AN ATCT LCL CTLR ATTEMPTED TO MAKE CONTACT WITH AN INBOUND DC9 FLC; AND WAS UNABLE TO DO SO; BECAUSE THE PNF HAD INADVERTENTLY TUNED THE GND CTL FREQ INTO THE ACTIVE FREQ CTL HEAD. THE FLC LANDED WITHOUT A CLRNC.

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WE WERE ON A VISUAL APCH TO RWY 12R AT STL. WE WERE HANDED OFF TO TWR FREQ OUTSIDE THE OM. I WAS HAND-FLYING THE ACFT. UNKNOWN TO ME; THE FO ACCIDENTALLY TUNED GND CTL FREQ ON THE ACTIVE CTL HEAD; INSTEAD OF THE STANDBY HEAD; AND I LANDED WHILE ON GND CTL FREQ; WITHOUT HAVING HEARD THE LNDG CLRNC. I AM DILIGENT ABOUT CONFIRMING LNDG CLRNC; BUT; THIS TIME; I DIDN'T. THIS CONFIRMS FOR ME THE NEED TO HAVE A SYS FOR REMINDING THE CREW THAT THEY DO OR DO NOT HAVE LNDG CLRNC; OR AT LEAST HAVE NOT HEARD THE LNDG CLRNC. SOME PLTS PUT THE GND CTL FREQ IN THE STANDBY HEAD AFTER THEY RECEIVE LNDG CLRNC. SOME USE THE NOSE GEAR LIGHT SWITCH. WHATEVER THE SYS; THOUGH; IT CAN'T TAKE THE PLACE OF PAYING ATTN TO WHAT YOU ARE DOING -- SITUATIONAL AWARENESS. SUPPLEMENTAL INFO FROM ACN 336078: TO PREPARE FOR THE TAXI PHASE AFTER LNDG; I TUNED GND CTL ON THE UNUSED HEAD. BUT IN THE DARK COCKPIT; I TUNED THE OPERATING HEAD BY MISTAKE. RESULT: TWR COULDN'T CONTACT US TO SWITCH TO THE PARALLEL RWY AND WE NEVER RECEIVED OUR LNDG CLRNC.