Narrative:

I was en route from smf to psp. I was PIC of aircraft. I was in right seat. A junior captain was flying in the left seat. We were in a slow descent from FL330. ATC was very busy. We were given dscnts in 1000 ft increments in a very rhythmic fashion just prior to leveling at the prescribed altitude. Prior to leveling at FL210. ATC gave us what we later found out was a traffic report. 'Traffic 10-11 O'clock, 2 mi, FL200.' we thought ATC told us to descend and maintain FL200. We provided the standard readback, 'roger, descend and maintain FL200.' ATC did not catch our mistake, nor did we. We proceeded past FL210 to FL200. After a few seconds of quiet, ATC asked us to verify our altitude of FL210. I replied, 'was cleared to FL200, but we will return to FL210.' he replied (ATC), 'maintain FL210, traffic 10-11 O'clock, B737, 2 mi.' we were back at FL210 by then and replied, 'no joy, level FL210.' bottom of altitude deviation was FL203. Autoplt was on the whole time except for altitude correction. As soon as we replied level FL210, ATC cleared us to 'descend and maintain FL200.' once on the ground, psp ATC asked us to call ZLA. ZLA operational supervisor spoke to me. Informed me of the details of the tape. Aircraft in loss of separation was lost. After reviewing tapes, it was clear we mistook the traffic report for another 1000 ft descent and the controller did not catch the faulty readback from me. The controller was immediately taken off duty for 'retraining.' my first officer and I were also taken off flight status and brought back to headquarters for debriefing and possible/certain retraining. I feel both parties are very proactive in remedying this situation.

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Original NASA ASRS Text

Title: CPR HS1000 DEV ALT AFTER ARTCC CTLR MISSES INCORRECT READBACK ARRIVING PSP.

Narrative: I WAS ENRTE FROM SMF TO PSP. I WAS PIC OF ACFT. I WAS IN R SEAT. A JUNIOR CAPT WAS FLYING IN THE L SEAT. WE WERE IN A SLOW DSCNT FROM FL330. ATC WAS VERY BUSY. WE WERE GIVEN DSCNTS IN 1000 FT INCREMENTS IN A VERY RHYTHMIC FASHION JUST PRIOR TO LEVELING AT THE PRESCRIBED ALT. PRIOR TO LEVELING AT FL210. ATC GAVE US WHAT WE LATER FOUND OUT WAS A TFC RPT. 'TFC 10-11 O'CLOCK, 2 MI, FL200.' WE THOUGHT ATC TOLD US TO DSND AND MAINTAIN FL200. WE PROVIDED THE STANDARD READBACK, 'ROGER, DSND AND MAINTAIN FL200.' ATC DID NOT CATCH OUR MISTAKE, NOR DID WE. WE PROCEEDED PAST FL210 TO FL200. AFTER A FEW SECONDS OF QUIET, ATC ASKED US TO VERIFY OUR ALT OF FL210. I REPLIED, 'WAS CLRED TO FL200, BUT WE WILL RETURN TO FL210.' HE REPLIED (ATC), 'MAINTAIN FL210, TFC 10-11 O'CLOCK, B737, 2 MI.' WE WERE BACK AT FL210 BY THEN AND REPLIED, 'NO JOY, LEVEL FL210.' BOTTOM OF ALTDEV WAS FL203. AUTOPLT WAS ON THE WHOLE TIME EXCEPT FOR ALT CORRECTION. AS SOON AS WE REPLIED LEVEL FL210, ATC CLRED US TO 'DSND AND MAINTAIN FL200.' ONCE ON THE GND, PSP ATC ASKED US TO CALL ZLA. ZLA OPERATIONAL SUPVR SPOKE TO ME. INFORMED ME OF THE DETAILS OF THE TAPE. ACFT IN LOSS OF SEPARATION WAS LOST. AFTER REVIEWING TAPES, IT WAS CLR WE MISTOOK THE TFC RPT FOR ANOTHER 1000 FT DSCNT AND THE CTLR DID NOT CATCH THE FAULTY READBACK FROM ME. THE CTLR WAS IMMEDIATELY TAKEN OFF DUTY FOR 'RETRAINING.' MY FO AND I WERE ALSO TAKEN OFF FLT STATUS AND BROUGHT BACK TO HEADQUARTERS FOR DEBRIEFING AND POSSIBLE/CERTAIN RETRAINING. I FEEL BOTH PARTIES ARE VERY PROACTIVE IN REMEDYING THIS SIT.

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.