Narrative:

Air carrier X at FL380 cleared to sjc via the ROBIE2 arrival and to maintain FL380. Good readback from pilot. Air carrier Y on a 250 degree heading; FL360; for sequencing to sjc to go behind the air carrier X (northbound). J-ring displayed for air carrier Y to show when the air carrier X would be laterally separated (and subsequently descended for sjc). I made a few contacts with other aircraft in the sector (ZOA sector 14); and during the sector san noticed air carrier X descending out of FL374 approximately 4.3 mi north of air carrier Y. Aircraft on diverging courses. I immediately questioned air carrier X about altitude assignment; at which time he said something unintelligible; so I restated the cleared altitude 'climb and maintain FL380.' air carrier X read back altitude clearance; but had by this time descended through FL362 and the next radar hit showed FL367 climbing. Air carrier Y had been issued a descent to FL350 in between xmissions to air carrier X. 5 mi lateral separation showed at the time air carrier X was climbing back through FL367. Pilot initiated descent without a clearance. Possibly the pilot believed that he could descend on the ROBIE2 arrival because he had been cleared via the ROBIE2; but he was also cleared to maintain FL380 which he correctly read back. He did not initiate the descent until several mins later after being cleared to sjc via the ROBIE2 arrival. The reason we restate the altitude after issuing the pertinent arrival is to prevent this from happening. There were no similar sounding call signs in the sector at the time of the incident.

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Original NASA ASRS Text

Title: ZOA CTLR DESCRIBED CONFLICT AT FL362 WHEN SJC ARR BEGAN UNAUTH DESCENT AND CONFLICTED WITH ANOTHER SJC ARR.

Narrative: ACR X AT FL380 CLRED TO SJC VIA THE ROBIE2 ARR AND TO MAINTAIN FL380. GOOD READBACK FROM PLT. ACR Y ON A 250 DEG HDG; FL360; FOR SEQUENCING TO SJC TO GO BEHIND THE ACR X (NBOUND). J-RING DISPLAYED FOR ACR Y TO SHOW WHEN THE ACR X WOULD BE LATERALLY SEPARATED (AND SUBSEQUENTLY DSNDED FOR SJC). I MADE A FEW CONTACTS WITH OTHER ACFT IN THE SECTOR (ZOA SECTOR 14); AND DURING THE SECTOR SAN NOTICED ACR X DSNDING OUT OF FL374 APPROX 4.3 MI N OF ACR Y. ACFT ON DIVERGING COURSES. I IMMEDIATELY QUESTIONED ACR X ABOUT ALT ASSIGNMENT; AT WHICH TIME HE SAID SOMETHING UNINTELLIGIBLE; SO I RESTATED THE CLRED ALT 'CLB AND MAINTAIN FL380.' ACR X READ BACK ALT CLRNC; BUT HAD BY THIS TIME DSNDED THROUGH FL362 AND THE NEXT RADAR HIT SHOWED FL367 CLBING. ACR Y HAD BEEN ISSUED A DSCNT TO FL350 IN BTWN XMISSIONS TO ACR X. 5 MI LATERAL SEPARATION SHOWED AT THE TIME ACR X WAS CLBING BACK THROUGH FL367. PLT INITIATED DSCNT WITHOUT A CLRNC. POSSIBLY THE PLT BELIEVED THAT HE COULD DSND ON THE ROBIE2 ARR BECAUSE HE HAD BEEN CLRED VIA THE ROBIE2; BUT HE WAS ALSO CLRED TO MAINTAIN FL380 WHICH HE CORRECTLY READ BACK. HE DID NOT INITIATE THE DSCNT UNTIL SEVERAL MINS LATER AFTER BEING CLRED TO SJC VIA THE ROBIE2 ARR. THE REASON WE RESTATE THE ALT AFTER ISSUING THE PERTINENT ARR IS TO PREVENT THIS FROM HAPPENING. THERE WERE NO SIMILAR SOUNDING CALL SIGNS IN THE SECTOR AT THE TIME OF THE INCIDENT.

Data retrieved from NASA's ASRS site as of January 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.