August 31, 1986, At approximately 1140 PDT, a Piper PA-28, N4891F, departed Torrance, CA on a VFR flight to big Bear, CA. After takeoff, the pilot turned eastbound toward the Paradise vortac with his x-ponder squawking 1200. At that time, Aero-Mexico Flight 498 (DC-8, Mexican registry XA-JED) was on arrival, receiving northbound vectors from LAX approach control (ar-1 ctlr) for an ILS approach to the LAX International Airport. At 1151:04, the control tower asked flight 498 to reduce speed to 190 knots & descend from 7000' to 6000'.

During this time, the control tower was clearing other traffic & providing radar advisories, but didn't see a display for n4891f on his scope. At 1152:09, N4891F & flight 498 converged & collided at aprox  6560', then fell to the ground. An inv revealed N4891f had inadvertently entered the lax terminal control area (TCA) & wasn't in radio contact with ATC. Lax Tracon wasn't equipped with an auto conflict alert sys & the analog beacon response fm n4891f's x-ponder wasn't displayed due to equip configuration. N4891F's psn was displayed by an alphanumeric triangle, but the primary target wasn't displayed due to an atmospheric inversion.