Narrative:

Sequencing air carrier X and air carrier Y inbound to pdx. Both assigned speeds to achieve separation. Air carrier Y at FL340 behind and below air carrier X at FL400 same course. Issued air carrier X pilot's discretion to FL350 with speed reduction. Issued air carrier Y pilot's discretion to FL240 with speed reduction. When air carrier Y started descent; leaving FL329; I issued FL330 to air carrier X. Loss of separation occurred. Issued air carrier Y to increase rate of descent for traffic. Air carrier Y responded. Prevent recurrence: issue air carrier Y descend and maintain FL240 instead of pilot's discretion clearance.

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Original NASA ASRS Text

Title: ZSE CTLR EXPERIENCED OPERROR AT FL330 WHEN ALLOWING TFC TO DSND VIA PLT'S DISCRETION RESULTING IN A LOSS OF SEPARATION.

Narrative: SEQUENCING ACR X AND ACR Y INBOUND TO PDX. BOTH ASSIGNED SPDS TO ACHIEVE SEPARATION. ACR Y AT FL340 BEHIND AND BELOW ACR X AT FL400 SAME COURSE. ISSUED ACR X PLT'S DISCRETION TO FL350 WITH SPD REDUCTION. ISSUED ACR Y PLT'S DISCRETION TO FL240 WITH SPD REDUCTION. WHEN ACR Y STARTED DSCNT; LEAVING FL329; I ISSUED FL330 TO ACR X. LOSS OF SEPARATION OCCURRED. ISSUED ACR Y TO INCREASE RATE OF DSCNT FOR TFC. ACR Y RESPONDED. PREVENT RECURRENCE: ISSUE ACR Y DSND AND MAINTAIN FL240 INSTEAD OF PLT'S DISCRETION CLRNC.

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.