This was an e-mail from June of 1989. In this e-mail reply I was questioning the NTAP data recording process, and dispelling the myth that the controllers involved in the San Luis Obispo midair collision (or any ARTCC controllers for that matter) can simply "flip a switch" to eliminate VFR targets from their scope. [Caution: This is long, and it gets pretty technical. Skip to the end for the synopsis.]
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Regarding your fellow MITREite's comments per your email of 21-Jun-89, I would like to respond to the first two issues.....
1."The only data recording of radar done on a routine basis by the NAS sites is periodic dumps of the track table ... and display channel outputs. NTAP, which is used for incident analysis by all sites, uses the display channel outputs..."
My reaction - Review MD-320 Chp 6 "Storage and Output of Radar Data", para. 6.2, which discusses "Data Stored For Display Channel Outputs". Subparagraph "e" looks as though it is saying that the MRDP function will format and output limited data blocks for "_all_ uncorrelated beacon data". It says nothing about selective rejection until 6.2.1.1 "Radar Data Displayed". These paragraphs led me to believe that the data stored may be different than the data displayed. (This was previously discussed in my email of 17-May-89). Also, my experience in running NTAPs at ZOB seems to support your theory that the only data stored is what was displayed to the controller, yet things are suspicious in that the mode C data of VFR TARGETs will NOT show up on the NTAPs I run. (That's right, there is absolutely NO altitude data on ANY VFR TARGETs within all of ZOB if we believe what the NTAP is telling us! This situation causes me to have a lack of trust in NTAP.)
2. The controllers could have used Display Filter Key settings at the PVD which prevented the display of the unidentified code 1200 aircraft. Since a NAS Stage A computer does not record what the settings of the Display Filter Keys are at any sector, how these keys were set around the time of the collision would only be known through "controller recollection."
My reaction - In order for the controller to filter out the VFR Rockwell Commander that had mode C, he would have had to set his PVD up rather abnormally. Today I took the time to see exactly what actions I would have to take in order to filter a 1200 beacon code with mode C data. I had many low altitude VFR 1200 beacons (with mode C) to view.
First of all, we must discuss the available filters which the controller has at his disposal. Five possible filters (we will label them A thru E) can affect the display of radar data of a 1200 beacon return _with_ mode C. We will only discuss a 1200 beacon reply with mode C, as the VFR Rockwell Commander was shown to have replied reliably to ZLA's Paso Robles ARSR with good mode C data.
A = Altitude Filter Keys - There are 8 altitude keys which cause a TARGET symbol to be displayed for _all_ Mode C aircraft within the selected stratum (locally adapted). At ZOB, the first Altitude Key (ALT1) ranges from 0 to 5,700 ft msl (I imagine it is very similar, if not exactly the same, at ZLA). The VFR returns I viewed today had mode C and were generally quite low and stayed within this range. ZLA's sector 15 would utilize this ALT1 Filter Key, as it was a "low altitude" sector. The controllers testified that the radar scope had been configured to display radar returns up to an altitude of 24,200 ft msl. This is the same as we do at ZOB for a low altitude sector that has jurisdiction to the base of controlled airspace (i.e. no approach control(s) underlying the entire sector).
B = Altitude Limits - Line 12 of the RCRD (Radar Controller's Computer Readout Device) contains the altitudes specified in a Modify Altitude Limits message in block altitude format, i.e. 000B242 is the normal limits set for a low altitude sector which has responsibility for traffic from the base of controlled airspace to 23,000 ft (flight level 230). Sector 15 at ZLA provided such service in the SBP area. Again, the controllers testified that the scope was set up for displaying radar returns up to 24,200 ft msl.
C = PVD Code Selection List - Each RCRD has the capability of displaying up to 55 discrete or nondiscrete beacon codes. The codes in use by the sector are displayed in the PVD Code Selection List. Codes may be entered automatically by the computer (as it does for IFR flights that are flight planed through the sector) or manually by the controller (the controller must enter the 1200 code himself, as the computer will NOT do this for him).
D = Select Beacon Filter Key - Selecting this filter key causes the display of a TARGET symbol for all aircraft squawking a code displayed in the PVD Code Selection List (C). If the code 1200 is in the PVD Code Selection List (C) and this Beacon Filter Key (D) is selected, no matter what altitude limits the controller has selected, the VFR TARGETs will be displayed.
E = Select Limited Data Block Filter Key - Selecting this filter key causes the display of Limited Data Blocks (LDBs) for all uncorrelated aircraft whose code is in the PVD Code Selection List (C). If the code 1200 is in the PVD Code Selection List, no matter what altitude limits the controller has selected, a LDB (showing altitude) will be displayed for all aircraft squawking 1200 and equipped with a functioning mode C.
