A CPR JET CAPT RPTED EXECUTING INSTRUCTIONS ISSUED BY D21 WHICH WERE MEANT FOR ANOTHER ACFT WITH A SIMILAR SOUNDING CALL SIGN.

Narrative

ON DOWNWIND FOR ILS RWY 21L AT DTW; I WAS THE PF; LEVEL AT 7000 FT AND HDG 030 DEGS. THE DETROIT APCH CTLR INSTRUCTED A DSCNT TO 3000 FT AND A L TURN (BASE LEG) 300 DEGS. NOT HEARING THE 'TAIL NUMBER' THE CTLR STATED; FOLLOWING THE PNF ACKNOWLEDGEMENT OF ATC INSTRUCTIONS...I PROCEEDED TO INQUIRE WITH PNF IF THE INSTRUCTIONS WERE INDEED FOR US. WITH AN AFFIRMATIVE; I INITIATED THE DSCNT AND THE TURN. PASSING THROUGH 5500 FT; ATC INSTRUCTED US TO LEVEL AT 5000 FT AND MAKE A TIGHT R TURN TO 070 DEGS THEN FURTHER R TO 090 DEGS. I WAS THEN WONDERING IF THOSE INSTRUCTIONS WERE MEANT FOR ACFT Y AS THE PNF LATER WONDERED AND NOTED TO ME. ATC LATER ADVISED US TO MAKE A TIGHT L TURN TO 250 DEGS; LATER DSNDING US TO 3000 FT FOR ILS RWY 21L 'VISUAL.' WE COMPLETED THE APCH AND THE DAY'S FLT WITHOUT FURTHER INCIDENT. CONTRIBUTING FACTORS: 1) FAILURE OF PNF TO CORRECTLY LISTEN TO (WHO-WHOM) INSTRUCTIONS WERE DIRECTED. 2) FAILURE OF PF TO ASK ATC; NOT PNF; IF INSTRUCTIONS WERE FOR PF OF THAT ACFT. CORRECTIVE ACTIONS: 1) IMPLEMENTED NEW FLT DEPT RULE. IF EITHER PF OR PNF DOES NOT HEAR THE ACFT IDENT WITHIN ATC INSTRUCTIONS; THEN PF OR PNF WILL RECONFIRM WITH ATC WHETHER INFLT OR ON THE GND.