WHILE ON SHORT FINAL LNDG STL; AN MD80 PIC INADVERTENTLY SWITCHED THE RADIO TO GND FREQ.

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I WAS FLYING THE ACFT. THE CAPT WAS THE PNF. WE WERE ON FINAL TO STL RWY 12R AND WERE CLRED FOR THE VISUAL. AT 1000 FT AGL; I HAD TO MANEUVER ABOVE THE GS AND S OF THE LOC TO AVOID A LARGE FLOCK OF GEESE. WHILE I RE-STABILIZED THE ACFT ON THE APCH PRIOR TO 500 FT AFL ; THE CAPT RPTED THE FLOCK OF BIRDS TO THE TWR CTLR AND THEN RECEIVED LNDG CLRNC FROM THE CTLR FOR RWY 12R. SINCE I WAS CONCENTRATING ON THE LNDG; I DID NOT NOTICE THAT THE CAPT THEN INADVERTENTLY DIALED IN THE GND CTL FREQ INTO THE ACTIVE SIDE OF THE #1 VHF COM INSTEAD OF THE INACTIVE SIDE; THUS REPLACING THE TWR FREQ WITH GND. AFTER I COMPLETED THE LNDG ROLL AND XFERRED CTL OF THE ACFT BACK TO THE CAPT; IT BECAME APPARENT THAT WE WERE NO LONGER ON TWR FREQ. AFTER WE TURNED OFF; TAXIED IN; AND PARKED AT THE GATE; THE CAPT CALLED THE TWR CTLR ON TWR FREQ TO CHK IF THERE HAD BEEN ANY PROBS AND TO APOLOGIZE FOR PREMATURELY SWITCHING OFF FREQ. THE TWR CTLR SAID THAT HE HAD ATTEMPTED TO CONTACT US SEVERAL TIMES; BUT THAT IT HAD CAUSED NO PROBS; AND THERE WERE NO TFC CONFLICTS. THE INACTIVE SIDE OF THE VHF COM IN THIS MD80 HAS A RED BAR OVER THE FREQ WINDOW. THE INACTIVE SIDE OF THE VHF COM IN OTHER MD80'S DOES NOT HAVE THIS RED BAR. ON THE OTHERS; THERE IS MERELY A SMALL GREEN LIGHT ABOVE THE FREQ WINDOW THAT LIGHTS UP TO INDICATE WHICH FREQ IS ACTIVE. THE HABIT PATTERN IS TO DIAL UP THE NEXT ANTICIPATED FREQ INTO THE WINDOW WITH THE RED BAR; NOT TO CHK FOR THE ACTIVE VERSUS INACTIVE FREQ WINDOW BY GREEN LIGHT OR SWITCH POS. A DISTR CAN EASILY CAUSE AN ERROR. IT IS IMPORTANT TO CONFIRM THE ACTIVE FREQ BY SWITCH POS FOR EITHER CONFIGN BEFORE CHANGING THE FREQ IN THE INACTIVE WINDOW.