ACFT MAKES UNCOMMANDED TURN OFF RWY DURING LNDG ROLL.

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WE WERE ON APCH TO SLC RWY 16. I; THE FO; WAS FLYING THE AIRPLANE. VISUAL CONTACT WITH THE RWY WAS MADE ABOUT 3-4 MI OUT. TWR RPTED WIND SHEAR; AND WE DECIDED TO INCREASE REF TO VREF PLUS 20. THE LNDG WAS NORMAL AND THE AIRPLANE WAS ON THE CTRLINE. AS THE SPD SLOWED TO APPROX 60 KIAS; THE CAPT SAID; 'I HAVE THE AIRPLANE;' AND I GAVE HIM THE CTLS. (THIS IS A NORMAL PROC WITH OUR COMPANY.) THE DIRECTIONAL CTL WAS STILL NORMAL. THE CAPT MOVED THE SPD LEVERS TO A LOW POS; AND THE ACFT SUDDENLY VEERED TO THE R. THE CAPT IMMEDIATELY PUT IN FULL RUDDER (L) AND FULL L REVERSE; BUT WAS UNABLE TO KEEP THE ACFT GOING STRAIGHT. WHEN I REALIZED WE WERE RUNNING OFF THE RWY; I IMMEDIATELY PULLED BOTH STOP AND FEATHERS. AS THE ACFT CAME TO A STOP; THE CAPT SHUT OFF FUEL AND HYD. ATC WAS NOTIFIED. WHEN WE OPENED THE CURTAIN; THE PAX HAD ALREADY STARTED EVAC THROUGH THE OVERWING EXITS. THE CAPT OPENED THE MAIN CABIN DOOR; AND NOTICED AT THIS TIME A TONGUE OF FLAMES COMING OUT OF THE L INTAKE. WE REACHED FOR THE FIRE EXTINGUISHER; BUT THE FLAME EXTINGUISHED ITSELF BEFORE WE EVEN GOT TO IT. THE PAX WERE NOW EVACED THROUGH THE MAIN CABIN DOOR. I CHKED WITH ALL PAX TO MAKE SURE NOBODY WAS INJURED. EVERYBODY WAS OK. IT IS HARD TO SAY EXACTLY WHAT WAS THE CAUSE OF THE INCIDENT; SINCE THE INVESTIGATION IS STILL ONGOING. HOWEVER; I DO SUSPECT THAT THE NOSEWHEEL STEERING SYS ON THE FAIRCHILD METROLINER MAY HAVE FAILED. I WAS NOT CTLING THE ACFT; AND IT IS THEREFORE HARD TO SAY; BUT THE FACT THAT THE AIRPLANE VEERED R JUST AS THE SPD LEVERS WERE BROUGHT LOW (NOSEWHEEL STEERING ACTIVATED) GIVES ME AN INDICATION OF A NOSEWHEEL STEERING PROB. CAPT TRIED TO OPPOSE THE R TURN BY L RUDDER AND L REVERSE; BUT WAS UNABLE. ALL I COULD DO WAS TO PULL THE STOP AND FEATHERS AS WE RAN OFF THE SIDE OF THE RWY. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE ACFT TYPE WAS A METROLINER SA 227. THE RPTR WAS SENT TO SCHOOL IN A NEW ACFT TYPE WITHIN DAYS OF THE INCIDENT SO WAS UNABLE TO LEARN THE CAUSE OF THE UNCOMMANDED TURN. THE ACFT WAS ON CTRLINE AND PERFORMING NORMALLY DOWN THE CTRLINE UNTIL THE CAPT CALLED FOR SPD LEVERS TO LOW POS. THIS ACTION ACTIVATES THE NOSEWHEEL STEERING IF IT IS ARMED. THE PROC AT THE AIRLINE IS NOT TO ARM THE NOSEWHEEL STEERING UNTIL NECESSARY. THE CAPT HAS STATED THAT HE DID NOT BELIEVE THAT IT WAS ARMED. THE COPLT SAYS THE CAPT HAD HIS HAND ON THE NOSEWHEEL ARMING SWITCH ABOUT THE TIME OF THE TURN AND DOES NOT KNOW WHETHER THE SWITCH WAS ARMED OR DISARMED PRIOR TO THE TURN. THE WX HAD GUSTY WINDS; BUT THE RPTR DOES NOT BELIEVE THE ACFT WAS RESPONDING TO WINDS WITH THE UNCOMMANDED TURN.