COMMUTER CLRING RWY AT TKOF END. MLG ON FINAL WENT AROUND FROM LOW ALT.

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WE WERE TAXIING FOR DEP ON 12L AT STL. A COMPANY ACFT WAS INSTRUCTED TO S-TURN TO MAKE ROOM FOR DEPARTING TFC ON 12L. AN MLG WAS CLRED FOR TKOF AND WE WERE TOLD TO POS AND HOLD AND BE READY. WE ACKNOWLEDGED AND COMPLIED. SOON AFTER GETTING INTO POS THE TWR SAID THAT THIS 'WASN'T GOING TO WORK' AND TO CLR 12L TO THE L SIDE. WE COMPLIED AND ASKED FOR; AND RECEIVED; PERMISSION TO TURN AROUND ON THE PARALLEL RWY 13. WE THOUGHT THE MLG WAS LNDG ON 12L AS WAS IMPLIED BY STL TWR. I LOOK OUT FOR THE TFC AND HE IS NOT ON FINAL FOR 12L. I LOOK FURTHER TO THE R REAR OF THE ACFT AND SEE HE IS LINED UP AND APPARENTLY LNDG ON RWY 13. I TELL THE CAPT THIS AND WE BECOME VERY CONCERNED BECAUSE THE HOLD SHORT LINE FOR 12L ON THE L SIDE PUTS PART OF OUR ACFT ON RWY 13. THE NEXT THING I HEAR IS THE MLG DECLARING A GAR. WE QUESTIONED THE TWR AND WERE TOLD THE MLG HAD GONE AROUND. FURTHER INVESTIGATION REVEALED THAT MLG ACFT ARE NOT ALLOWED TO USE RWY 13 AT ALL DUE TO THE WING SPAN. I FEEL THAT THERE WAS SOME CONFUSION BTWN STL TWR AND THE MLG CREW AND THE TWR WAS TRYING TO PUSH OUT TOO MANY DEPS. I BELIEVE; IN MY OPINION THAT THE MLG LINED UP ON THE WRONG RWY AND REALIZED THE MISTAKE AT THE LAST MOMENT AVERTING A POTENTIAL ACCIDENT.