Date: 201209
Local time of day: 1201-1800
Location: ZLA.ARTCC, CA
Aircraft: P180 Avanti
Flight plan: IFR
Flight phase: Descent
Airspace: Class A ZLA
ATC / advisory: Center ZLA
Second aircraft: EMB ERJ 135 ER/LR
Reporter organization: Government
Reporter function: Enroute
Reporter qualification: Air Traffic Control Developmental
Human factors: Situational Awareness
Anomaly: ATC Issue All Types; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued New Clearance
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: I had RADAR on an Embraer level at FL260 and routed via RZS..SNS..KMRY. I took RADAR to change the routing and ease the workload of Sector 26; since they had 3 other fast jets to sequence with the Piaggio. The Piaggio checked in and I told him to fly heading 140; expect revised routing for traffic. When the Piaggio said he was ready; I cleared him to proceed direct VTU; direct SXC; direct SLI; direct SNA; descend and maintain FL270. He read back my clearance as I issued it. There were a few other calls and clearances I issued; and then I returned to look at the Piaggio and the Embraer to assess if they were still traffic. I updated the route for the Piaggio to see how he was established to VTU and it looked like he was progressing direct after two minutes had elapsed since the clearance and clear of the Embraer. As he departed FL280 for FL270 I cleared the Piaggio to descend at pilot's discretion to FL240 and he read it back as well. I took a couple more calls and issued a couple more clearances and the machine went off. It no longer looked like the Piaggio was going to clear the Embraer. I told the Piaggio to report passing FL250 as the computer showed him passing FL260. He read that back. I turned the Embraer left to a 280 heading to ensure there was no way they would come together. When they were clear; I cleared the Embraer back to SNS. The Piaggio reported leaving FL250 for FL240 and asked if he could go direct VTU. I cleared him direct VTU; and he read it back again. Even though I looked at the Piaggio's route and history projection twice before I cleared him to FL240; if there's a doubt in my mind that the pilot may still be on a different heading; I should verify with him his heading and turn him if necessary to guarantee I clear traffic before I descend him. In this case I should have asked the Piaggio to say heading for VTU before issuing a pilot's discretion to FL240. This would have clarified the ambiguity that seems to have existed in the pilot's mind. Also; since it looked clear; but close; I should have ensured I had a back up plan so that if it didn't work; I could still keep the two aircraft five miles and 1;000 FT clear. In this case; it was too close for such a backup plan; so in my judgment (in retrospect); I should have just waited and let the Piaggio level at FL270 until he passed the Embraer. Maybe there's a better; less ambiguous way to clear a pilot from an initial heading to aim in the direction of a NAVAID other than to 'Fly heading; when able proceed direct (to fix). This may be ambiguous to some pilot's because apparently this pilot did not recognize he was cleared direct to the fix already.




Synopsis: ZLA Controller experienced a loss of separation when issuing a clearance that did not provide adequate lateral separation from another conflicting aircraft; the reporter listing a number of alternative actions that would have prevented this occurrence.