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Aeromexico flight 498 left Tijuana, Mexico on the morning of August 31, 1986 bound for Los Angeles. 58 passengers and six crew members were on board the DC-9 as it made it's approach to LAX. Meanwhile, a Piper Archer carrying a family of three had departed Torrance Municipal Airport bound for Big Bear.

It was a clear day in the Los Angeles basin with visibility reported at 15 miles. The Archer was on an easterly climb while 498 was descending to the northwest. At an approximate altitude of 6,500ft, the two aircraft collided, the horizontal stabilizer of the DC-9 slicing through the cockpit of the Archer. 498 rolled onto it's back and fell inverted into a residential area, the Archer impacting in a school yard about a quarter of a mile away. All aboard both planes were killed in the accident along with 15 people on the ground. 18 houses were destroyed in the impact.

......The Archer was equipped with a non-encoding transponder, meaning it's radar return did not include it's altitude. The controller working the aircraft at the time did not remember seeing the return from the Archer, though review of the ATC tapes showed that it was visible. The Archer was operating VFR and had not received a clearance to enter the Los Angeles Terminal Control Area. Radar data showed that the aircraft had penetrated the LA TCA eight minutes after departing Torrance, straying into the path of Aeromexico 498.

At about the same time the Archer entered the LA TCA, another aircraft also strayed into the airspace in the path of a commuter aircraft. The controller was temporarily distracted during it's communication with the aircraft and when he had returned his attention, the collision had already occurred. Because there was no altitude reported for the Archer and it had neither requested nor received a clearance to enter the LA TCA, the controller had no idea that it was not staying below of the floor of the airspace.

Examination of the wreckage showed that the pilot of the Archer had the LA TCA chart open during the flight, so no clues were given as to the deviation. In addition, by operating VFR, the pilot of the Archer was assuming responsibility for collision avoidance. Based on the flight paths of the aircraft, both pilots should have been able to see each other. Because of this accident, the FAA now requires that all aircraft flying in a 30nm radius of the primary airport of a TCA/Class B airspace be equipped with an altitude-encoding transponder.