AN ACR MLG SLID OFF OF THE EDGE OF A TXWY IN ICY CONDITIONS.

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ALTHOUGH USING A SLOWER THAN NORMAL SPD ON TAXI OUT FOR DEP; THE ACFT LOST TOTAL BRAKE EFFECTIVENESS AND NOSEWHEEL STEERING AUTHORITY AFTER ENCOUNTERING A SECTION OF TXWY COVERED WITH CLR ICE. DESPITE LOCKED BRAKES; IDLE ENG THRUST WAS ENOUGH TO CONTINUE ACFT MOTION AT LEAST 1500 FT DOWN THE TXWY. FEARING DEP FROM THE PAVED SURFACE; REVERSE THRUST WAS UTILIZED. HOWEVER; THE USE OF THIS THRUST; AND A QUARTERING TAILWIND; HELPED INDUCE AN UNCTLED YAWING THAT LED TO A SLOW SKID. AS THE SKID AND WXVANING CONTINUED; THE AIRPLANE BEGAN TO MOVE LATERALLY FROM THE PARALLEL TXWY; ON WHICH ORIGINALLY LOCATED; ONTO THE 90 DEG TXWY LEADING TO THE DEP END OF THE INTENDED TKOF RWY. NOSEWHEEL STEERING WAS HELD INTO AND LATER AGAINST THE SKID; BUT PROVED TO BE TOTALLY INEFFECTIVE. DESPITE REVERSE THRUST AGAIN BEING APPLIED; THE NOSEWHEEL DEPARTED THE PAVED SURFACE OF THE TXWY; ON A HDG OF APPROX 140 DEGS; AND ACFT MOVEMENT FINALLY CEASED. THE ORIGINAL AIRPLANE HDG WAS APPROX 010 DEGS WHILE NBOUND ON THE TXWY. NO DAMAGE WAS INCURRED TO THE ACFT OR ANY SURFACE STRUCTURES. ATIS AND GND CTL RPTS INDICATED THAT THE RWYS HAD BEEN CHEMICALLY TREATED AND SANDED AS HAD SOME OF THE TXWYS. A PLT RPT OF 'FAIR TO GOOD' BRAKING ACTION EXISTED ON RWY 19L. LIGHT FREEZING RAIN HAD BEEN FALLING IN THE AREA FOR SOME TIME PRIOR TO; AND DURING; ENG START AND TAXI. WITH THE COMPLETION OF FINAL ACFT DEICE; TAXI CLRNC TO RWY 19L WAS RECEIVED. INITIAL ACFT MOVEMENT WAS EXTREMELY CAUTIOUS. THE RAMP AREAS HAD RECEIVED NO DEICE ATTN AND SIMPLY WALKING WAS DIFFICULT. AFTER MOVING ONTO THE MAIN TXWY; NOSEWHEEL STEERING AND BRAKE APPLICATIONS WERE EFFECTIVE BUT DEGRADED. AS THE TAXI PROGRESSED; THE LIGHT GROSS WT OF THE ACFT; COMBINED WITH THE ADDED THRUST OF BOTH ENGS OPERATING; CAUSED THE AIRPLANE TO WANT TO ACCELERATE. SO; AS STATED EARLIER; WHEN THE AREA OF ICE WAS ENCOUNTERED; THE LOSS OF BRAKE EFFECTIVENESS WAS READILY APPARENT AND CAUSED IMMEDIATE CONCERN. I AM OF THE OPINION THAT THE ARPT AUTHORITY UNDERESTIMATED THE SEVERITY OF THE WX CONDITIONS THAT EXISTED; AND WAS NOT CAPABLE OF MONITORING AND MAINTAINING MULTI-RWY OPS GIVEN THE EQUIP AND PERSONNEL ON HAND.