Narrative:

Lax center issued clearance to us. 'Air carrier XX32 descend to FL290 for traffic.' copilot read it back and I started down at 1500 FPM. Then the controller said 'air carrier XX32 verify you are level at FL330.' as were at FL320. I immediately leveled at FL320 to avoid any conflicts at 31000 or whatever alerted the ATC computer. The controller was talking to another air carrier Y at 31000'. We asked what he wanted us to do. He said the clearance to 29000' was for air carrier XY32. We said we read back XX32 to 290 and started down appropriately, he said continue on down to 29000' and he'd check it out. The controller left the frequency soon thereafter and we were issued the next frequency change by another controller. We read back and executed the clearance as issued to air carrier XX32 to descend to FL290. We saw no other aircraft or heard about any actual conflicts. Supplemental information from acn 158246. Air carrier XX32 was cleared by lax center to descend from FL330 to FL290 now for crossing traffic. I read back the clearance 'descend from FL330 to FL290 air carrier XX32 leaving now.' approaching FL320 a second controller requested verification of our altitude, I responded air carrier XX32 descending through FL320 to FL290 as cleared.' the controller requested we level at FL320 and instructed air carrier Y to make a turn and look for traffic. Air carrier Y advised they had us 'in sight no problem'. After approximately 15 seconds with no response from lax center, I inquired as to whether we should continue our descend to FL290 or return to FL330. The second controller requested we continue our descend to FL290. We were never advised by lax center there was another flight with a similar number and because I read the clearance back and received no correction from lax center we had no reason to believe the clearance was not for us.

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Original NASA ASRS Text

Title: ACR XX32 HAD LESS THAN STANDARD SEPARATION FROM ACR Y. ACR XX32 UNAUTH ALT CHANGE. PLTDEV.

Narrative: LAX CTR ISSUED CLRNC TO US. 'ACR XX32 DSND TO FL290 FOR TFC.' COPLT READ IT BACK AND I STARTED DOWN AT 1500 FPM. THEN THE CTLR SAID 'ACR XX32 VERIFY YOU ARE LEVEL AT FL330.' AS WERE AT FL320. I IMMEDIATELY LEVELED AT FL320 TO AVOID ANY CONFLICTS AT 31000 OR WHATEVER ALERTED THE ATC COMPUTER. THE CTLR WAS TALKING TO ANOTHER ACR Y AT 31000'. WE ASKED WHAT HE WANTED US TO DO. HE SAID THE CLRNC TO 29000' WAS FOR ACR XY32. WE SAID WE READ BACK XX32 TO 290 AND STARTED DOWN APPROPRIATELY, HE SAID CONTINUE ON DOWN TO 29000' AND HE'D CHK IT OUT. THE CTLR LEFT THE FREQ SOON THEREAFTER AND WE WERE ISSUED THE NEXT FREQ CHANGE BY ANOTHER CTLR. WE READ BACK AND EXECUTED THE CLRNC AS ISSUED TO ACR XX32 TO DSND TO FL290. WE SAW NO OTHER ACFT OR HEARD ABOUT ANY ACTUAL CONFLICTS. SUPPLEMENTAL INFO FROM ACN 158246. ACR XX32 WAS CLRED BY LAX CTR TO DSND FROM FL330 TO FL290 NOW FOR XING TFC. I READ BACK THE CLRNC 'DSND FROM FL330 TO FL290 ACR XX32 LEAVING NOW.' APCHING FL320 A SECOND CTLR REQUESTED VERIFICATION OF OUR ALT, I RESPONDED ACR XX32 DSNDING THROUGH FL320 TO FL290 AS CLRED.' THE CTLR REQUESTED WE LEVEL AT FL320 AND INSTRUCTED ACR Y TO MAKE A TURN AND LOOK FOR TFC. ACR Y ADVISED THEY HAD US 'IN SIGHT NO PROB'. AFTER APPROX 15 SECS WITH NO RESPONSE FROM LAX CTR, I INQUIRED AS TO WHETHER WE SHOULD CONTINUE OUR DSND TO FL290 OR RETURN TO FL330. THE SECOND CTLR REQUESTED WE CONTINUE OUR DSND TO FL290. WE WERE NEVER ADVISED BY LAX CTR THERE WAS ANOTHER FLT WITH A SIMILAR NUMBER AND BECAUSE I READ THE CLRNC BACK AND RECEIVED NO CORRECTION FROM LAX CTR WE HAD NO REASON TO BELIEVE THE CLRNC WAS NOT FOR US.

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.