A METROLINER OPERATING A SCHEDULED PAX FLT FROM HVR TO LWT DEPARTED THE RWY AFTER ATTEMPTING TO APPLY TKOF PWR WHICH RESULTED IN DAMAGE TO THE R PROP.

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AS DESCRIBED IN INCIDENT RPT; DEP POINT HVR DEST LWT. AFTER LATE DEP OUT OF BIL; WENT THROUGH COMPLETE CHKLIST -- AS IF FIRST FLT OF THE DAY. ALL SYS CHKED OK. NORMAL TAXI; TKOF AND DEP OUT OF BIL. AFTER VFR APCH INTO LWT AND A NORMAL LNDG. TAXIED TO RAMP AND PARKED. QUICK TURNED; NORMAL TAXI TO RWY. LINED UP; PWRED UP THROUGH 25%. CAPT RELEASED BRAKES. TORQUE CAME UP WITH ABOUT A 5% SPLIT THROUGH 40%. AT THAT POINT; CAPT PULLED PWR BACK AND STARTED REVERSING. I LOOKED UP; SAW THE ACFT STARTING TO THE R WITH REVERSE AND BRAKES HAD NO EFFECT. ACFT STRUCK A 3 FT SNOW BANK. THERE WAS DAMAGE TO THE R PROP. THE ACFT WAS IN THE FIRST 300 FT OR SO OF THE ROLL. THE BEGINNING OF THE ROLL SEEMED NORMAL AND THE TORQUE RISE WAS NORMAL. THE ONLY THING I CAN THINK OF IS THE NOSEWHEEL STEERING. AFTER TALKING WITH THE FLT OPS DIRECTOR AND OUR CHIEF PLT; I'M SURE THE ACFT WAS OFF THE START LOCKS. METROS WON'T GO THROUGH 20% IF THEY'RE ON THE LOCKS. I'M NOT SURE IF THE CAPT RELEASED THE BUTTON OR IF THEY CONTINUED TO TRY TO STEER IT TO THE L. THERE WAS NO INDICATION OF A NOSEWHEEL STEERING FAIL LIGHT. WE WERE ALSO THE FIRST CREW TO FLY AFTER REPAIRS HAD BEEN MADE TO THE NOSEWHEEL STEERING SYS THAT AFTERNOON. THE WORK WAS SIGNED OFF BY MAINT AND NOSEWHEEL STEERING TESTS WERE COMPLETED BY MAINT IN A TAXI CHK. WE ALSO COMPLETED SAS; NTS; AND NOSEWHEEL STEERING TEST BEFORE LEAVING BILLINGS. AS I TOLD THE FAA; I'M FROM TEXAS. I HAVE NO ICE AND SNOW EXPERIENCE BEFORE COMING TO MONTANA. I DON'T THINK I WOULD HAVE DONE ANYTHING DIFFERENT. AFTER THE ACFT CAME TO A STOP; THE MAINS WERE STILL ON THE RWY. THE NOSE WAS OFF IN THE GRASS (AFTER PLOWING). THE RWY HAD NOT BEEN COMPLETELY PLOWED.