Date: 201709
Local time of day: 0001-0600
Location: ZOA.ARTCC, CA
Aircraft: Commercial Fixed Wing
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class A ZOA
ATC / advisory: Center ZOA
Second aircraft: Commercial Fixed Wing
Reporter organization: Government
Reporter function: Enroute
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Communication Breakdown; Situational Awareness; Human-Machine Interface
Anomaly: ATC Issue All Types; Conflict Airborne Conflict; Deviation - Altitude Crossing Restriction Not Met; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Air Traffic Control Separated Traffic
Contributing factors: Airspace Structure; Procedure; Company Policy; Human Factors
Primary problem: Procedure

Narrative: I was working Sector by myself with 12 aircraft because we do not have enough staffing. Aircraft X was descending via a STAR on the established Automated Information Transfer (AIT) procedure. [Another] Sector descends STAR arrivals via; hands off to [me]. [My] sector accepts handoff; and hands off to Approach. When Approach accepts handoff; [another] Sector ships the aircraft to Approach. On the STAR the aircraft are supposed to cross a fix at or below 21000 feet because other arrivals are descending via a STAR to 22000 feet at that point. These two procedures are separated by 5.1 miles at a common fix. That is the biggest problem. There is no extra room for any error or problem. This is not safe.I had descended Aircraft Y via a STAR. When I looked at these two aircraft; Aircraft X was at 25000 feet with 1 minute or less to make his restriction 4000 feet lower. Aircraft Y was at 25100 feet and had been descending at 6000 then 5000 feet per minute and was going to be level at 22000 feet. I told Aircraft Y to' maintain current altitude and turn 30 degrees; actually turn right heading 310'. This was a 60 to 70 degree turn one minute apart. Aircraft Y turned about two miles north and Aircraft X missed his restriction by a little over a mile and I had 5.1 miles when Aircraft X leveled at 21000 feet. Normally we would have shipped Aircraft Y to Approach before this; so two different Approach sectors would be taking to these aircraft. Even if we are still talking to one arrival; we are not talking to the other arrival so we have no way of knowing when they are not going to make any of the restrictions. I would estimate that we have at least one STAR a week miss a restriction. We need to ask ourselves; if I had done nothing; at what point does Aircraft Y get TCAS alert telling him to descend and what is Aircraft X TCAS resolution? And what would happen to the rest of our traffic in the area?The crossing restrictions on the STARs need to be moved at least 3 miles south so that we have at least 8 miles separation between the one arrival and the other arrivals. Even at 8 miles this is not a great situation but at least it gives a little more time for a pilot to still get under the traffic. It also gives us more time and more information to determine if we need to take action.




Synopsis: Center Air Traffic Controller reported two aircraft 'descending via' on crossing STARs were going to have less than required separation due to one aircraft not complying with its crossing restriction.