Narrative:

Aircraft 1 was level at 10000' on a vector for the ILS 32. Aircraft 2 was handed off to me at 9000', overflying westbound. I was not yet talking to aircraft 2. When I gave aircraft 1 a left turn to 180 degrees I meant to restate his altitude of 10000' because by this time aircraft 1 and 2 were less than 3 mi apart. Looking directly at aircraft 2 I said to aircraft 1, 'maintain 9000'.' aircraft 1 read back 9000'. Because aircraft 1 read back the altitude my mouth said, my ears heard it, but my brain did not. I was very surprised when I looked back and saw aircraft 1 descending. When I went back and told him to maintain 10000' he was already through 9800'. When the pilot questioned me I in no uncertain terms told him I did not clear him to 9000'. I was extremely surprised to hear my voice on the tape say 9000'. My human error was compounded by the fact that I didn't hear anything wrong with the readback. Listening closely to readbacks is the only way I can catch my human errors.

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Original NASA ASRS Text

Title: 2 ACFT APCHING ON PARALLEL HEADINGS SEPARATED BY 1000'. APCH CTLR TURNS THE TOP ACFT AND MISTAKENLY DESCENDS THE ACFT TO THE SAME ALT OF THE OTHER ACFT. ACFT PAS WITH LESS THAN STANDARD SEPARATION.

Narrative: ACFT 1 WAS LEVEL AT 10000' ON A VECTOR FOR THE ILS 32. ACFT 2 WAS HANDED OFF TO ME AT 9000', OVERFLYING WBND. I WAS NOT YET TALKING TO ACFT 2. WHEN I GAVE ACFT 1 A LEFT TURN TO 180 DEGS I MEANT TO RESTATE HIS ALT OF 10000' BECAUSE BY THIS TIME ACFT 1 AND 2 WERE LESS THAN 3 MI APART. LOOKING DIRECTLY AT ACFT 2 I SAID TO ACFT 1, 'MAINTAIN 9000'.' ACFT 1 READ BACK 9000'. BECAUSE ACFT 1 READ BACK THE ALT MY MOUTH SAID, MY EARS HEARD IT, BUT MY BRAIN DID NOT. I WAS VERY SURPRISED WHEN I LOOKED BACK AND SAW ACFT 1 DSNDING. WHEN I WENT BACK AND TOLD HIM TO MAINTAIN 10000' HE WAS ALREADY THROUGH 9800'. WHEN THE PLT QUESTIONED ME I IN NO UNCERTAIN TERMS TOLD HIM I DID NOT CLR HIM TO 9000'. I WAS EXTREMELY SURPRISED TO HEAR MY VOICE ON THE TAPE SAY 9000'. MY HUMAN ERROR WAS COMPOUNDED BY THE FACT THAT I DIDN'T HEAR ANYTHING WRONG WITH THE READBACK. LISTENING CLOSELY TO READBACKS IS THE ONLY WAY I CAN CATCH MY HUMAN ERRORS.

Data retrieved from NASA's ASRS site as of July 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.