RWY INCURSION. LNDG WITHOUT CLRNC FROM TWR.

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RPT DAMAGED IN MAIL. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: ACFT WAS CLRED FOR VISUAL APCH TO RWY 31C BY APCH CTL AND TO CONTACT TWR AT THE OM. ATTN WAS DIVERTED BY A MESSAGE TO CONTACT THE FBO WHERE THEY WOULD BE PARKING. FLC FAILED TO CONTACT THE TWR AT OM AND LANDED WITHOUT CLRNC AND; THEY DIDN'T REALIZE IT UNTIL THE RPTR FO LOOKED AT THE FREQS ON THE APCH PAGE AND SAW THAT APCH WAS STILL SET. AFTER CONTACTING GND CTL FOR TAXI INSTRUCTIONS; THEY DIDN'T SAY ANYTHING ABOUT THE LNDG. DURING A SUBSEQUENT FLT THE RPTR AGAIN ALMOST FORGOT TO CONTACT TWR AND MADE A COMMENT TO THE CTLR TO THAT EFFECT; TO WHICH THE CTLR RESPONDED 'IT IS A COMMON OCCURRENCE HERE AT MDW.' RPTR THINKS PERHAPS ONE OF THE CONTRIBUTING CAUSES IS BEING CLRED FOR A VISUAL APCH AND NOT TELLING THE FLC TO CONTACT TWR AT THAT TIME. INSTEAD; BEING INSTRUCTED TO CONTACT TWR AT OM WHEN ON A VISUAL APCH; LEADS TO FORGETTING TO CALL. WHEREAS; ON AN IFR APCH IT IS NOT LIKELY TO BE MISSED. ANOTHER POSSIBLE FACTOR THAT MAY CONTRIBUTE TO THE PROB IS THE SIMILARITY OF FREQS OF APCH CTL AND TWR; APCH IS 118.4 AND TWR IS 118.7.