Narrative:

While setting up an arrival stream, sep was lost between air carrier X and air carrier Y. Normal procedures requires air carrier X type to be given to approach control at an altitude lower than arriving turbo jet aircraft. X requested to remain at FL230 as long as possible. The sector was not busy. The controller did not think X's altitude would be a problem with 2 turbo jet converging arrs, and if it was there would be time to resolve it. X was issued restriction to clear another air carrier Z. Situation developed further and X was in possible conflict with the other arrival Z. In the controller's mind, air carrier X had already been separated from air carrier Y. When action was taken to resolve the conflict with air carrier Z. This negated the earlier restriction (sep had not yet been effected) and air carrier X turned toward air carrier Y, and sep was lost. The controller initially did not think these aircraft would require control restrictions. The sector was not busy. The situation developed and the controller was not ready. Moral: always think, 'what if?', and have plans a & B ready. Callback conversation with reporter revealed the following: reporter (controller) clarified location. Said aircraft were same altitude when 5 mi latitude and when 3 mi latitude they had about 1000' vertical. Air carrier X came from over gill VOR and enters keann arrival route at about 90 degree angle, and this type is to cross keann at 16000' or below. Normally would have had altitude separation established well before aircraft got to den 046 degree right. Reporter gave yrs in ATC. Air carrier X rate of descent was much slower the reporter expected.

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Original NASA ASRS Text

Title: LESS THAN STANDARD SEPARATION BETWEEN 2 ACR. OPERATIONAL ERROR.

Narrative: WHILE SETTING UP AN ARR STREAM, SEP WAS LOST BTWN ACR X AND ACR Y. NORMAL PROCS REQUIRES ACR X TYPE TO BE GIVEN TO APCH CTL AT AN ALT LOWER THAN ARRIVING TURBO JET ACFT. X REQUESTED TO REMAIN AT FL230 AS LONG AS POSSIBLE. THE SECTOR WAS NOT BUSY. THE CTLR DID NOT THINK X'S ALT WOULD BE A PROB WITH 2 TURBO JET CONVERGING ARRS, AND IF IT WAS THERE WOULD BE TIME TO RESOLVE IT. X WAS ISSUED RESTRICTION TO CLEAR ANOTHER ACR Z. SITUATION DEVELOPED FURTHER AND X WAS IN POSSIBLE CONFLICT WITH THE OTHER ARR Z. IN THE CTLR'S MIND, ACR X HAD ALREADY BEEN SEPARATED FROM ACR Y. WHEN ACTION WAS TAKEN TO RESOLVE THE CONFLICT WITH ACR Z. THIS NEGATED THE EARLIER RESTRICTION (SEP HAD NOT YET BEEN EFFECTED) AND ACR X TURNED TOWARD ACR Y, AND SEP WAS LOST. THE CTLR INITIALLY DID NOT THINK THESE ACFT WOULD REQUIRE CONTROL RESTRICTIONS. THE SECTOR WAS NOT BUSY. THE SITUATION DEVELOPED AND THE CTLR WAS NOT READY. MORAL: ALWAYS THINK, 'WHAT IF?', AND HAVE PLANS A & B READY. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: RPTR (CTLR) CLARIFIED LOCATION. SAID ACFT WERE SAME ALT WHEN 5 MI LAT AND WHEN 3 MI LAT THEY HAD ABOUT 1000' VERT. ACR X CAME FROM OVER GILL VOR AND ENTERS KEANN ARR ROUTE AT ABOUT 90 DEG ANGLE, AND THIS TYPE IS TO CROSS KEANN AT 16000' OR BELOW. NORMALLY WOULD HAVE HAD ALT SEPARATION ESTABLISHED WELL BEFORE ACFT GOT TO DEN 046 DEG R. RPTR GAVE YRS IN ATC. ACR X RATE OF DSCNT WAS MUCH SLOWER THE RPTR EXPECTED.

Data retrieved from NASA's ASRS site as of August 2007 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.