FLC OF AN MLG LANDED ON THE RWY AT NIGHT DUE TO DISTR ON ATTEMPTING TO TUNE IN THE NEWLY ASSIGNED RWY ILS FREQ.

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DFW APCH CLRED US DIRECT TO THE OM FOR ILS RWY 17L AT DFW (DFW HAS 4 PARALLEL RWYS) AND TURNED US OVER TO TWR. WE WERE APCHING FROM THE NE AT APPROX 45 DEGS TO THE ILS WHEN WE THOUGHT DFW TWR SWITCHED US TO RWY 17R. UNLIKE APCH; THE TWR XMISSIONS WAS VERY GARBLED AND BROKEN. WE DIALED IN THE ILS FOR RWY 17R BUT WERE UNABLE TO RECEIVE A SIGNAL. AFTER SOME TIME AND DISCUSSION; WE ELECTED TO RE-ENTER THE FREQ FOR RWY 17L. BY NOW; WE HAD PASSED THE OM AND WE TURNED ONTO FINAL; VISUALLY LINING UP ON WHAT WE THOUGHT WAS RWY 17R. IT WAS ACTUALLY RWY 18R AS WE HAD UNKNOWINGLY PASSED BOTH E SIDE RWYS WHILE ATTEMPTING TO IDENT RWY 17R ILS. WE CONCENTRATED ON THE VISUAL LNDG PROFILE AND MADE A NORMAL AND UNEVENTFUL LNDG. WE WERE AWARE OF ONLY ONE OTHER ACFT THAT LANDED AHEAD OF US. IT WAS ONLY AFTER EXITING THE RWY AND BEGINNING THE TAXI DID WE REALIZE WE HAD LANDED ON RWY 18R INSTEAD OF RWY 17R. NO MENTION WAS EVER MADE BY TWR OR GND CTL (WHICH WAS ALSO BROKEN AND GARBLED AND STAFFED BY THE SAME INDIVIDUAL WHO WAS WORKING TWR) AND WE ARE STILL NOT SURE WE DID INDEED LAND ON THE INCORRECT RWY. IN RETROSPECT; IT IS CLR WE BECAME DISTRACTED AND ABSORBED IN TRYING TO ENTER AND USE THE ILS FREQ FOR THE RWY OF INTENDED USE. ONLY AFTER THE EVENT DID WE LEARN THAT OUR ACFT NAV RADIO WAS NOT EQUIPPED TO RECEIVE FREQS WITH THE 0.05 SPACING REQUIRED FOR RWY 17R ILS FREQ OF 111.35. THIS NAVIGATIONAL HANDICAP HAD NOT BEEN DISCLOSED NOR ADDRESSED BY OUR COMPANY. THIS LED US TO DIVERT TOO MUCH TIME AND ATTN; USED PRECIOUS TIME; AND SET US UP FOR THE FALL.