GA-SMT DEVIATED FROM ASSIGNED HEADING AND HAD TO REQUEST 360 DEGREE TURN TO AVOID OVERTAKING ACR-MLG AHEAD ON APCH.

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SUN (A BEAUTIFUL DAY); 8/88; ABOUT XA00 LCL TIME. SMT WAS CLRED FROM TORONTO ISLAND TO BUF AT 3500'. TORONTO CTL TURNED US OVER TO BUF APCH. TRANSPONDER AND ALTIMETER SETTINGS WERE SET AND WE WERE IDENTED. I HAD A COURSE SET UP TO TAKE SMT TO THE OM FOR RIGHT TURN INTO RWY 23 AT BUF. APCH ADVISED WE WOULD BE FOLLOWING AN MLG. THE THOUGHT THAT SMT WOULD CATCH UP TO MLG DIDN'T SEEM A POSSIBILITY. SHORTLY AFTER THAT WE WERE CLRED TO A HDG OF 140 DEGS AND QUERIED TO SEE IF THE ARPT WAS IN SIGHT AT 1-2 O'CLOCK. THAT WAS VERIFIED AND I REQUESTED DSCNT TO 2000'. THAT WAS ACKNOWLEDGED BY 'DSND AT PLT'S DISCRETION.' BASED ON THE QUERY OF SEEING THE ARPT; AND THE 'DSND PLT DISCRETION;' TRIGGERED THE THOUGHT WE WERE CLRED VISUALLY TO THE ARPT. WHILE SETTING #2 COM TO TWR FREQ; AND SELECTING THE OTHER COM BACK TO APCH; WE WERE DSNDING ON A HDG TO MAKE A RIGHT TURN WITH A SHORT BASE. I COULD SEE THE MLG ON FINAL AND REALIZED WE WERE TOO CLOSE TO FOLLOW. THE RADIO MIXUP HAD DISCONNECTED US FROM APCH AND TO EXPEDITE IMMEDIATE COM I CALLED THE TWR AND REQUESTED A LEFT 360 DEG TO GET RE-ESTABLISHED ON FINAL. THAT WAS GRANTED AND SMT WAS CLRED TO LAND. GND CTL ASKED ME TO CALL APCH WHEN I ARRIVED AT THE TERMINAL PHONE. THIS I DID AND EXPLAINED THE RADIO MIX-UP IN THE COCKPIT. THE CTLR TOLD HIS SIDE OF THE STORY WHICH I UNDERSTOOD. TWO THINGS WENT WRONG: THE UNDERSTANDING BY THE PLT OF WHAT HE THOUGHT APCH INTENDED 'DSND AT PLT'S DISCRETION;' PLUS THE FUMBLING OF RADIO SELECTORS; PUT SMT WHERE APCH HAD NOT INTENDED IT TO BE. FORTUNATELY THE VIS WAS EXCELLENT AND NO 2 ACFT TRIED TO OCCUPY THE SAME AIRSPACE. AFTER THE CONVERSATION AND EXPLANATION TO THE CTLR; I SUSPECT MY SINS HAD BEEN CLEANED. I DON'T RECALL AN ADDENDUM TO THE DSND AT PLT DISCRETION; STATING SMT WAS TO MAINTAIN THE ASSIGNED HDG; WHICH WOULD HAVE BEEN MORE DEFINITIVE. ON THE OTHER HAND; A PLT SHOULD NOT ASSUME; HE SHOULD KNOW BY SPECIFIC INFO OR ASK! AFTER ALMOST 50 YRS OF THIS; I KNOW FLT IS SURROUNDED BY BOOBY TRAPS. IT BEHOOVES ALL OF US IN THE CIRCUIT TO SPEAK AND TO LISTEN AS THOUGH OUR LIVES DEPEND ON IT--THEY DO.