UNAUTHORIZED LNDG BY ACR. LNDG WITHOUT ATC CLRNC.

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I WAS PF (FO); CAPT WAS OPERATING RADIO. WE WERE CLRED FOR VISUAL APCH TO THE VFR RWY (PARALLEL) 18; TO CALL TWR ON FINAL. NO OTHER TFC FOR THAT RWY; BUT TFC LNDG ON 17R AND L. AFTER LNDG; I NOTICED WE HAD NOT SWITCHED TO LCL CTL. CONTRIBUTING FACTORS: REDUCED REST THE NIGHT BEFORE; (LESS THAN 9 HRS) AFTER A LONG DUTY DAY; VFR WX WHICH CONTRIBUTED TO A RELAXED -- TOO RELAXED -- STATE OF MIND. MY OWN MENTAL STATE THAT WRONGLY ASSUMED THAT SINCE WE WERE HEARING RADIO CHATTER (APCH CTL) AND OUR FLT NUMBER WASN'T BEING CALLED; WE WERE OK. THE CAPTAIN AND I ARE BOTH HIGHLY EXPERIENCED; ALTHOUGH NOT AT THIS ARPT. NEITHER OF US HAD EVER LANDED WITHOUT A CLRNC BEFORE. THE SIMPLE THINGS DO COUNT! CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR GAVE INFO ON HIS ACFT. GAVE RUNDOWN ON WORK SCHEDULE. DESCRIBED WORK/REST CYCLE ON 4 DAY SEQUENCE. CHKLIST PROC REQUIRES PNF TO PERFORM MOST OF CHKLIST WHILE ALSO COMMUNICATING WITH ATC DURING LNDG PHASE. WAS FIRST APCH AND LNDG ON RWY 18 FOR BOTH PIC AND FO. PIC HAD HAD VERY LONG DAY ON PREVIOUS DAY. THINKS FATIGUE WAS PRIMARY FACTOR IN INCIDENT. 2 RADIOS WERE SET ON CENTER FREQ AND APCH CTL FREQ DURING LNDG.