Both a supervisor and controller filed detailed reports regarding a loss of separation event; listing causal and preventative actions that differed; providing diverse perspectives from two different vantage points.
Synopsis
Both a supervisor and controller filed detailed reports regarding a loss of separation event; listing causal and preventative actions that differed; providing diverse perspectives from two different vantage points.
Narrative
Initially; the Denver flow requirement was 5 MIT (Miles In Trail) SAYGE and 10 MIT over LANDR (normally the requirement is 5 MIT SAYGE and 20 MIT LANDR). This subsequently changed due to the high volume of Denver arrivals where TMU told me were at risk of overfeeding Denver Approach and asked if we could extend the offload MIT requirement to 15 MIT or 20 MIT. I informed the controllers on 35; 16 and 9 that the offload to LANDR restriction was now 15 MIT. I could have split sector 15 off from sector 16 or at the least put a tracker on the 16 position. One reason I did not was that of the controllers available to me. I could also have reemphasized to the 16R controller that offloading Denver arrivals to LANDR was a viable option to alleviate the compression of the arrival stream on the SAYGE arrival. Right before the error(s) occurred I had received calls from TMU and from the OMIC that the arrivals were being fed to Denver Approach too bunched up and they were too high. I told the 16RA controller that the arrivals needed to be taken down or we would run the risk of spinning aircraft. I also told the 16RA controller to ensure that a point out to Denver approach was accomplished on Air Carrier (A) who was nearing the approach boundary and was still above FL190. Air Carrier (A) was in front of Air Carrier (B); Air Carrier (C) and Air Carrier (D); in that order. I told the 16RA controller; and not the 16R controller; because I could see the 16R controller was very busy. At this point I could have told the 16R controller to put Air Carrier (A) over the LANDR arrival gate. I remained behind the sector position while the error occurred. The error occurred as follows: Air Carrier (A) had been vectored to the north for his descent and to provide spacing between preceding aircraft. The 16R controller vectored Air Carrier (B) to the south by 20 degrees for additional spacing. After several moments; Air Carrier (B) was in or near the lateral confines of Denver approach when the 16R controller re-cleared Air Carrier (B) back to SAYGE. This resulted in an almost 90 degree turn to the north at which point separation was lost between Air Carrier (B) and Air Carrier (C) and then Air Carrier (B) and Air Carrier (D). A turn direct Denver for Air Carrier (B) instead of SAYGE may have prevented the loss of separation. The 16R controller attempted to stop Air Carrier (D)'s descent at FL200 but the clearance did not come soon enough to prevent a loss in separation. I did not hear the clearance issued to Air Carrier (B) direct SAYGE that turned him north into Air Carrier (C) and Air Carrier (D). After I had pulled the 16R controller off position; he remarked to me that he should have turned Air Carrier (B) to direct Denver and not direct SAYGE. Recommendation; I would have had sector 15 decombined from sector 16 sooner or had assigned a third person to sector 16. I should have instructed the 16R controller to offload Air Carrier (A) to the LANDR arrival.
Second reporter narrative
I was working the sector 15/16 radar position; sectors combined for the morning rush. In trail requirements for DEN Approach Control had been set at 2 routes; SAYGE Arrival as primary (5MIT) with LANDR Arrival offload (10MIT). LANDR spacing was increased to 15MIT at some point toward the front/middle of the push. Arrivals from the north were pre-coordinated to be on the LANDR Arrival with sectors 33/35/9. Several aircraft were vectored for spacing; nearly all had speeds assigned. Air Carrier (B) was given a 20 degree left turn to maintain spacing with Air Carrier (A)-which had been turned out to the right to gain separation with preceding traffic and been cleared on course to SAYGE. When separation was ensured; Air Carrier (B) was cleared on course direct SAYGE. SAYGE intersection was abeam to behind of Air Carrier (B). I believed I had cleared Air Carrier (B) to FQF (the next fix on SAYGE6 Arrival). Pilot requested verification of direct SAYGE; I confirmed; and shipped the aircraft to DEN Approach. Skw6764 had earlier been vectored 15 degrees right for spacing and was cleared direct SAYGE. Air Carrier (B) appeared to be in a right hand turn. I called Air Carrier (B) to confirm he had made the frequency change and received no response. I believed Air Carrier (B) to be in turn to downwind by Denver Approach. As Air Carrier (B) continued his turn; Denver Approach called to direct Air Carrier (C) be climbed to Fl200. It was at this time I realized that I had cleared Air Carrier (B) to SAYGE instead of FQF. I complied with Denver Approach's request- issuing an expeditious climb clearance to Air Carrier (C) to Fl200. Air Carrier (C) reported the traffic in site; acknowledged the clearance; and reported to responding to an RA. Air Carrier (D) was in trail of Air Carrier (C); just above Fl200 in the descent. I directed Air Carrier (D) to maintain Fl200. Air Carrier (D) acknowledged the clearance climbing back to Fl200; reported the traffic in sight and responding to an R.A. Recommendation; I am somewhat at a loss to provide a cut-and-dried solution for this incident. I believe that my plan was working and that I had solved almost all the traffic situations; until I misspoke and sent Air Carrier (B) to the SAYGE intersection; instead of to FQF; as I had intended. I admit to being preoccupied at the beginning of the rush due to an incident earlier. I don't believe this had a great impact; as I was pretty well concentrating on the task at hand. I know that I was thinking of things that needed to be done right away after I had ensured separation for Air Carrier (B). I needed to get Air Carrier (C) re-cleared to SAYGE and monitor some speeds at the back of the pack. I can't say definitively that thinking of the clearance to SAYGE caused me to say that instead of FQF to Air Carrier (B). I felt that I was busy; but that the situation was not unworkable as complexity and volume were only moderately high. Having the sectors combined didn't have a great impact; even though I had not worked the morning rush with sectors 15/16 combined in a while; as I had a limited Radar Associate. A CPC might have recognized the fallacy of sending Air Carrier (B) to SAYGE instead of FQF; but the D-side was good help. Frequency congestion was only a problem after I realized I had cleared Air Carrier (B) to SAYGE and needed to speak to three aircraft immediately instead of one aircraft three times.I can say that in the future I would give greater consideration to turning an aircraft right up behind traffic in the stream; as I did Air Carrier (A); but might require additional succeeding aircraft be vectored to follow. Though the 20 degree left turn to Air Carrier (B) put him more directly into the wind and achieved the matching speeds quicker; the side effect of this action was the necessity of more coordination with surrounding sectors and Denver Approach with less time to get things to work. I would also give greater consideration to offloading traffic to the LANDR Arrival. I believed at the time that I didn't have the necessary 15MIT and might have to work harder for less return of effort to fit Air Carrier (B) in; but I would have had more time to let things develop.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.