Narrative:

Small aircraft Y was level at 11,000' at somto intersection southwest of ard VOR. Cga X was given descent to 12,000. Cga X descended to 11,000 thus causing a conflict with small aircraft Y. After listening to the tapes, cga X was given descent to 12,000, but read back 11,000 and the incorrect readback was not caught by controller. Supplemental information from acn 113783: we were given a descent to 12,000'. However, we both thought we heard 11,000'. 11,000' is the normal crossing altitude and it was raining with 'P' static so 11,000' felt right. I am positive we read back 11,000' when we acknowledged the descent. We set 11,000' in the alerter window and went to 11,000'. Upon arriving at 11,000' the controller said our descent limit was 12,000' and climb back to 12,000'. We did. No mention of diverted traffic was mentioned and we did not get a vector, etc. The problem as I see it is that we as pilots must listen and read back all clrncs to ATC. However, they very often don't listen to our readback. It is possible, and in fact often occurs, that in heavy radio traffic you mishear a clearance, that is why we read back. But I don't feel ATC has time to listen. In this case the radio traffic was very heavy and the controller was very busy. But, I am very sure (positive) that we read back 11,000' when we set it in the alerter.

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Original NASA ASRS Text

Title: CGA READ BACK AN INCORRECT DESCENT ALT WHICH WAS NOT CAUGHT BY THE CTLR. LESS THAN STANDARD SEPARATION RESULTED.

Narrative: SMA Y WAS LEVEL AT 11,000' AT SOMTO INTXN SW OF ARD VOR. CGA X WAS GIVEN DSCNT TO 12,000. CGA X DESCENDED TO 11,000 THUS CAUSING A CONFLICT WITH SMA Y. AFTER LISTENING TO THE TAPES, CGA X WAS GIVEN DSCNT TO 12,000, BUT READ BACK 11,000 AND THE INCORRECT READBACK WAS NOT CAUGHT BY CTLR. SUPPLEMENTAL INFORMATION FROM ACN 113783: WE WERE GIVEN A DSCNT TO 12,000'. HOWEVER, WE BOTH THOUGHT WE HEARD 11,000'. 11,000' IS THE NORMAL XING ALT AND IT WAS RAINING WITH 'P' STATIC SO 11,000' FELT RIGHT. I AM POSITIVE WE READ BACK 11,000' WHEN WE ACKNOWLEDGED THE DSCNT. WE SET 11,000' IN THE ALERTER WINDOW AND WENT TO 11,000'. UPON ARRIVING AT 11,000' THE CTLR SAID OUR DSCNT LIMIT WAS 12,000' AND CLIMB BACK TO 12,000'. WE DID. NO MENTION OF DIVERTED TFC WAS MENTIONED AND WE DID NOT GET A VECTOR, ETC. THE PROBLEM AS I SEE IT IS THAT WE AS PLTS MUST LISTEN AND READ BACK ALL CLRNCS TO ATC. HOWEVER, THEY VERY OFTEN DON'T LISTEN TO OUR READBACK. IT IS POSSIBLE, AND IN FACT OFTEN OCCURS, THAT IN HEAVY RADIO TFC YOU MISHEAR A CLRNC, THAT IS WHY WE READ BACK. BUT I DON'T FEEL ATC HAS TIME TO LISTEN. IN THIS CASE THE RADIO TFC WAS VERY HEAVY AND THE CTLR WAS VERY BUSY. BUT, I AM VERY SURE (POSITIVE) THAT WE READ BACK 11,000' WHEN WE SET IT IN THE ALERTER.

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.