACR #2 AND ONT APCH OR BTWN THE CTLRS OF ONT APCH IN TFC MGMNT; IF ACR #2 WAS IN CONTACT WITH ONTARIO. ADDITIONALLY THE POSSIBLE MID-AIR WAS AVOIDED BY THE STERLING DILIGENCE OF ACR #1 SECTOR CTLR; HER RECOMMENDATION FOR THE IMMEDIATE DSCNT; AND THE IMPLEMENTATION THEREOF.