COMMUTER SMT CLRNC RESPONSE TO WRONG CALL SIGN CAUSES HEADING DEVIATION AND EXCURSION FROM CLRNC ALT.

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I WAS THE CAPT FROM LGA NONSTOP TO MVY. OUR LAST ASSIGNED CLRNC WAS FLY DIRECT TO MVY AND MAINTAIN 5000'. I WAS THE PNF. APPROX 20 MI FROM MVY I TOLD THE COPLT TO GUARD COM #1 (ATC FREQ) AS I NEEDED TO CONTACT OUR OPS IN MVY ON COM #2 TO ASK SEVERAL QUESTIONS CONCERNING PAX LOAD; FUEL; ETC. AT ABOUT HALFWAY THROUGH MY CONVERSATION WITH COMPANY THE COPLT STARTED A DSCNT AND TURNED TO A HDG OF 180 DEGS (LAST HDG WAS 070 DEGS). I FINISHED MY COMS WITH COMPANY AND ASKED THE COPLT ABOUT OUR NEW CLRNC. HE STATED WE WERE CLRED TO 2000' AND TO FLY A 180 DEG HDG. AT APPROX 3700' MSL THE APCH CTLR SAID VERIFY LEVEL AT 5000'. I THEN STATED I THOUGHT WE WERE CLRED TO 2000'. THE CTLR THEN SAID MAINTAIN 3000'. ABOUT 15 SECS LATER THE CTLR SAID VERIFY HDG. I SAID 180 DEGS ASSIGNED. THE CTLR SAID 'YOU TOOK THE CLRNC FOR ANOTHER FLT; YOURS WAS TURN RIGHT 10 DEGS; MAINTAIN 5000'.' THIS OF COURSE HAPPENED WHILE I WAS OFF FREQ; AND THE COPLT WAS WORKING THE RADIO. WE HAD NO CONFLICT WITH ANY OTHER ACFT THAT I KNOW ABOUT. WE WERE THEN CLRED DIRECT TO THE ARPT AND COMPLETED A NORMAL APCH AND LNDG. I'M STILL DEBATING HOW TO AVOID THIS SITUATION IN THE FUTURE. IT IS MY OPINION THAT A PLT SHOULD NOT TRY TO MONITOR 2 COM RADIOS AT THE SAME TIME. PERHAPS I WILL REVERIFY THE CLRNC WITH CTLR BEFORE ALLOWING THE ACFT TO TURN OR DSND. HUMAN PERFORMANCE CONSIDERATIONS: THE CTLR SHOULD HAVE STOPPED US WHEN THE COPLT READ BACK THE WRONG CLRNC. COPLT WAS EXPERIENCED AND I TENDED TO TRUST HIM. FIRST TIME I HAD FLOWN WITH THIS COPLT. I WAS NEW IN THIS ACFT. ACR X IS MERGING WITH ACR Y. I AM ACX CREW. COPLT WAS ACR Y. I DID NOT WANT TO OFFEND HIM BY REVERIFYING OUR CLRNC. TENSION BTWN THE 2 PLT GROUPS IS VERY HIGH DUE TO ONGOING LITIGATION IN COURT OVER SENIORITY MATTERS.