ATCT LCL CTLR CLEARED ACR-MLG TO LAND ON RIGHT RWY WHEN HE SHOULD HAVE STATED LEFT RWY. MLG EXECUTED GO AROUND ACCOUNT TRAFFIC ON RWY.

Narrative

CLRED LDA RWY 12L APCH AT STL IN VISUAL CONDITIONS TO FOLLOW AN SMT ON APCH FOR RWY 12R. WE CALLED THE SMT IN SIGHT (ABOUT 3 MI AHEAD) AND WE WERE TOLD TO FOLLOW HIM TO THE ARPT; MAINTAIN VISUAL SEP. HE WAS FOR RWY 12R; WE WERE FOR RWY 12L. WE WERE SWITCHED TO THE TWR ABOUT 6 MI OUT (N TWR 120.05) AND RPTED IN FOR LNDG. TWR CTLR SAID; 'CLRED TO LAND RWY 12R;' AND CAPT READ BACK; 'CLRED TO LAND 12R.' EMPHASIZING THE 'RIGHT' TO MAKE SURE. I FELT WE WERE TOO CLOSE TO THE SMT (ABOUT 2 MI AHEAD); BUT CONTINUED; THINKING HE WOULD MAKE AN EARLY TURN OFF. ON SHORT FINAL WE ASKED TWR IF THE SMT WAS GOING TO CLEAR. TWR SEEMED CONFUSED BY OUR REQUEST; SO WE INITIATED A GO AROUND AT 100' OVER THE THRESHOLD. TWR THEN TOLD US WE WERE SUPPOSED TO BE ON RWY 12L (MY FEELING ALL ALONG); BUT NEVER CORRECTED US WHEN WE READ BACK OUR LNDG CLRNC FOR RWY 12R; AND NEVER NOTICED OUR SIDESTEP AND APCH TO THE RIGHT. WE SHOULD HAVE INITIATED ANOTHER CALL TO THE TWR TO CONFIRM THE RWY CHANGE BEFORE ASSUMING THE TWR CTLR HAD SWITCHED OUR RWY. THE ABSENCE OF A FREQ CHANGE TO THE S TWR SHOULD HAVE BEEN ANOTHER CLUE. I LOOKED BEHIND ME VERY CAREFULLY BEFORE SIDESTEPPING TO THE RIGHT AND THERE WAS NO CONFLICT; AND THAT GAVE ME MORE REASON TO BELIEVE THE CTLR HAD DONE US A FAVOR AND SWITCHED US TO RWY 12R FOR A SHORTER TAXI. PERHAPS IT WAS A LESS EXPERIENCED CREW WORKING TWR/APCH ON THIS HOLIDAY--EVERYONE WAS GETTING A 25 MI FINAL ON A VFR/VMC DAY.