Narrative:

Aircraft #1 level at FL380 landing okc. Aircraft #2 level at FL370 nebound. My radar associate reminded me that aircraft #1 was landing okc and would need lower. I acknowledged the statement and did other things that needed to be done. By the time I got back to descending aircraft #1 and looking for possible traffic; the data block for aircraft #2 had been covered up by a third aircraft data block. Unable to see aircraft #2's data block; I descended aircraft #1 to FL360. I went on to make other clrncs until the pilot of aircraft #1 asked if the target at his 4 O'clock position was real. After moving the data block covering up aircraft #2's data block; I responded 'affirmative.' separation had already been lost; and since the aircraft were already tail-to-tail; I did not take action. Separation was re-established 5 seconds later.

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Original NASA ASRS Text

Title: ZFW CTLR EXPERIENCED OPERROR AT FL370 DURING DATA BLOCK OVERLAP WHEN DSNDING ACFT THROUGH OCCUPIED ALT.

Narrative: ACFT #1 LEVEL AT FL380 LNDG OKC. ACFT #2 LEVEL AT FL370 NEBOUND. MY RADAR ASSOCIATE REMINDED ME THAT ACFT #1 WAS LNDG OKC AND WOULD NEED LOWER. I ACKNOWLEDGED THE STATEMENT AND DID OTHER THINGS THAT NEEDED TO BE DONE. BY THE TIME I GOT BACK TO DSNDING ACFT #1 AND LOOKING FOR POSSIBLE TFC; THE DATA BLOCK FOR ACFT #2 HAD BEEN COVERED UP BY A THIRD ACFT DATA BLOCK. UNABLE TO SEE ACFT #2'S DATA BLOCK; I DSNDED ACFT #1 TO FL360. I WENT ON TO MAKE OTHER CLRNCS UNTIL THE PLT OF ACFT #1 ASKED IF THE TARGET AT HIS 4 O'CLOCK POS WAS REAL. AFTER MOVING THE DATA BLOCK COVERING UP ACFT #2'S DATA BLOCK; I RESPONDED 'AFFIRMATIVE.' SEPARATION HAD ALREADY BEEN LOST; AND SINCE THE ACFT WERE ALREADY TAIL-TO-TAIL; I DID NOT TAKE ACTION. SEPARATION WAS RE-ESTABLISHED 5 SECONDS LATER.

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.