This was an eye opener! The VFR aircraft, during the entire period that it was established on a straight inbound course to San Luis Obispo airport, was entirely within a separate radar sort box, a completely different sort box than the Wings West aircraft! The aircraft collided nearly directly on the edge of these two sort boxes.
If sort box #1114 had been somehow assigned to provide preferred coverage from the Los Angeles long-range radar, with Paso Robles as supplemental, it would be completely obvious that the radar returns of the VFR aircraft would not have been displayed.
However, there is absolutely no good or logical reason why Los Angeles radar would be assigned as preferred in sort box 1114. It would make absolutely no sense whatsoever to have it this way. (It should be, and when checked for me by others, it was the assigned supplemental radar site for this sort box.) However, if somehow it had been assigned as preferred, by some quirk or oddball chain of events, it certainly would account for the controllers not seeing the radar returns of the VFR aircraft.
Also, if such adaptation had been utilized, it could also account for any reports of transponders appearing unreliable in that area. Remember, a tracked aircraft utilizes supplemental coverage, an untracked does not. So, if a low-altitude "tracked" aircraft would fly from sort box 1115 to 1114, the aircraft's target would still be processed for display. Yet, a low-altitude untracked (typically VFR 1-2-0-0 squawk) aircraft, flying the same exact course, would drop off the display the instant it entered sort box 1114.
However, from what I have been told many times, and as best I can understand from reading the manuals, if a target symbol is not displayed, the radar data for that symbol will not be recorded on the data storage tapes. [Note: Having radar data go unrecorded carries serious implications for the support of search and rescue operations, especially when attempting to determine the LKP (last known position) of an overdue aircraft.] My tests at Cleveland Center confirmed for me that VFR targets are not stored if they're not displayed. So I am still left with a feeling of puzzlement about San Luis Obispo. Since the VFR radar returns were stored, according to how I know things work now, it must have been displayed. I certainly wish the issue of selective rejection and radar sort box programing had been discussed and looked into back then.
However, without a doubt, as I hope you understand from this review, due to the nature of today's radar data processing, THERE ARE DEFINITELY REASONS WHY AN AIRCRAFT THAT IS DETECTED BY RADAR WILL NOT BE DISPLAYED, EVEN IF THAT RADAR SITE FEEDS THE FACILITY.
Given the above, I have an extremely difficult time accepting the NTSB's analysis in the San Luis Obispo midair collision report that says...
Since the manager of the FAA En Route Automation Program had testified that, under these conditions, there was no functional reason why the Rockwell Commander's radar return, or any other VFR transponder return, should not have been displayed on the R-15 radar scope, the Safety Board concludes that the Rockwell radar return had been displayed and that the reason the controllers did not observe it cannot be attributed to any failure or malfunction of the Nas Stage A's computers or associated equipment.
Being as the NTSB report made no mention of the software process of selective rejection, and that there was no mention of radar sort box adaptation for that airspace, I feel that there exists reasonable doubt about that analysis. (For more on this subject, jump to "Reasonable Doubt.")
Regardless of what occurred at San Luis Obispo, the manner in which radar data is processed today must be improved upon. I call upon the industry to correct this problem and improve upon radar data processing methods. As I have suggested in the past, here are my recommendations...
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