Large Transport
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 6.6
Communication Breakdown; Distraction; Situational Awareness
ATC Issue All Types; Conflict Airborne Conflict; Deviation / Discrepancy - Procedural Clearance
Human Factors; Procedure
Aircraft X; a PHX departure was climbing out on the SNOBL departure climbing to FL360. Due to a ZLA LOA requirement; I also had 4 KLAS arrivals descending to FL300. I noticed that Aircraft X was an issue with the last of the KLAS arrivals; and turned Aircraft X 15 degrees right; to fall behind. During the shift we had bad rides anywhere from light chop to moderate chop to moderate turbulence. Aircraft Y who was further east entered the airspace at FL340. Halfway through the sector (about 5 minutes prior to turning Aircraft X); Aircraft Y asked for a better altitude; I had then stated that I no complaints at FL300. Aircraft Y asked for FL300 and I descended the aircraft.While Aircraft X began his turn; I noticed that Aircraft Y was also traffic; and was planning to get Aircraft X on course at the earliest time. I cleared Aircraft X on course; and had he done the turn right there; this would be a non-issue. Aircraft X slopped his turn (possibly due to the winds) and I recognized that I was not going to have separation with the current status (just slightly over 2 minutes out). I called ZLA and APPREQED Aircraft Y descending to FL280 for traffic; they said approved. I keyed up to issues the descent; 'Aircraft Y descend and maintain FL280 for traffic.' I believe I issued a 2;000 feet per minute descent rate (I know this is not 'valid' but I know that it helps convey a rate needed; and is used as an advisory). I waited; and received no response. I keyed up and said the same thing; then waited; no response. I keyed up a 3rd time and issued the descent. After the 3rd call; Aircraft Y asked if I was calling him. I said yes; then issued the clearance for the 4th time. The pilot waited a second; then responded with; 'Can you confirm the altitude for Aircraft Y?' I reissued the clearance; and told the aircraft to expedite and that I needed him 'Out of 29 in a minute of less.' Aircraft Y waited a second; then read back the clearance. I then told Aircraft X to expedite his climb and to report out of FL310. At that point; Aircraft Y said that he had the traffic in sight; 'Traffic in sight 6 miles in front 400 feet above.' And Aircraft X on his own said he had the 4 o'clock traffic. Aircraft X was at FL305 climbing; Aircraft Y level at FL300; 2 hits later; I lost separation at circa 4.8 miles; the next hit was around 4 miles; I saw as close as 3.75 nm between the targets.All the time; ZLA had the hand off on Aircraft Y; after APPREQING the descent; ZLA never changed the data block. I know that around 3.75 miles I had 1;000+ feet; because I finally saw Aircraft Y out of FL295. But I cannot tell you what I really had. After Aircraft X was out of FL295; ZLA changed the data block to reflect FL280.I firmly believe that I would have had separation had Aircraft Y responded to the first clearance. It took 5 transmissions to get the clearance to Aircraft Y (in my opinion that is unacceptable). My back up plan was to descend Aircraft Y. I went to execute the backup plan and Aircraft Y didn't listen. I really feel that I let the ATC system down. I trusted that the pilots would listen and follow directions. I have had a handful of close calls in my career; but up to this point; I maintained separation because the pilots responded on the first call. This situation really caught me off guard. I didn't expect so much effort to relay a descent clearance to Aircraft Y. I am truly sorry. I could have stopped Aircraft X at FL280 and never climbed him above the traffic. But I was trying to provide the best service possible. And providing that service caught me.Having pilots that listen. Change data blocks when altitudes are APPREQED.
ZAB Controller describes a loss of separation he attributes to one of the aircraft taking five transmissions to receive information.