AN ACR FLC FLYING AN L1011 NEAR GLD MISTAKENLY ACCEPTS A DSCNT CLRNC MEANT FOR ANOTHER ACFT WITH A SIMILAR CALL SIGN.

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I WAS IN THE COCKPIT OF #1 ACR FLT ON SEP/XA/99 ACTING AS A LINE CHK PLT FOR THE FLT. I WAS DIVIDING MY TIME AND ATTN BTWN THE PLTS AND THE SO WHO WAS ON HIS SECOND TRIP. NEARING THE CHANGEOVER FROM ZKC TO ZDV; THE CREW ACKNOWLEDGED A CLRNC FROM ZKC FOR #1 ACR; 'DSND TO FL310.' THERE WAS NO REASON GIVEN FOR THE CLRNC. WE WERE IN CRUISE AT FL350; VMC; ON OUR FLT PLAN RTE AND HAD NOT REQUESTED NOR EXPECTED A CHANGE OF ALT. THE CREW ROGERED THE CLRNC BY THE CALL SIGN #1 ACR AND BEGAN A GENTLE DSCNT OUT OF FL350. SHORTLY AFTER BEGINNING THE DSCNT; ZKC GAVE US A FREQ CHANGE TO ZDV. THE CREW CHKED IN USING #1 ACR AND RPTED DSNDING TO FL310. DURING THIS TIME; EITHER FROM ZKC OR ZDV; THE CTLR XMITTED TO #2 ACR; 'CONFIRM YOU ARE DSNDING TO FL310.' I DID NOT HEAR THE REPLY. CTR ASKED OUR FLT FOR ITS ALT AND THE CREW RPTED AN ALT NEAR FL340. WE WERE SHORTLY GIVEN A CLRNC TO CLB TO FL350 WHICH THE CREW ACKNOWLEDGED AND COMPLIED WITH. I HAD VISUAL CONTACT WITH ANOTHER ACFT ON THE L SIDE OF THE ACFT AND NOTICED ON THE CAPT'S TCASII DISPLAY THAT THE ACFT WAS APPROX 1200 FT BELOW AND 3-5 MI TO OUR L MOVING IN ROUGHLY THE OPPOSITE DIRECTION TO OUR FLT. THERE WAS NO TA OR RA. I DO NOT RECALL ANY CTLR ISSUING A CAUTION TO #1 ACR OF ANOTHER FLT WITH A SIMILAR CALL SIGN AT ANY TIME DURING THIS TIME FRAME. NOR DO I RECALL HEARING A CTLR WARNING TO ANOTHER FLT ABOUT SIMILAR CALL SIGNS ON THE FREQ. THERE WAS NO PERCEIVED DANGER OR IMMEDIATE ACTION TAKEN OR NEEDED BY #1 ACR DURING THIS EVENT. THE CREW WAS CONCERNED THEY HAD TAKEN ANOTHER FLT'S CLRNC BUT ALL IN THE COCKPIT BELIEVE THE INITIAL CLRNC TO DSND ISSUED BY ZKC WAS FOR #1 ACR. IT WAS ONLY WHEN CTR QUERIED #2 ACR AS TO ITS ALT DID OUR FLT BECOME AWARE OF ANY POSSIBLE CONFUSION. AT NO TIME DID EITHER ZKC AND ZDV ADVISE ACFT OF THE OTHER FLTS WITH SIMILAR CALL SIGNS.