Narrative:

On sat at approximately XX30 local, I was working sector right/D 11 in oak ARTCC's airspace. I had 2 single engine propellers to oak on V301. Small aircraft X was level at 8000'. I descended small aircraft Y to 9000' and quoted traffic on small aircraft X at 1 O'clock and 4 mi level at 8000'. Small aircraft Y read back, 'roger, descending to 8 we're IMC.' I missed his incorrect readback. About 5 mins later I observed the small aircraft Y descending through 9000'. I asked him to verify level at 9000'. He said negative, 'we understood 8, we read back 8.' I said, 'negative, you were to maintain 9000, maintain 9000.' the pilot of the small aircraft Y responded, 'roger, back to 9.' sep was lost 2 NM southeast of karnn intersection at XX37 vtc. Closest proximity was 1.5 mi and 600'. Small aircraft Y descended through 9000 to 8600', then back to 9000'.

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Original NASA ASRS Text

Title: ACFT DESCENDED BELOW ASSIGNED ALT DUE TO HEARBACK PROBLEM.

Narrative: ON SAT AT APPROX XX30 LCL, I WAS WORKING SECTOR R/D 11 IN OAK ARTCC'S AIRSPACE. I HAD 2 SINGLE ENG PROPS TO OAK ON V301. SMA X WAS LEVEL AT 8000'. I DSNDED SMA Y TO 9000' AND QUOTED TFC ON SMA X AT 1 O'CLOCK AND 4 MI LEVEL AT 8000'. SMA Y READ BACK, 'ROGER, DSNDING TO 8 WE'RE IMC.' I MISSED HIS INCORRECT READBACK. ABOUT 5 MINS LATER I OBSERVED THE SMA Y DSNDING THROUGH 9000'. I ASKED HIM TO VERIFY LEVEL AT 9000'. HE SAID NEGATIVE, 'WE UNDERSTOOD 8, WE READ BACK 8.' I SAID, 'NEGATIVE, YOU WERE TO MAINTAIN 9000, MAINTAIN 9000.' THE PLT OF THE SMA Y RESPONDED, 'ROGER, BACK TO 9.' SEP WAS LOST 2 NM SE OF KARNN INTXN AT XX37 VTC. CLOSEST PROX WAS 1.5 MI AND 600'. SMA Y DSNDED THROUGH 9000 TO 8600', THEN BACK TO 9000'.

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.