Narrative:

During descent from cruise, air carrier X was reclred to maintain what we thought was 12000'. We read back 12000'. The controller had apparently cleared us to maintain 16000' and didn't catch it till we were leaving 15000', as he had conflicting traffic at 15000'. We saw the other aircraft, and since we were at 14300' and descending at 2500 FPM, we continued descending as it was faster than climbing back to 16000'. No evasive action necessary. Distance approximately 4-5 mi. Supplemental information from acn 174891: during descent from cruise, we were originally cleared to 11000'. At approximately 20800', controller asked us to stop the descent at what we both understood and read back as 12000'. Passing through 15000' at about 2000 FPM, the controller advised us of a traffic conflict with an aircraft at 15000', and asked why we hadn't leveled off at 16000'. The controller had not previously made us aware of the other aircraft. He told us to climb back to 16000'. At this time, though, we were already passing through 14300' with other aircraft visually in sight. We told the controller we thought there would be a greater traffic conflict if we climbed back to 16000'. No evasive action was necessary. We passed approximately 4-5 mi of the other aircraft. The controller seemed fairly busy, possibly working more than 1 frequency.

Related reports:

Google
 

Original NASA ASRS Text

Title: ACR X DESCENT TO WRONG ALT THROUGH OCCUPIED ALT HAD LESS THAN STANDARD SEPARATION FROM ACR Y. SYSTEM ERROR. PLT DEVIATION.

Narrative: DURING DSNT FROM CRUISE, ACR X WAS RECLRED TO MAINTAIN WHAT WE THOUGHT WAS 12000'. WE READ BACK 12000'. THE CTLR HAD APPARENTLY CLRED US TO MAINTAIN 16000' AND DIDN'T CATCH IT TILL WE WERE LEAVING 15000', AS HE HAD CONFLICTING TFC AT 15000'. WE SAW THE OTHER ACFT, AND SINCE WE WERE AT 14300' AND DSNDING AT 2500 FPM, WE CONTINUED DSNDING AS IT WAS FASTER THAN CLBING BACK TO 16000'. NO EVASIVE ACTION NECESSARY. DISTANCE APPROX 4-5 MI. SUPPLEMENTAL INFO FROM ACN 174891: DURING DSNT FROM CRUISE, WE WERE ORIGINALLY CLRED TO 11000'. AT APPROX 20800', CTLR ASKED US TO STOP THE DSNT AT WHAT WE BOTH UNDERSTOOD AND READ BACK AS 12000'. PASSING THROUGH 15000' AT ABOUT 2000 FPM, THE CTLR ADVISED US OF A TFC CONFLICT WITH AN ACFT AT 15000', AND ASKED WHY WE HADN'T LEVELED OFF AT 16000'. THE CTLR HAD NOT PREVIOUSLY MADE US AWARE OF THE OTHER ACFT. HE TOLD US TO CLB BACK TO 16000'. AT THIS TIME, THOUGH, WE WERE ALREADY PASSING THROUGH 14300' WITH OTHER ACFT VISUALLY IN SIGHT. WE TOLD THE CTLR WE THOUGHT THERE WOULD BE A GREATER TFC CONFLICT IF WE CLBED BACK TO 16000'. NO EVASIVE ACTION WAS NECESSARY. WE PASSED APPROX 4-5 MI OF THE OTHER ACFT. THE CTLR SEEMED FAIRLY BUSY, POSSIBLY WORKING MORE THAN 1 FREQ.

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.