Date: 201405
Local time of day: 0001-0600
Location: ZLA.ARTCC, CA
Light: Night
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class A ZLA
ATC / advisory: Center ZLA
Second aircraft: B737 Undifferentiated or Other Model
Reporter organization: Government
Reporter function: Instructor; Enroute
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Human-Machine Interface; Communication Breakdown
Second reporter function: Enroute; Trainee
Anomaly: ATC Issue All Types; Conflict Airborne Conflict; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Separated Traffic; Air Traffic Control Provided Assistance
Contributing factors: Human Factors; Procedure
Primary problem: Procedure

Narrative: Aircraft X was coming from the north at FL280 southbound. Aircraft Y coming from the east westbound at FL300. The rides were choppy from FL290 through FL330 so we knew in advance that the Aircraft Y would be asking for lower. Trainee got control from Sector 39 to descend Aircraft X in anticipation of this. Descended Aircraft X from FL280 to FL240 just then Aircraft Y asked for lower for the ride. I saw Aircraft X leave FL280 and passing FL278 when trainee issued descent to FL280 to Aircraft Y. Trainee has been recommended for certification skills check at this sector and is very competent. I was then distracted from the sector and took my attention away from the scope. I heard the trainee issue the next clearance to FL260 to Aircraft Y. I then noticed conflict alert going off and I questioned the trainee. He informed me that he used vacated altitudes and I figured it was all good. After being relieved from the sector we were doing a debriefing on his training from the day and he informed me that he may have messed up on the vacated altitudes. He wasn't sure if he transposed the altitude form FL265 for FL256. That is when I informed him to fill out a report as I would do the same.No recommendation I can think of. I let my guard down as traffic was slow and I have worked with this trainee on numerous occasions. I saw him use vacated altitudes earlier with these two aircraft and did not observe him the second time when he descended Aircraft Y to FL260. When I saw the conflict alert go off and I asked him and he replied he used vacated; I assumed he did but I was not 100 percent certain. As traffic was slow I let my attention wander.


Second narrative: I was Sector 40/60 training on the radar. Aircraft Y was at FL300 requesting lower for the rides. So then I descended Aircraft X from FL280 to FL240. Once Aircraft X vacated FL280 I descended Aircraft Y to FL280. When I looked at Aircraft X I thought I saw FL256 and continued to descend Aircraft Y to FL260. Conflict alert went off and I disregarded the alert believing I had used vacated altitudes. After I was relieved from position I went over the scenario in my head and realized I may have transposed FL265 for FL256 for Aircraft Y.No recommendations; just was a human error.


Synopsis: Reporters write about loss of separation due to improper use of Vacated Altitude separation rule; leading to a loss of separation.