FLC OF AN UNSCHEDULED SA227 TURBOPROP CARGO ACFT LANDED GEAR UP DUE TO DISTRS CAUSING THE LNDG GEAR DOWN NORMAL PRE LNDG CHKLIST TO BE OVERLOOKED.

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FLT TUL TO FYV; AR. WHILE ON THE DSCNT FROM CRUISING ALT 9000 FT TO 4000 FT; PLT'S DISCRETION; I WAS PF; CAPT WAS PNF. WE WERE CLRED FOR LOC 16 APCH AT 3000 FT AT FYV; AR. I WAS A LITTLE HIGH; PULLED PWR BACK; GEAR WARNING HORN SOUNDED. AFTER APCH CHKLIST WAS COMPLETED; I ASKED FOR FLAPS 25 DEGS. ATIS WAS CALLING RWY 100 FT WIDE. ALL TXWYS WERE CLOSED EXCEPT 2 BECAUSE SNOW/ICE. PICKED UP THE VASI WHILE ON THE APCH AT 2-3 MI FROM FAF AT 2400 FT. AT THAT TIME; PNF TALKED TO TWR ABOUT RWY CONDITIONS AND DURING THAT TIME WE LACKED DOING THE NORMAL CREW CALLOUTS. I ASKED FOR FLAPS 50 DEGS; 1 MI BEFORE FAF AND I SAID I WAS GOING TO CONTINUE THE APCH; WHILE I BRIEFED; EXPECTING THE VISUAL ON THE CREW BRIEFING PRIOR TO APCH CHKLIST. PNF ASKED TWR TO TURN UP RWY LIGHTS WHEN WE SAW THE VASI. THE LIGHTS THEN WENT OUT AND THEN TO HIGH AGAIN. THAT OCCURRED RIGHT AT THE FAF WHEN I CALLED FOR GEAR DOWN AND BEFORE LNDG CHKLIST; BUT ALSO LISTENING TO THE TWR WHAT WAS GOING ON WITH THE LIGHTS AND PREPARING FOR A LNDG ON AN ICY RWY. PNF NEVER DID THAT FLOW NOR DID I CHK HIM ON THAT. WE DID NOT FINISH THE BEFORE LNDG CHKLIST. I WAS PREOCCUPIED WITH THE LNDG SO I DID NOT CHK AND FORGOT ABOUT IT. I THEN ASKED FOR FULL FLAPS AND AT 100 FT I DID NOT CHK 3 GREEN; NOR DID THE PNF CALL OUT THE ALTS. NORMALLY 3 GREEN WOULD OF COURSE BE COVERED ON THE CHKLIST. WE THEN LANDED GEAR UP; SKIDDED TO THE L SIDE; AND WENT OFF THE RWY AND TURNED COUNTERCLOCKWISE APPROX 110 DEGS AND MORE THAN 1/2 WAY DOWN THE RWY. CONTRIBUTING FACTORS: PLT FATIGUE; LONG DAYS PRIOR TO THAT DAY AVERAGED 13 HRS 1 DAY; SATURDAY 16 HR DUTY. LACK OF CREW COORD; CALLOUTS. INTERCOM WAS NOT WORKING PROPERLY. PNF COULD NOT HEAR ME 30-50% OF THE TIME. WINTER WX. RWY/TXWY CONDITIONS. PNF TALKING WITH THE TWR DURING ESSENTIAL PHASES OF THE APCH DURING ESSENTIAL CALLOUTS. PF NOT CORRECTING OR CHKING ANY MISSED ITEMS INCLUDING CHKLIST/FLOWS. RWY LIGHTS HIGH TO MEDIUM; OFF AND THEN HIGH AGAIN.