I decided to go through all the possible combinations for these five filters while I observed low altitude 1200 beacon returns with mode C (TARGETs below 5,700 ft). The following table is the result of this exercise.
-------------------------------------------------------------------------------
A thru E represent the filters.
0 = filter de-selected or incorrect, 1 = filter selected or correct.
T represents the display of a TARGET symbol
O = NO TARGET symbol displayed, X = TARGET symbol displayed
L represents the display of a limited data block (LDB)
O = NO LDB displayed, X = LDB displayed.
FILTER SELECTION >>>>> RESULTS
Scenario A B C D E T L
#
01 0 0 0 0 0 >>>>> O O
02 0 0 0 0 1 >>>>> O O
03 0 0 0 1 0 >>>>> O O
04 0 0 0 1 1 >>>>> O O
05 0 0 1 0 0 >>>>> O O
06 0 0 1 0 1 >>>>> O X
07 0 0 1 1 0 >>>>> X O
08 0 0 1 1 1 >>>>> X X
09 0 1 0 0 0 >>>>> O X
10 0 1 0 0 1 >>>>> O X
11 0 1 0 1 0 >>>>> O X
12 0 1 0 1 1 >>>>> O X
13 0 1 1 0 0 >>>>> O X
14 0 1 1 0 1 >>>>> O X
15 0 1 1 1 0 >>>>> X X
16 0 1 1 1 1 >>>>> X X
17 1 0 0 0 0 >>>>> X O
18 1 0 0 0 1 >>>>> X O
19 1 0 0 1 0 >>>>> X O
20 1 0 0 1 1 >>>>> X O
21 1 0 1 0 0 >>>>> X O
22 1 0 1 0 1 >>>>> X X
23 1 0 1 1 0 >>>>> X O
24 1 0 1 1 1 >>>>> X X
25 1 1 0 0 0 >>>>> X X
26 1 1 0 0 1 >>>>> X X
27 1 1 0 1 0 >>>>> X X
28 1 1 0 1 1 >>>>> X X
29 1 1 1 0 0 >>>>> X X
30 1 1 1 0 1 >>>>> X X
31 1 1 1 1 0 >>>>> X X
32 1 1 1 1 1 >>>>> X X
-------------------------------------------------------------------------------
Now lets review this data with regard to the midair collision of August 24, 1984, between a VFR Rockwell Commander and a commuter flight, Wings West Flight 628. This midair collision occurred about 8 miles northwest of the San Luis Obispo, CA (SBP) airport.
First of all, if the ALT1 Filter Key (Item "A" of our table) is depressed, there is _always_ a TARGET displayed if the mode C falls within the range of the ALT1 key (scenarios 17 thru 32). The rest of the PVD filters may be incorrectly adjusted, yet if this ALT1 key is depressed, A TARGET WILL BE DISPLAYED. It is hard to imagine that the ALT1 key was not depressed, however, I have seen cases (rare) in which the software didn't sample the key correctly, or the key had a hardware failure and didn't make the proper electrical connection. So, let's assume the controller was telling the truth and this ALT1 Filter Key was selected, but the key had a mechanical failure and appeared to the software as though it was not selected......
We are now left with scenarios 1-16. If the controller had the Altitude Limits Filter (B) of the RCRD correctly set (i.e. 000B242), at the _minimum_ he would have seen the limited data block (LDB) of the VFR Rockwell Commander (scenarios 9-14), or both a TARGET and LDB (scenarios 15 & 16).
Now let's stretch the imagination even more and assume that throughout that fateful morning all the controllers who worked this position at ZLA's sector 15 failed to set _both_ the Altitude Filter Keys (A) and the Altitude Limits (B) correctly. This could be possible if this sector position had been used on the mid-shift as a high altitude sector, with both the Altitude Filter Keys (A) and the Altitude Limits (B) being set to 228B999, but I must say that this scenario is hard to imagine. Most controllers realize such an improper selection almost immediately, and such a gross error as this would have had to of gone on for over four (4) hours, as the accident occurred at 11:17a local time. Several rotations of controllers would have occurred by then, and surely someone would realize they were working a low altitude sector with improper altitude filter selections! But, that's right, we are stretching the imagination here......
So, let's assume the controllers never set A or B correctly. We are now left with C, D & E. Whenever a controller works a low altitude sector, according to the controller's handbook (7110.65), the controller shall always "monitor the appropriate IFR code...and...code 1200....except that during periods when ring-around or excessive VFR TARGET presentations derogate the separation of IFR traffic, the monitoring of VFR code 1200...may be temporarily discontinued" (7110.65E 5-33a). Again, the controllers (4) all testified that the PVD was set up correctly for monitoring VFR radar returns. I have seen controllers forget to put 1200 into the PVD Code Selection List (C), but it never is a problem since they have the rest of the scope set up correctly (as in scenarios 25-28).
