Small Aircraft
Air Traffic Control Fully Certified
Time Pressure; Communication Breakdown
ATC Issue All Types; Conflict NMAC
Air Traffic Control Separated Traffic; Flight Crew FLC complied w / Automation / Advisory
Airspace Structure
Aircraft X was IFR inbound to CNO on RNAV RWY 26R. At about 6 miles out when he contacted me; there was traffic at his 1 o'clock and 1-2 miles same altitude Aircraft Y. Aircraft Y was not on my frequency and was just outside the Delta airspace just about 5 miles east. I gave numerous traffic calls for Aircraft Y. At about a mile and half separation; I issued the IFR to descend which led him to continue on the approach with an early descent. Aircraft Y passed behind and about 400 higher than Aircraft X. Just as Aircraft Y passed behind Aircraft X; said he had the traffic in sight and said that was scary. due to the pilot saying it was scary an NMAC would be done. Since I wasn't talking to the transitioning aircraft who was last talked to by ONT with just one traffic call then a frequency change; I was never talking to Aircraft Y.Extend the Class Delta more east so aircraft would be forced to contact CNO. have better traffic calls for aircraft from previous facility to eliminate NMAC or having ONT give the aircraft a frequency change to CNO Tower to have better control and communication with all aircraft.
CNO Tower Controller reported issuing inbound IFR a descent to avoid NORDO traffic resulted in a NMAC.