SINGLE TWR CTLR DESCRIBED GEAR-UP LNDG EVENT; NOT OBSERVED; BECAUSE OF ADMIN DUTIES; PLT LISTED CHECKLIST/FATIGUE AND STATE OF MIND AS CAUSAL.

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CESSNA REQUESTED 3 FULL-STOP LNDGS AND I ASSUME THE PLT WAS GETTING NIGHT CURRENCY. ON THE THIRD AND LAST LNDG RWY Y; CTLR HAD BACK TO OP AND WAS TAKING CARE OF SOME PAPERWORK WHEN HE HEARD WHAT SOUNDED LIKE METAL SCRAPING THE GND. THE CTLR OBSERVED X CESSNA HAD COME TO A COMPLETE STOP AND APPEARED TO HAVE LANDED WITH ITS GEAR UP. THE CTLR; WHO WAS WORKING ALONE AT THE TIME; IMMEDIATELY NOTIFIED RESCUE PERSONNEL AND ATTEMPTED TO ASCERTAIN IF THE PLT WAS OK. THE PLT ACKNOWLEDGED THAT HE WAS NOT HURT AND RWY Y WAS CLOSED. THERE WAS ONLY 1 PERSON ON BOARD -- THE PIC. STAFFING FOR THIS TIME OF NIGHT AT ZZZ TYPICALLY IS ONLY 1 CTLR AFTER XA45. FORTUNATELY; THE OTHER CTLR; WHO WAS SCHEDULED TO GO HOME; HAD NOT LEFT THE BUILDING AND WAS ABLE TO ASSIST THE CTLR IN HANDLING THIS SITUATION. THE NTSB IN CONFERENCE WITH THE MGR ON DUTY FOR THE ARPT AND THE REGIONAL DUTY OFFICER FOR THE FAA DETERMINED THIS TO BE AN INCIDENT. SUPPLEMENTAL INFO FROM ACN 786527: GEAR UP LNDG ON THIRD LNDG (NIGHT CURRENCY FLT). CONTRIBUTORS COULD HAVE BEEN AN INCOMPLETE USE OF CHKLIST AND A TIRED PLT. STATE OF MIND MAY ALSO HAVE BEEN A FACTOR.