RWY EXCURSION AFTER LOSS OF ACFT CTL. NIGHT OP.

Narrative

UPON TOUCHDOWN ON RWY 30L AT CPS; ACFT IMMEDIATELY SLID OFF L SIDE OF RWY IN ABOUT A 20 DEG R YAW. DEPARTED RWY ABOUT 1500 FT DOWN AND SKIDDED 2500 FT PARALLEL TO RWY; AND THEN GOT ACFT BACK ON RWY. NO ONE WAS INJURED AND ONLY MINOR DAMAGE TO ACFT. TEMP AT TIME OF OCCURRENCE WAS RPTED ABOUT 35 DEGS F; HOWEVER; IT WAS FOUND OUT LATER THAT LOW GND TEMPS HOVERED AT FREEZING. DEW POINT/TEMP SPREAD 1 DEG. WIND 4 KTS QUARTERING TAILWIND. SNOW AND ICE HAD BEEN REMOVED FROM ACTIVE RWY SEVERAL DAYS BEFORE BUT REMAINED IN LOW PILES AT SIDES OF RWY. ATIS WAS RPTING BRAKING ACTION ON UNUSED OTHER RWY; BUT NO BRAKING ACTION WAS GIVEN ON ACTIVE RWY; LEADING US TO BELIEVE BRAKING ACTION GOOD. WE LATER FOUND OUT THAT BRAKING ACTION HAD NOT BEEN TESTED FOR AT LEAST 3 HRS PRIOR TO OUR LNDG. WE ALSO HAD HEARD FROM THE LINEMAN; WHEN WE PARKED; THAT A LEAR JET HAD LANDED PRIOR TO US AND HAD A 'WILD RIDE' AND SKIDDED SIDEWAYS ON RWY. I GUESS THE LEAR DID NOT RPT THIS TO TWR AND THE TWR COULD NOT SEE THE RWY DUE TO FOG; SO WE DID NOT HAVE THIS INFO ON LNDG. MY GUESS IS THAT WE HIT BLACK ICE WITH 1 GEAR FIRST WHICH YAWED US TO THE R AND THEN WE SLID OFF RWY. IT ALL HAPPENED SO FAST THERE WAS NO CTLING THE ACFT UNTIL IT WAS OFF THE SIDE OF RWY. ONCE IN THE GRASS I COULD CTL DIRECTION OF ACFT AND PARALLEL RWY UNTIL I GOT BACK ON RWY.