We will now assume that these controllers had correctly put 1200 in the PVD Code Selection List (C), yet had both altitude limits incorrectly set. This is covered in scenarios 5-8. Only in scenario 5 do we get absolutely NO data displayed for the VFR Rockwell Commander. Scenario 5 assumes the controller had neither the Select Beacon Filter Key (D) nor the Select Limited Data Block Filter Key (E) selected. It is extremely difficult to imagine such a gross error as not selecting D or E, because no matter what position a controller works, a "high" or a "low" sector, the Select Beacon Filter Key (D) and the Select Limited Data Block Key (E) are _always_ selected. Now let's assume that the controllers never put 1200 in the PVD Code Selection List (C), while also totally screwing up their altitude limits in A & B. I would consider this a case of gross negligence, where all VFR TARGETs would not be displayed. I consider these scenarios (1-4) highly, highly unlikely. Keep in mind that the controllers observed other VFR TARGETs, even that of the Wings West flight before a discreet code was issued. More on this later......
Let's not forget that the controller working the traffic (Bill Simons) was a developmental (trainee). This unfortunate day was picked as his sector certification day......his _first_ opportunity to be certified to operate this radar position without the constant and watchful eye of a full performance level controller critically monitoring his every move. His instructor (Gary Hobbs), a seasoned controller, was assuming full responsibility of the sector, and had complete override capability and full view of the PVD. Bill Simons knew that he must do his absolute best that day, as a failed certification could mean NO JOB! The area supervisor (James Smith) was also monitoring Bill's every move, listening critically to each and every clearance, as it was he who must decide whether or not Bill measures up to the high standards of being a controller at ZLA, especially in His area. Leaving the PVD set up in a completely incorrect manner would automatically be grounds for a failed certification.
After a moderate workload situation had passed (the sequencing of some inbounds), the Wings West Flight 628 departed SBP (squawking VFR) and called for his IFR clearance while climbing outbound on the localizer. Bill issues a discrete code to the pilot, and observed a VFR TARGET just northwest of SBP change from a "V" symbol, to an uncorrelated symbol with a limited data block displaying the code he just issued to the Wings West pilot, as well as his altitude. The TARGET then changed to a correlated symbol with a full data block. Bill then advised the pilot "radar contact six northwest of San Luis Obispo airport". When a controller states "radar contact", ADVISING THE PILOT OF HIS GEOGRAPHICAL position, THE CONTROLLER MUST look directly at that TARGET. At the time Bill advised "radar contact," the VFR Rockwell Commander TARGET should have been displayed approximately one-half inch (0.5") to the west-northwest of the Flight 628. IT IS INCONCEIVABLE TO ME THAT ALL FOUR CONTROLLERS AT THE SECTOR WOULD NOT HAVE SEEN THE VFR TARGET OF THE ROCKWELL, HAD THE VFR TARGET BEEN DISPLAYED!
If the controllers had not set the PVD up to correctly display VFR (beacon code 1200) radar TARGETs, why would Bill have seen the Wings West TARGET as a VFR TARGET before the pilot dialed in the assigned transponder code? I believe the PVD was set up correctly. I feel it is beyond reason that the PVD could have been set so as to cause the Rockwell Commander to NOT be displayed, yet at the same time allow the Wings West to be displayed. It would have taken too many gross errors to filter out VFR TARGETs. Also, it makes absolutely no sense that the filter keys would affect one VFR target, but not another VFR TARGET within ten miles.
I feel that the only thing that could have caused the VFR Rockwell Commander to NOT be displayed was the fact that it was located in a different Radar Sort Box (which would have had to of been programmed differently than that in which the Wings West was located). It is interesting to note that the midair collision occurred on the boundary of where two Radar Sort Boxes are located this day (RSBs 1114 & 1115).
If adaptation data for ZLA's sort boxes was archived, we could possibly answer the question of how these sort boxes were programmed. Unfortunately, such data is NOT archived. For example, at ZOB, when a new software build comes on-line, the old adaptation listing (the ACES listing) is thrown out! Such a procedure is an unfortunate method of operation, as valuable information is then lost.
It is my hope that this discussion helps to dispel the myth of an en route radar controller simply pushing a single button in an effort to eliminate VFR radar returns from his or her display. It just isn't quite so simple, nor have I ever seen an en route radar controller go through the immense amount of effort required to do such in a low altitude sector.
Thomas G. Lusch
June 22, 1989
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