What happened
A Beech 95, registration N9750Y, was traveling south at approximately 185 knots when it converged with a Cessna 180, registration N42695, which was traveling on a heading of 118 degrees at approximately 147 knots. Both aircraft were operating in Visual Meteorological Conditions (VMC) and receiving radar services from Stockton TRACON as VFR flight following.
During the automated radar handoff to the Tracy sector controller, the pilots reported an altitude of 5,500 feet MSL. The two aircraft subsequently converged and collided at approximately 5,500 feet MSL, resulting in both aircraft entering uncontrolled descents and crashing.
The investigation
The investigation revealed that the air traffic controller was occupied with controlling two other aircraft operating under IFR, which required significant attention to maintain separation. At the time of the collision, the controller was concentrating on the separation of those two IFR aircraft.
Regarding the radar capabilities at the time, the Stockton ARTS II radar was not equipped with a conflict alert system. Furthermore, the radar was programmed such that non-beacon (VFR) call signs would be moved to a tab list after 5 scans (approximately 20 seconds) without a transponder reply. After an additional 33 scans—totaling approximately 2.5 minutes—the call sign would disappear from the tab list entirely.