CL60 CREW LOST ACFT CTL AFTER LNDG AND WENT OFF THE R SIDE OF THE RWY. THE NOSE STEERING SYS WAS MEL'ED INOP.

Narrative

AFTER FLYING A NORMAL APCH TO LNDG AT WAUKEGAN REGIONAL (UGN); I DEPLOYED SPOILERS AND THRUST REVERSERS TO SLOW THE ACFT. AFTER THE ACFT SLOWED FROM 126 KTS (VREF) TO ABOUT 100 KTS; THE ACFT BEGAN TO TURN FROM R TO L. I TRIED TO CTL THE ACFT'S UNCOMMANDED TURNS WITH DIFFERENTIAL BRAKING; AS THE NOSEWHEEL STEERING WAS INOP AND MEL'ED. HOWEVER; I WAS UNABLE TO REGAIN CTL AND THE ACFT DEPARTED THE R SIDE OF RWY 5 AT UGN; ABOUT 3000 FT FROM THE APCH END. WX AT UGN WAS 040 DEGS AT 13 KTS; GUSTING TO 18 KTS; CLR AND 10 SM VISIBILITY; ALTIMETER 30.28. UNDER THESE CONDITIONS AND COMPLYING WITH ALL MEL REQUIREMENTS; LNDG WAS BELIEVED TO BE SAFE. ALTHOUGH I DON'T BELIEVE THE NOSEWHEEL STEERING INOP WAS THE CAUSE OF THE INCIDENT; I DO BELIEVE IT LIMITED MY ABILITY TO REGAIN CTL OF THE ACFT. THERE WAS NO DAMAGE TO ACFT OR PROPERTY. NO ONE WAS INJURED. SUPPLEMENTAL INFO FROM ACN 573898: AFTER TOUCHDOWN AT UGN; THE CAPT AND I WERE USING STANDARD COMPANY CALLS WITH THE SPOILERS BEING DEPLOYED AND THE THRUST REVERSERS BEING DEPLOYED. THE FIELD CONDITIONS WERE DRY AND CLR. WE HAD TO ADD AN ADDITIONAL 2% TO THE LNDG DISTANCE DUE TO THE NOSEWHEEL STEERING BEING MEL'ED. ALL FACTORS WERE BRIEFED AND VERIFIED TO LNDG WITH NO NOSE STEERING. I THINK THE FACTORS THAT CONTRIBUTED TO THIS INCIDENT WERE THE NOSEWHEEL BEING MEL'ED AND THE UNEXPLAINABLE REASON IT STARTED TO SWAY UNCTLABLY.