ACR B727 GAR WHEN UNABLE TO COM WITH TWR ON EITHER OF 2 PUBLISHED FREQS. TWR WAS USING 2 UNPUBLISHED FREQS. RPTR STATED THIS INFO WAS NOT ON THE ATIS.

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I AM THE FO AND THE PF. CLRED RWY 27L APCH. APCH CTL TOLD US TO CONTACT TWR AT WILLT; 120.75. AT WILLT WE CALLED; NO ANSWER. WE TRIED 126.9; NO ANSWER. UNABLE TO CONTACT TWR ON ANY OF THE PUBLISHED FREQS. INITIATED A GAR. VECTORS AROUND TO AN UNEVENTFUL APCH AND LNDG; USING TWR FREQ 127.92. WX WAS VFR AND NO CONFLICT OCCURRED TO OUR KNOWLEDGE. ORD WAS USING TWR FREQ 132.7 AND 127.92 -- NEITHER OF WHICH ARE PUBLISHED EITHER BY NOTAM; ATIS; OR FIELD RPT. IS THERE A WAY WHEN TWR IS USING UNPUBLISHED FREQS THAT THEY BE BROADCAST ON ATIS OR NOTAM? CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR INDICATED THE REASON FOR THE USE OF THE UNPUBLISHED FREQS WAS; ACCORDING TO THE GND CTLR; DUE TO INTERFERENCE PROBS ON THE TWR FREQS. RPTR SAID THEY HAD HEARD THE APCH CTLR GIVE THE OLD FREQ AND THEN CORRECT IT WITH THE NEW FREQ TO A COUPLE OF ACFT AHEAD OF THEM. ANALYST MADE A CALLBACK TO THE FACILITY AND LEARNED THAT THE ONLY TIME THE UNPUBLISHED FREQS ARE USED IS WHEN THERE IS A STUCK MIKE OR SOME OTHER SIT CAUSING A TEMPORARY INTERRUPTION TO THE FREQS. RPTR INDICATED THEY DO NOT PUT IT ON THE ATIS BECAUSE OF THE SHORT TERM USE.