B727 CREW SLID OFF A TXWY WITH NIL BRAKING ACTION AT COS.

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AFTER LNDG AT COS ON RWY 17R AND AFTER SLOWING TO TAXI SPD TURNOFF WAS ACCOMPLISHED AT TXWY A5. AFTER APPROX 70 DEGS OF TURN THE ACFT BEGAN TO SLIDE STRAIGHT AHEAD AND ALL CTL OVER NOSEWHEEL STEERING WAS LOST. ALL BRAKING ACTION WAS LOST AND THE ACFT SLOWLY SLID OFF THE TXWY AND CAME TO REST WITH THE L MAIN GEAR AND THE NOSE GEAR IN THE GRASS WITH THE R MAIN GEAR STILL ON THE TXWY. COS ATIS (1 HR OLD) WAS CALLING CONDITIONS CALM 8 SM 50 SCATTERED -3/-3 PATCHY FOG WITH NO MENTION OF ANY PROBS WITH SURFACE CONDITIONS OR BRAKING ACTION. BRAKING ACTION WAS GOOD ON ROLLOUT WITH A LIGHT DUSTING OF SNOW ON THE RWY WITH RWY MARKINGS PLAINLY VISIBLE. TXWY A5 LOOKED THE SAME TO US AS THE RWY DID AND NO PROBS COMPLETING TURNOFF AND TAXI WAS ANTICIPATED. TO PREPARE THE TXWY FOR OUR TUG TO TOW US OUT OF THE GRASS IT TOOK 2 DEICE TRUCKS AND A SAND TRUCK TO TREAT TXWY A5. PEOPLE WERE HAVING TROUBLE JUST WALKING ON THE SLICK SURFACE. IF WE HAD BEEN ALERTED TO THESE CONDITIONS PRIOR TO LNDG BY TWR OR ATIS; WE WOULD HAVE SLOWED TO A CRAWL BEFORE ENTERING ANY TXWY AS OPPOSED TO SLOWING TO A SPD CONSISTENT WITH THE RWY CONDITIONS THAT WE FOUND UPON LNDG WITH TWR ASKING US ABOUT BRAKING ACTION. SUPPLEMENTAL INFO FROM ACN 605639: TWR ASKED US TO RPT ON THE RWY BRAKING CONDITIONS AND WE RESPONDED WITH GOOD. EVEN THOUGH WE DID NOT ANTICIPATE ANY PROBS TAXI SPD WAS VERY SLOW. I'M NOT SURE IF THE TXWY HAD BLACK ICE; BUT PERSONNEL ON THE GND WORKING TO PULL THE ACFT BACK ONTO THE TXWY TALKED ABOUT HOW SLIPPERY IT WAS JUST WALKING. ONE OF OUR RAMP AGENTS ALSO MENTIONED THAT THE ARPT AUTH OFTEN OVERLOOKS THE W SIDE OF THE AIRFIELD WHEN IT COMES TO CLRING THE RWY AND TXWYS. I THINK THE ARPT AUTH; WHICH SHOULD BE AWARE OF WHAT VARIOUS WX PHENOMENON CAN DO; SHOULD BE MORE PROACTIVE IN CHKING FIELD CONDITIONS WHEN THE WX CHANGES. THE FLT WE WERE ON ARRIVED AT THE SAME TIME ON A DAILY BASIS SO THE ARPT SHOULD KNOW WHEN TO EXPECT AN ARR THAT WOULD TRIGGER AN EXTRA FIELD INSPECTION AND SUBSEQUENT CLRING ACTIONS. I THINK THE TXWY SURFACE WAS JUST EXTRAORDINARILY SLICK AT THE TIME. A CAUTION/WARNING ON ATIS OR FROM THE TWR WOULD SURELY HELP ALERT US TO THE POTENTIAL PROB.