FLC OF A BAE146 LANDED WITHOUT CONTACTING TWR FOR CLRNC. THEY NOTICED THEIR ERROR WHEN AFTER LNDG ATTEMPTING TO OBTAIN TAXI INSTRUCTIONS AND DISCOVERED THAT THEY WERE STILL ON APCH CTL FREQ.

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CLRED FOR VISUAL APCH IN DES MOINES; FOR RWY 31R. AUTOPLT FLEW THROUGH FINAL APCH COURSE; EVEN THOUGH IT SHOWED IT HAD CAPTURED THE LOC. CAPT AND I WERE TRYING TO FIGURE OUT WHAT AND WHY THE AUTOPLT HAD DONE WHAT IT DID. AS APCH WAS CLRING US FOR THE VISUAL APCH WE WERE NOT SURE IF THEY HAD TOLD US TO CONTACT THE TWR OR NOT. EITHER WAY AFTER WE LANDED I REALIZED WE WERE STILL ON APCH FREQ. I IMMEDIATELY SWITCHED FREQ AND TWR SAID 'CLRED TO YOUR GATE WITH ME.' NO FURTHER MENTION WAS MADE AS TO WHY WE DID NOT REPLY WHEN HE CLRED US TO LAND OR IF HE DID CLR US TO LAND. WE ASSUMED HE DID CLR US TO LAND BECAUSE NOTHING MORE WAS SAID. ON APCH IN DSM WE WERE FOLLOWING A CPR JET THAT WAS 5-10 MI AHEAD AND WAS WELL CLR BY THE TIME WE LANDED. NO OTHER PLANES WERE WAITING FOR TKOF OR CONFLICTS IN THE AREA EXISTED. THE CAPT AND I WERE DISTRACTED BY THE AUTOPLT AND WHEN WE DECIDED TO FORGET ABOUT IT AND CONCENTRATE ON THE JOB AT HAND (CHKLISTS; SETTING AND CALLING SPDS) WE HAD FORGOTTEN TO SWITCH FREQS. I WAS PNF ON REDUCED REST OVERNIGHT PREVIOUSLY WITH APPROX 5 HRS SLEEP. FATIGUE MAY HAVE BEEN A FACTOR IN MY ALERTNESS. APCH DID NOT QUERY TO SEE IF WE WERE STILL ON FREQ.